Datasheet COM20051I Datasheet (Standard Microsystems Corporation)

COM20051I

Integrated Microcontroller and

ARCNET (ANSI 878.1) Interface

FEATURES

!"

High Performance/Low Cost

!"

Microcontroller Based on Popular 8051
Architecture

!"
!"
!"
!"
!"
!"
!"

The COM20051I is a low-cost, highly-integrated microcontroller incorporating a high-performance network controller
based on the ARCNET Token Bus Standard (ANSI 878.1). The COM20051I is based around the popular Intel 8051
architecture. The device is implemented using a microcontroller core compatible with the Intel 80C32 ROMless
version of the 8051 architecture. The COM20051I is ideal for distributed control networking applications such as
those found in industrial/machine controls, building/factory automation, consumer products, instrumentation and
automobiles.

The COM20051I contains many features that are beneficial for embedded control applications. T he microcontroller is
a fully-functional 16MHz 80C32 that is comparable to the Intel 80C32 with 2 timers. In contrast to other embedded
controller/networking solutions, the COM20051I adds a fully-featured, robust, powerful, and simple network interface
while retaining all of the basic 8051 peripherals, such as the serial port and counter/timers.

In addition, the COM20051I supports an Emulation Mode that permits the use of a standard 80C32 emulator in
conjunction with the COM20051I to develop software drivers for the network core.

256-byte page of the External Data Memory Space of the 80C32. This provides for an easy interface between the
CPU and the ARCNET core.

that provides highly-reliable and fault tolerant message delivery at data rates ranging from 5Mbps down to 156
Kbps with message sizes varying from 0 to 507 bytes. The ARCNET protocol offers a simple, standardized, and
easily-understood networking solution for any application. The network interface supports several media interfaces,
including RS-485, coaxial, and twisted pair in either bus or star topologies. The network interface incorporates
powerful diagnostic features for network management and fault isolation. These include duplicate node ID detection,
reconfiguration detection, receive all (monitor) mode, receiver activity, and token detection.

8051 Instruction Set Compatible

Intel
Drop-In Replacement for 80C32 PLCC
Network Supports up to 255 Nodes
Powerful Network Diagnostics
Maximum 507 Byte Packets
Duplicate Node ID Detection
Self-Configuring Network Protocol

The networking core is based around an ARCNET Token Bus protocol engine

GENERAL DESCRIPTION

ORDERING INFORMATION

Order Number: COM20051ILJ P

44 Pin PLCC Package

!"

Retains all 8051 Peripherals Including Serial I/O
and Two Timers

!"

Utilizes ARCNET Token Bus Network Engine

!"

Requires No Special Emulators

!"

5 Mbps to 156 Kbps Network Data Rate

!"

Network Interface Supports RS-485, Twisted Pair,
Coaxial, and Fiber Optic Interfaces

!"

Receive All Mode Allows Any Packet to Be
Received

ARCNET core is mapped to a

SMSC DS – COM20051I Rev. 03/27/2000

© STANDARD MICROSYSTEMS CORPORATION (SMSC) 2000

80 Arkay Drive
Hauppauge, NY 11788
[phone]
FAX [phone]

Standard Microsystems is a registered trademark of Standard Microsystems Corporation, and SMSC is a trademark of Standard Microsystems
Corporation. Product names and company names are the trademarks of their respective holders. Circuit diagrams utilizing SMSC products are
included as a means of illustrating typical applications; consequently complete information sufficient for construction purposes is not necessarily given.
Although the information has been checked and is believed to be accurate, no responsibility is assumed for inaccuracies. SMSC reserves the ri ght to
make changes to specifications and product descriptions at any time without notice. Contact your local SMSC sales office to obtain the latest
specifications before placing your product order. The provision of this information does not convey to the purchaser of the semiconductor devices
described any licenses under the patent rights of SMSC or others. All sales are expressly conditional on your agreement to the terms and conditions of
the most recently dated version of SMSC's standard Terms of Sale Agreement dated before the date of your order (the "Terms of Sale Agreement").
The product may contain design defects or errors known as anomalies which may cause the product's functions to deviate from published
specifications. Anomaly sheets are available upon request. SMSC products are not designed, intended, authorized or warranted for use in any life
support or other application where product failure could cause or contribute to personal injury or severe property damage. Any and all such uses
without prior written approval of an Officer of SMSC and further testing and/or modification will be fully at the risk of the customer. Copies of this
document or other SMSC literature, as well as the Terms of Sale Agreement, may be obtained by visiting SMSC’s website at http://www.smsc.com.

SMSC DISCLAIMS AND EXCLUDES ANY AND ALL WARRANTIES, INCLUDING WITHOUT LIMITATION ANY AND ALL IMPLIED WARRANTIES
OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, TITLE, AND AGAINST INFRINGEMENT, AND ANY AND ALL
WARRANTIES ARISING FROM ANY COURSE OF DEALING OR USAGE OF TRADE.

IN NO EVENT SHALL SMSC BE LIABLE FOR ANY DIRECT, INCIDENTAL, INDIRECT, SPECIAL, PUNITIVE, OR CONSEQUENTIAL DAMAGES,
OR FOR LOST DATA, PROFITS, SAVINGS OR REVENUES OF ANY KIND; REGARDLESS OF THE FORM OF ACTION, WHETHER BASED ON
CONTRACT, TORT, NEGLIGENCE OF SMSC OR OTHERS, STRICT LIABILITY, BREACH OF WARRANTY, OR OTHERWISE; WHETHER OR
NOT ANY REMEDY IS HELD TO HAVE FAILED OF ITS ESSENTIAL PURPOSE; AND WHETHER OR NOT SMSC HAS BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.

SMSC DS – COM20051I Page 2 Rev. 03/27/2000

TABLE OF CONTENTS

FEATURES................................................................................................................................................................... 1

GENERAL DESCRIPTION............................................................................................................................................ 1

PIN CONFIGURATION ................................................................................................................................................. 4

OVERVIEW................................................................................................................................................................... 5

DESCRIPTION OF PIN FUNCTIONS........................................................................................................................... 5

BASIC ARCHITECTURE............................................................................................................................................... 8

PROTOCOL DESCRIPTION....................................................................................................................................... 13

NETWORK PROTOCOL............................................................................................................................................. 13

DATA RATES.............................................................................................................................................................. 13

NETWORK RECONFIGURATION..............................................................................................................................13

BROADCAST MESSAGES......................................................................................................................................... 15

EXTENDED TIMEOUT FUNCTION............................................................................................................................ 15

LINE PROTOCOL ....................................................................................................................................................... 15

SYSTEM DESCRIPTION............................................................................................................................................18

MICROCONTROLLER INTERFACE........................................................................................................................... 18

TRANSMISSION MEDIA INTERFACE........................................................................................................................18

ARCNET CORE FUNCTIONAL DESCRIPTION.........................................................................................................23

MICROSEQUENCER.................................................................................................................................................. 23

INTERNAL REGISTERS............................................................................................................................................. 23

INTERNAL RAM.......................................................................................................................................................... 35

SOFTWARE INTERFACE........................................................................................................................................... 35

COMMAND CHAINING............................................................................................................................................... 38

RESET DETAILS......................................................................................................................................................... 40

INITIALIZATION SEQUENCE..................................................................................................................................... 40

IMPROVED DIAGNOSTICS........................................................................................................................................ 41

COM20051I APPLICATIONS INFORMATION............................................................................................................ 43

USING ARCNET DIAGNOSTICS TO OPTIMIZE YOUR SYSTEM............................................................................. 60

CABLING THE COM20051I........................................................................................................................................64

USING THE COM20051I'S EMULATION MODE........................................................................................................ 65

OPERATIONAL DESCRIPTION.................................................................................................................................. 66

MAXIMUM GUARANTEED RATINGS........................................................................................................................ 66

DC ELECTRICAL CHARACTERISTICS ..................................................................................................................... 66

TIMING DIAGRAMS.................................................................................................................................................... 68

PACKAGE DIMENSIONS ........................................................................................................................................... 74

SMSC DS – COM20051I Page 3 Rev. 03/27/2000

PIN CONFIGURATION

P1.4

P1.3

P1.2

P1.1

P1.0

RXIN

VCC

P0.0/AD0

P0.1/AD1

P0.2/AD2

6 5 4 3 2 1 44 43 42 41 40

P0.3/AD3

P1.5
P1.6
P1.7

RST
P3.0/RXD
nPULSE1

P3.1/TXD
P3.2/INT0
P3.3/INT1

P3.4/T0
P3.5/T1

7
8
9
10
11
12
13
14
15
16
17

COM20051I

COM20051

18 19 20 21 22 23 24 25 26 27 28

VSS

XTAL2

P3.6/nWR

XTAL1

P3.7/nRD

nPULSE2

P2.0/A8

P2.1/A9

39
38
37
36
35
34
33
32
31
30
29

P2.2/A10

P2.3/A11

P2.4/A12

P0.4/AD4
P0.5/AD5
P0.6/AD6
P0.7/AD7
nEA/EMUL
TXEN
ALE
nPSEN
P2.7/A15
P2.6/A14
P2.5/A13

SMSC DS – COM20051I Page 4 Rev. 03/27/2000

OVERVIEW

The COM20051I is essentially a network board-in-a-chip. It takes an 80C32 microcontroller core and an ARCNET
controller and integrates them into a single device. ARCNET is a token passing-based protocol that combines
powerful flow control, error detection, and diagnostic capabilities to deliver fast and reliable messages. The
COM20051I supports a variety of data rates (5 Mbps to 156 Kbps), topologies (bus, star, tree), and media types (RS­485, coax, twisted pair, fiber optic, and powerline) to suit any type of application.

The ARCNET network core of the COM20051I contains many features that make network development simple and
easy to comprehend. Diagnostic features, such as Receive All, Duplicate ID Detection, Reconfiguration Detection,
Token, and Receiver Detection, all combine to make the COM20051I simple to use and to implement in any
environment. The ARCNET protocol itself is relatively simple to understand and very flexible. A wide variety of
support products are available to assist in network development, such as software drivers, line drivers, boards, and
development kits. The COM20051I implements a full-featured 16MHz, Intel-compatible 80C32 microcontroller with all
of the standard peripheral functions, including a full duplex serial port, two timer/counters, one 8-bit general purpose
digital I/O port, and interrupt controller. The 8051 architecture has long been a standard in the embedded control
industry for low-level data acquisition and control. ARCNET and the 8051 form a si mple solution for many of today's
and tomorrow's low-level networking solutions.

In addition to the 80C32 and the ARCNET network core, the COM20051I contains all the address decoding and
interrupt routing logic to interface the network core to the 80C32 core. The integrated 8051/ARCNET combination
provides an extremely cost-effective and space-efficient solution for industrial networking applications. The
COM20051I can be used in a stand-alone embedded application, executing control algorithms or performing data
acquisition and communicating data in a master/slave or
handling communication tasks in a multi-processing system.

peer-to-peer

configuration, or used as a slave processor

DESCRIPTION OF PIN FUNCTIONS

PIN NO. NAME SYMBOL DESCRIPTION

1 Receive In RXIN Input. Network receiver input.
2-9 P1.0-1.7 P1.0-1.7 Input/Output. Port 1 of the 8051. General purpose

digital I/O port.
10 Reset RESET Input. Active high reset.
11 P3.0 P3.0 Input/Output. Port 3 bit 0 of the 8051. RX input of

serial port.
12 nPulse 1 nPULSE1 Output. Network output. Open-drain when

backplane mode is invoked, otherwise it is a push-

pull output.
13-19 P3.1-3.7 P3.1-3.7 Input/Output. Port 3 bits 1-7 of the 8051.

20, 21 Crystal Oscillator XTAL1,

XTAL2

22 Ground VSS Ground pin.
23 nPulse 2 nPULSE2 Output. Network output. Outputs a synchronous

24-31 P2.0-2.7 P2.0-2.7 Input/Output. Port 2 of the 8051. High order address

32 nProgram Store

Enable

33 Address Latch

Enable

34 Transmit Enable TXEN Output. This signal is used to enable the drivers for

nPSEN Output.

ALE Output.

Input. Oscillator inputs 1 and 2.

clock at 2x the data rate when backplane mode is

invoked.

bus.

transmitting. The polarity of this signal is

programmable by grounding the nPULSE2 pin prior

to the POWER-UP.

nPULSE2 floating prior to the power-up = TXEN

active high

nPULSE2 grounded prior to the power-up = TXEN

active low. (This option is available only in the

Backplane mode).

SMSC DS – COM20051I Page 5 Rev. 03/27/2000

DESCRIPTION OF PIN FUNCTIONS

(

)

PIN NO. NAME SYMBOL DESCRIPTION

35 nEnable nEA Input. When high, causes the 8051's outputs to tri-

state. When low, allows the 8051 to address external

memory. Must be low to execute code from the

embedded 8051.
36-43 P0.7-0.0 P0.7-0.0 Input/Output. Port 0 of the 8051. Multiplexed low

order address/data bus.
44 Power Supply VCC +5V power supply.

RESET CIRCUIT FOR THE COM20051I

The power on reset circuit for the COM20051I should be designed to provide a clean, fast transition time TTL input to
the COM20051I. Sufficient signal high time on RST (pin 10) should be provided after Vcc reaches +5V DC. The
following circuit, which provides an 8ms power-on reset pulse, is recommended:

Vcc (+5V)

22uF/
10V

RST

PIN 10

220

74LS14

SMSC DS – COM20051I Page 6 Rev. 03/27/2000

EMULALE

CONTROL
BUS

ALE

nPSEN

80C32

PORT1

PORT0
TX
RX

PORT3
T0

T1

PORT2

A8-A15

INT 0 INT1

FIGURE 1 – INTERNAL ARCHITECTURE OF THE COM20051I

ALE

ARCNET CORE (COM20010)

AD0-AD7

ADDRESS
DECODER

INTERRUPT

ROUTER

IN T 0 IN T1

IN TnCSRD/WR

nPULSE1
nPULSE2
TXEN
RXIN

SMSC DS – COM20051I Page 7 Rev. 03/27/2000

BASIC ARCHITECTURE

The COM20051I consists of four functional blocks: the 80C32 microcontroller core, ARCNET network cell (includes
1K of buffer RAM), programmable address decoder, and programmable interrupt router. The internal architecture of
the COM20051I is shown in Figure 1.

The 80C32 microcontroller is a full ROMless implementation of the popular Intel 8051 series. The ARCNET network
core is similar in arc hitecture to SMSC's popular COM20020 family of ARCNET controllers and retains the same
command and status flags of previous ARCNET controllers. The programmable address decoder maps the ARCNET
registers into a 256-byte page anywhere within the External Data Memory space of the 80C32. The ARCNET core
was mapped to the External Data Memory space to simplify software and application development and for
production test purposes.

ARCNET core is available to the developer when working with the 8051 emulator.

When
the COM20051I is put into Emulate mode, the internal microcontroller is put into a high impedance state, thus
allowing an external In-Circuit Emulator (ICE) to program the ARCNET core. The advantage of this approach versus
mapping the ARCNET registers into the internal memory (Special Function) area of the 80C32 is that dedicated
software development tools will not be necessary to debug application software. Since a majority of 8051
applications use only a small portion of the Data Memory space, there is no penalty paid for used address space.
There will also be no penalty in execution time, since cycle times for external data memory accesses and internal
direct memory moves are identical. The network interrupt can be routed to either of the two external interrupt ports or
can be assigned to one of the general purpose I/O ports. The ARCNET interrupt is internally wire ORed with the
external interrupt pin to allow greater system flexibility.

80C32 ARCHITECTURE AND INSTRUCTION SET

The 80C32 microcontroller core is identical to the 16MHz Intel 80C32 in all respects

2.

Please refer to the Intel Embedded Microcontrollers and Processors Databook

except for the absence of Timer

, Volume 1, for details regarding
the 8051 architecture, peripherals, instruction set, and programming guide. Note that any access to the internal
ARCNET core or any external memorry access is

visible

on the pins of the COM20051I.

The following differences apply to the COM20051I:

1. Oscillator frequency is 40MHz instead of 16MHz. This is necessary to derive a 20MHz clock for the ARCNET

core. The processor still operates at 16MHz.

2. nEA pin - This pin must be tied to ground for normal internal processor operation. When tied to VCC, the

COM20051I will enter the Emulate mode.

3. Unused pins - The COM20051I is packaged in a 44-pin PLCC. Network I/O is generated on the four unused pins

of the standard 80C32 PLCC package. No DIP package is available.

4. Power Down operation - The Power Down mode can only be used in conjunction when the internal oscillator is

being used. If an external oscillator is used and the Power Down mode is invoked, damage may result to the
oscillator and to the COM20051I.

Clock Speed

The COM20051I processor operates at 16MHz and the network controller at a maximum 40MHz clock rate. A single
crystal oscillator is used to supply the two clocks: a 16MHz processor clock and a 20MHz network clock for the
nominal 2.5 Mbps data rate. Pins 20 and 21 are designated as crystal inputs. When clocking with an external
oscillator, pin 21 (XTAL1) functions as the clock input.

Emulate Mode

The COM20051I contains a unique feature called the Emulate mode that most 8051-based peripheral devices do
not accommodate. TheEmulate mode permits developers to access and program the internal ARCNET core using a
standard low-cost 8032 emulator. This feature eliminates the need for expensive dedicated development equipment
needed for other types of 8051-based peripheral devices. The Emulate mode is invoked by connecting the nEA pin
to VCC. This causes the internal 80C32 processor to enter a HI-Z state and changes the state of the COM20051I
pins according to the following table:

SMSC DS – COM20051I Page 8 Rev. 03/27/2000

Table 1 - Emulate Mode

SIGNAL NAME EMUL = 0 EMUL = 1

PORT 0 Bidirectional Bidirectional
PORT 1 Bidirectional HI-Z (except for pins

designated as interrupt
destinations)

PORT 2 Output Input

INT0,1

Input Output

(P3.2, P3.3)

RD/WR

Output Input

(P3.6, P3.7)

ALE Output Input

TX,T0, T1

Output HI-Z

(P3.1,3.4,3.5)

nPSEN

Address Decoding

The COM20051I, as described previously, maps the ARCNET registers into the 80C32's External Data Memory
space. This provides system flexibility because the location of the ARCNET registers can be located anywhere within
the 64K External Data Memory space. The precise location can be resolved with a 256-byte page. The location of
that page in the External Data Memory space is pointed to by the

read/write

Address Decode Register, as shown in
Figure 2. The Address Decode Register is located at FFFFh of the External Data Memory space. It holds the upper
8 bits of the 16-bit address at which the 256 page boundary will start. This register must be programmed prior to any
access to the ARCNET core. The default value is 0000h.

The ARCNET core register page must be mapped away from the external RAM prior to any access to the external
RAM by the software. Failure to do this may result in incorrect data write and read operations to the external RAM as
well as the core registers.

SMSC DS – COM20051I Page 9 Rev. 03/27/2000

FFFFh

ADDRESS DECODE
REGISTER

(FIXED LO C ATION)

V ALU E x 10 0h

256 BYT ES

64K

BYTES

ARCNET CORE

PAGE
LOCATION CAN
VARY

0000h

FIGURE 2 – COM20051I EXTERNAL DATA ADDRESS SPACE

ADDRESS DECODE REGISTER

NAME BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0

ADR DEC A15 A14 A13 A12 A11 A10 A9 A8

LOCATION: FFFFh of the External Data Memory space. Default: 00h
EXAMPLE: Address Decode Register = 80h

ARCNET core registers

Configuration Register offset = 06h, physical address = 8006h).

will be located at 8000h + Register offset (e.g. ARCNET

SMSC DS – COM20051I Page 10 Rev. 03/27/2000

COM20051I MEMORY MAPPING

The COM20051I maps the Arcnet core into a 256 byte page of data memory space. This memory is physically
located internally to the device and itrquote s default base address on power up is 0000h. This 256 byte page can be
logically located anywhere within the 64K external data memory space while physically remaining on board. The
location of this 256 byte page is pointed to by the Address Decode Register in the device. This Address Decode
Register holds the upper 8 bits of the 16 bit address at which the 256 byte page boundary will start. The address of
this Address Decode Register is FFFFh. This register is also logically located in external data memory space but
physically located on the device. This register must be written to on power-up to properly locate the Arcnet core. .

The user must ensure that the Arcnet corerquote s 256 byte page does not conflict with external memory, otherwise
data bus contention will result. As an example, if the user has 32K of external data memory located from 0000h to
7FFFh then the Arcnet core should be mapped above this area, 8000H is suggested. The user will write 80h to
address FFFFh on power up to properly map the core to this location.

ARCNET Network Core - Overview and Architecture

ARCNET is a baseband token passing network protocol (ANSI 878.1). ARCNET features deterministic behavior,
hardware-based network configuration, flexible topologies, several data rates, and multiple media support. Data
rates varying from 5 Mbps to 156 Kbps and message sizes from 0 to 507 bytes are supported. Supported media
includes RS-485, twisted pair, coax, fiber optic, and powerline in bus, star or tree topologies. ARCNET has enjoyed
widespread use in the industrial community, finding a home in such applications as I/O control/acquisition, multi­processor communications, point-of-sale terminals, in-vehicle navigation systems, data acquisition systems, remote
sensing, avionics, machine control, embedded computing, building automation, robotics, consumer products, and
security systems.

The ARCNET core used in the COM20051I is similar in architecture to SMSC's 200XX series of Industrial ARCNET
Controllers. The ARCNET core of the COM20051I contains a 1K x 8 internal RAM for packet buffering, Duplicate ID
Detection, Receive All Mode, New Next ID Indicator, Excessive NACK Interrupt, Programmable Data Rates,
Backplane Mode, Programmable Transmitter Enable, Polarity Receive Activity, Reconfiguration, Token Seen
Indicators, and Network Mapping hooks. The ARCNET core of the COM20051I uses a software-programmable node

enabling the user to program the Node ID according to the application needs. The Node ID can be stored in an

ID,

electronic medium or changed with the switch.

SMSC DS – COM20051I Page 11 Rev. 03/27/2000

Power On

Start
Reconfiguration
Time r (8 40 m S)

Reconfigure
Timer has
Timed Out

Y

Send

Reconfigure

Burst

Read Node ID

Write ID to

RAM Buffer

Set NID=ID

Invitation

to Transmit to

this ID?

N

YN

TA?

Broadcast?

Y

Send

Packet

Was Packet
Broadcast?

N

No

Y

Activity

for 74.7

us?

N

Y

N

ACK? Set TMA

YN

Trans mi t

NAK

Trans mit

ACK

N

Y

Set TA

YN

RI?

Trans mit

Free Buffer

Enquiry

YN

ACK?

NID

Free Buff er

Enquiry to

this ID?

N

Y

NAK?

No
Activity
for 74.7

us?

N

No

Activity

for 74.7

us?

1

ID refers to the identification number of the ID assigned to this node.

­NID ref e rs to th e n e x t ide n tification nu mber that r ec e iv es the to ke n a fter

­after this ID p a s se s it.

SID ref er s to the s ou r ce ide n tification .

­DID refers to the destination identification.

-

SOH refers to the start of header character; preceeds all data packets.

-

Note*: Time values pertain to the default 2.5 Mbps operation

FIGURE 3 - DETAILED ARCNET CORE OPERATION

Y

Set TA

Pass the

Token

NYIncrement

RI?

1

YN

SOH?

NY

Write SID
to B uffer

Y

DID
=0?

N

Broadcast

DID

=ID?

Y

Write Buffer
with Packet

CRC

OK?

LENGTH

OK?

DID
=0?

N

DID

=ID?

Y

SEND ACK

Enabled?

N

N

Y

N

Y

Y

N

No Activity

for 82

uS?

Set NID=ID

N

Y

Set RI

Start Time r:

T=(255-ID)

x 146 us

Activity

On Line?

N

T=0?

N

Y

Y

N

Y

SMSC DS – COM20051I Page 12 Rev. 03/27/2000

PROTOCOL DESCRIPTION

NETWORK PROTOCOL

Communication on the network is based on a token passing protocol. Establishment of the network configuration
and management of the network protocol are handled entirely by

called ARCNET network core .

The 80C32 controller core transmits data by simply loading a data packet and its
destination ID into the network core's RAM buffer, and issuing a command to enable the transmitter. When the
ARCNET core next receives the token, it verifies that the receiving node is ready by first transmitting a FREE
BUFFER ENQUIRY message. If the receiving node transmits an ACKnowledge message, the data packet is
transmitted followed by a 16-bit CRC. If the receiving node cannot accept the packet (typically its receiver is
inhibited), it transmits a Negative AcKnowledge message and the transmitter passes the token. Once it has been
established that the receiving node can accept the packet and transmission is complete, the receiving node verifies
the packet. If the packet is received successfully, the receiving node transmits an ACKnowledge message (or
nothing if it is not received successfully) allowing the transmitter to set the appropriate status bits to indicate
successful or unsuccessful delivery of the packet. An interrupt mask permits the ARCNET core to generate an
interrupt to the processor when selected status bits become true. Figure 4 is a flow chart illustrating the internal
operation of the ARCNET core.

data rate.

All timing details in th e discuss ion of ARCNET protocol are based on the 2.5 Mbps

DATA RATES

The ARCNET core is capable of supporting data rates from 156.25 Kbps to 5 Mbps. For slower or faster data rates,
an internal Programmable clock divider scales down the clock frequency. Thus all timeout values are scaled up
as shown in the following table:

DATA

CLOCK

PRE-

SCALER

8

!

16

!

32

!

64

!

128

!

40 MHz CLOCK

DIV. TO 20 MHz

25Mbps

1.25Mbps
625Kbps

3125Kbps

156.25
Kbps

RATE

40 MHz UN-

DIVIDED

5Mbps

25Mbps

1.25Mbps
625Kbps

3125Kbbps

the COM20051I's internal microcoded sequencer

TIMEOUT SCALING

FACTOR

(MULTIPLY BY)

40 MHz CLOCK

DIV. TO 20 MHz

1
2
4
8

16

40 MHz UN-

DIVIDED

.5

1
2
4
8

NETWORK RECONFIGURATION

A significant advantage of the ARCNET is its ability to adapt to changes on the network. Whenever a new node is
activated or deactivated, a NETWORK RECONFIGURATION is performed. When a new ARCNET node is turned on
(creating a new active node on the network), or if the COM20051I has not received an INVITATION TO TRANSMIT
for 840ms, or if a software reset oc curs, the ARCNET node causes a NETW ORK RECONFIGURATION by sending
a RECONFIGURE BURST consisting of eight marks and one space repeated 765 times. The purpose of this burst is
to terminate all activity on the network. Since this burst is longer than any other type of transmission, the burst will
interfere with the next INVITATION TO TRANSMIT, destroy the token and keep any other node from assuming
control of the line.

When any ARCNET node senses an idle line for greater than 82 S, which occurs only when the token is lost, it starts
an internal timeout equal to 146 s times the quantity 255 minus its own ID.

reconfiguration by sending an invitation to transmit (TOKEN) first to itself and then to

The COM20051I starts network

all other nodes by
incrementing the destination Node ID value. If the timeout expires with no line activity, the ARCNET core starts
sending INVITATION TO TRANSMIT with the Destination ID (DID) equal to the currently stored NID. Within a given
network, only one node will timeout (the one with the highest ID number). After sending the INVITATION TO
TRANSMIT, the COM20051I waits for activity on the line. If there is no activity for 74.7 S, the COM20051I
increments the NID value and transmits another INVITATION TO TRANSMIT using the NID equal to the DID. If
activity appears before the 74.7 S timeout expires, the COM20051I releases control of the line. During NETWORK
RECONFIGURATION, INVITATIONS TO TRANSMIT are sent to all NIDs

(1-255).

SMSC DS – COM20051I Page 13 Rev. 03/27/2000

HARDWARE OR
SOFTWARE RES ET
OR NO TOKEN FOR

840 MS

NODE DRO PS

OFF TH E

NETWORK

SEND RECON

BURST

(765 times 111111110)

TIMEOUT FOR

146µs x

(255 - NODE ID)

NO ACTIVITY

WITHIN 82µs?

N

Y

TRANSMIT TOKEN

TO NODE = OWN ID

LINE ACTIVITY

DETECTED

WITHIN 7 4 .7µs

N

Y

N

SET RECON BIT

IN THE STATUS

REGISTER &

NEW NEXT ID

BIT IN THE

DIAGNOSTIC

STATUS

REGISTER

END NO DE

RECONFIGURATION

INCREMENT TOKEN

VALUE AND

TRANSMIT

FIGURE 4 - ARCNET RECONFIGURATION PROCESS

840ms
TIMER

EXPIRED?

Y

SMSC DS – COM20051I Page 14 Rev. 03/27/2000

Each COM20051I on the network will finally have saved a NID value equal to the ID of the ARCNET node that it
released control to. This is called the Next ID Value. At this point, control is passed directly from one node to the
next with no wasted INVITATIONS TO TRANSMIT being sent to ID's not on the network, until the next NETWORK
RECONFIGURATION occurs. W hen a node is powered off, the previous node attempts to pass the token to it by
issuing an INVITATION TO TRANSMIT. Since this node does not respond, the previous node times out and
transmits another INVITATION TO TRANSMIT to an incremented ID and eventually a response will be received.

The NETWORK RECONFIGURATION time depends on the number of nodes in the network, the propagation delay
between nodes, and the highest ID number on the network, but is typically within the range of 24 to 61 ms for 2.5
Mbps operation.

BROADCAST MESSAGES

Broadcasting gives a particular node the ability to transmit a data packet to all nodes on the network simultaneously.

NID=0

is reserved for this feature and no node on the network can be assigned
the transmitting node's processor simply loads the RAM buffer with the data packet and sets the DID
equal to zero. Figure 12 illustrates the position of each byte in the packet with the DID residing at address

NID=0.

To broadcast a message,

(Destination ID)

1Hex

of
the current page selected in the "Enable Transmit from Page fnn" command. Each individual node has the ability to
ignore broadcast messages by setting the most significant bit of the "Enable Receive to Page fnn" command (see
Table 8) to a logic "0".

EXTENDED TIMEOUT FUNCTION

There are three timeouts associated with the COM20051I operation. The values of these timeouts are controlled by
bits 3 and 4 of the Configuration Register

and bit 5 of the Setup Register (see register description for details).

Response Time

(ET1, ET2)

The Response Time determines the maximum propagation delay allowed between any two nodes, and should be
chosen to be larger than the round trip propagation delay between the two furthest nodes on the network plus the
maximum turn around time (the time it takes a partic ular ARCNET node to start sending a message in respons e to a
received message) which is approximately 12.7 S. The round trip propagation delay is a function of the transmission
media and network topology. For a typical system using RG62 coax in a baseband system, a one way cable
propagation delay of 31 S translates to a distance of about 4 miles. The flow chart in Figure 3 uses a value of 74.7 S
(31 + 31 + 12.7) to determine if any node will respond.

Idle Time

(ET1, ET2)

The Idle Time is associated with the NETWORK RECONFIGURATION. Figure 3 and Figure 4 illustrate that during a
NETWORK RECONFIGURATION one node will continually transmit INVITATIONS TO TRANSMIT until it
encounters an active node. All other nodes on the network must distinguish between this operation and an entirely
idle line. During NETW ORK RECONFIGURATION, activity will appear on the line every 82 S. This 82 S is equal to
the Response Time of 74.7 S plus the time it takes the COM20051I to start retransmitting another message (usually
another INVITATION TO TRANSMIT).

Reconfiguration Time

(ET1, ET2)

If any node does not receive the token within the Reconfiguration Time, it will initiate a NETWORK
RECONFIGURATION. The ET2 and ET1 bits of the Configuration Register allow the network to operate over longer
distances than the 4 miles stated earlier. The logic levels on these bits control the maximum distances over which
the COM20051I can operate by controlling the three timeout values described above. For proper network operation,
all nodes connected to the same network must have the same Response Time, Idle Time, and Reconfiguration Time.

LINE PROTOCOL

The ARCNET line protocol is considered isochronous because each byte is preceded by a start interval and ended
with a stop interval. Unlike asynchronous protocols, there is a constant amount of time separating each data byte.
Each byte takes exactly 11 clock intervals that are defined by nPULSE1 and nPULSE2 signals (at 2.5 Mbps one byte
takes 4.4 ms). As a result a time to transmit a message can be precisely determined. The line idles in a spacing
(logic "0") condition. A logic "0" is defined as no line activity and a logic "1" is defined as a negative pulse of 200nS
duration.

SMSC DS – COM20051I Page 15 Rev. 03/27/2000

A transmission starts with an ALERT BURST consisting of 6 unit intervals of mark (logic "1"). Eight bit data
characters are then sent, with each character preceded by 2 unit intervals of mark and one unit interval of space.
Five types of transmission can be performed as described below:

Invitations To Transmit

An Invitation To Transmit is used to pass the token from one node to another and is sent by the following sequence:

!"

An ALERT BURST

!"

ITT (Invitation To Transmit:

An

!"

Two (repeated) DID (Destination ID) characters

Free Buffer Enquiries

A Free Buffer Enquiry is used to ask another node if it is able to accept a packet of data. It is sent by the following
sequence:

!"
!"
!"

Data Packets

A Data Packet consists of the actual data being sent to another node. It is sent by the following sequence:

!"
!"
!"
!"
!"

!"

!"

An ALERT BURST
An FBE - Free Buffer Enquiry: ASCII code 85H)
Two (repeated) DID (Destination ID) characters

(PAC)

An ALERT BURST

PAC (Data Packet

An
An SID (Source ID) character
Two (repeated) DID (Destination ID) characters

A single COUNT character which is the 2's complement of the number of data bytes to follow if a short
packet is sent, or 00

N data bytes where COUNT = 256-N (or 512-N for a long packet)
Two CRC (Cyclic Redundancy Check) characters. The CRC polynomial used is: X16 + X15 + X2 + 1.

(ITT)

ASCII code 04H)

(FBE)

--ASCII code 01H)

Hex

followed by a COUNT character if a long packet is sent

ALERT
BURST

ALERT
BURST

ITT

FBE

DID DID

DID DID

ALERT

BURST

Acknowledgements

An Acknowledgement is used to acknowledge reception of a packet or as an affirmative response to FREE BUFFER
ENQUIRIES and is sent by the following sequence:

!"

An ALERT BURST

!"

An ACK (ACKnowledgement--ASCII code 86H) character

SMSC DS – COM20051I Page 16 Rev. 03/27/2000

PAC

(ACK)

SID

DID

DID

ALERT
BURST

COUNT

data

ACK

data

CRC

CRC

Negative Acknowledgements

A Negative Acknowledgement is used as a negative response to FREE BUFFER ENQUIRIES and is sent by the
following sequence:

!"

An ALERT BURST

!"

A NAK (Negative Acknowledgement--ASCII code 15H) character

Figure 5 illustrates the flow of events on a 5-node network where a node with the NID=1 transmits a data packet to
a node with the NID=5, and a node with the NID=3 tries to transmit a data to a node with the NID=4 but node 4
cannot accept it. All other nodes are just passing the tokens.

(NAK)

ALERT
BURST

NAK

#5

ITT to #1

#1 FBE to #5 #5 ACK to #1 #1

PAC t o #5

#5 ACK to #1

#3 ITT to #4

#4 NAK to #3

#3 FBE to #4

#1 ITT to #2#2 ITT to #3

#4 ITT to # 5

SEQUENC E O F L INE EVENTS

1) NODE 1 RECEIVES TOKEN FR OM NOD E 5

2) NODE 1 TRANSM ITS TO NODE 5
A) ISSUES FBE TO NOD E 5
B) NODE 5 IS READ Y TO R ECEIVE SO IT ISSUES AN ACK
C) NODE 1 NOW TRANSM ITS THE DATA
D) NODE 5 RECEIVES THE DATA ERR OR FREE AN D ISSUES AN ACK

3) NODE 1 P ASSES T OKEN TO NOD E 2

4) NODE 2 DOES NOT NEED TO TRAN SMIT AND P ASSES THE TO KEN TO N ODE 3

5) NODE 3 NEEDS T O TRANSM IT TO NODE 4
A) ISSUES AN FBE TO NOD E 4
B) NODE 4 IS NOT READY TO RECEIVE AND IT ISSUES A NAK

6) NODE 3 P ASSES THE T OKEN TO NO DE 4

7) NODE 4 P ASSES THE T OKEN TO NO DE 5

8) GO T O S T E P 1

FIGURE 5 – AVERAGE SEQUENCE OF LINE EVENTS FOR A FIVE-NODE NETWORK

SMSC DS – COM20051I Page 17 Rev. 03/27/2000

SYSTEM DESCRIPTION

MICROCONTROLLER INTERFACE

COM20051I ARCNET network core contains 1K byte of RAM and 11 registers.

pointer-based scheme (refer to the Sequential Access Memory section), and the internal registers are accessed via
direct addressing. The ARCNET core bus interface is designed to be flexible so that it is independent of the 80C32
speed.

The COM20051I provides for no wait state arbitration via direct addressing to its internal registers and a pointer
based addressing scheme to access its internal RAM.

slowed down using the SLOWARB bit of the Setup Register.

for typical sequential buffer emptying or loading, or it can be taken out of the auto-increment mode to perform out of
sequence accesses to the RAM. The data within the RAM is accessed through the Data Register. Data being read
is prefetched from memory and placed into the Data Register for the microcontroller to read. During a write
operation, the data is stored in the Data Register and then written into memory. Whenever the pointer is loaded for
reads with a new value, data is immediately prefetched to prepare for the first read operation.

TRANSMISSIO N MEDIA INTERFACE

Figure 6 illustrates the COM20051I interface to the transmission media used to connect the node to the network.
Table 2 lists different types of cable which are suitable for ARCNET applications.1 The user may interface to the

cable of choice in one of three ways:

1

Please refer to TN7-5 -

distance, termi nat i on, and node count for ARCNET nodes.

Traditional Hybrid Interface

The Traditional Hybrid Interface is that which is used with previous ARCNET devices. The Hybrid Interface is
recommended if the node is to be placed in a network with other Hybrid-Interfaced nodes. The Traditional Hybrid
Interface is for use with nodes operating at 2.5 Mbps only. The transformer coupling of the Hybrid offers isolation for
the safety of the system and offers high Common Mode Rejection. The Traditional Hybrid Interface uses circuits like
SMSC's HYC9068 or HYC9088 to transfer the pulse-encoded data between the cable and the COM20051I. The
COM20051I transmits a logic "1" by generating two 100nS non-overlapping negative pulses, nPULSE1 and
nPULSE2. Lack of pulses indicates a logic "0". The nPULSE1 and nPULSE2 signals are sent to the Hybrid, which
creates a 200nS dipulse signal on the
coupled through the transformer of the LAN Driver, which produces a positive pulse at the RXIN pin of the
COM20051I. The pulse on the RXIN pin represents a logic "1". Lack of pulse represents a logic "0". Typically, RXIN
pulses occur at multiples of 400nS. The COM20051I can tolerate distortion (bit jitter) of plus or minus 100nS and still
correctly capture and convert the RXIN pulses to NRZ format. Figure 8 illustrates the events which occur in
transmission or reception of data consisting of 1, 1, 0.

Backplane Configuration

The Backplane Configuration is recommended for cost-sensitive, short-distance applications like backplanes and
instrumentation. This mode is advantageous because it saves components, cost, and power.

Cabling Guidelines for the COM20020 ULANC

The internal RAM is accessed via a

Note that at 5 Mbps data rate, the internal arbiter must be

The pointer may be used in the auto-increment mode

medium.

During reception, the 200nS dipulse appearing on the media is

, available from SMSC, for recommended cabling

SMSC DS – COM20051I Page 18 Rev. 03/27/2000

RXIN

HYC9068 or

HYC9088

RXIN

+5V

6

10

uF

+

0.47

uF

COM20051I

nTXEN

nPULSE1

nPULSE2

GND

N/C

nPULSE1
nPULSE2

17, 19,

4, 13, 14

0.47
uF

12
11

3

+

10

uF

-5V

5.6K

1/2W

5.6K
1/2W

Traditional Hybrid

Configuration

0.01 uF
1KV

FIGURE 6 – DIPULSE HYBRID CONFIGURATION

RT

+VCC

RBIAS

75176B or

Equiv.

RBIAS

RT

+VCC

RBIAS

+VCC

COM20051I

COM20051I

COM20051I

FIGURE 7 – COM20051I NETWORK USING RS-485 DIFFERENTIAL TRANSCEIVERS

SMSC DS – COM20051I Page 19 Rev. 03/27/2000

(

)

10

20M HZ
CLOCK

FOR

REF.
ONLY

100ns

nPULSE1

1

100ns

nPU LSE2

200ns

DIPULSE

400ns

RXIN

FIGURE 8 – DIPULSE WAVEFORM FOR BIT DATA OF 1-1-0

SMSC DS – COM20051I Page 20 Rev. 03/27/2000

Since the Backplane Configuration encodes data differently than the traditional Hybrid Configuration, nodes utilizing
the Backplane Configuration cannot communicate directly with nodes utilizing the Traditional Hybrid Configuration.

The Backplane Configuration does not isolate the node from the media nor protect it from Common Mode noise, but
Common Mode Noise is less of a problem in short distances.

The COM20051I supplies a programmable output driver for Backplane Mode operation. A push/pull or open drain
driver can be selected by programming the P1MODE bit of the Setup Register (see register descriptions for details.)
The COM20051I defaults to an open drain output.

The Backplane Configuration provides for direct connection between the COM20051I and the

open drain configuration of the output driver.

Only one pull-up resistor (for open drain only) is required somewhere

physical medium in

on the media (not on each individual node). The nPULSE1 signal, in this mode, is an open drain or push/pull driver
and is used to directly drive the media. It issues a 200nS negative pulse to transmit a logic "1". Note that when used
in the open-drain mode, the COM20051I does not have a fail/safe input on the RXIN pin.

The nPULSE1 signal actually contains a weak pull-up resistor. This pull-up should not take the place of the resistor
required on the media for open drain mode. In typical applications, the serial backplane is terminated at both ends
and a bias is provided by the external pull-up resistor.

The RXIN signal is directly connected to the cable via an internal Schmitt trigger. A negative pulse on this input
indicates a logic "1". Lack of pulse indicates a logic "0". For typical single-ended backplane applications, RXIN is
connected to nPULSE1 to make the serial backplane data line. A ground line (from the coax or twisted pair) should
run in parallel with the signal. For applications requiring different treatment of the receive signal (like filtering or
squelching), nPULSE1 and RXIN remain as independent pins. External differential drivers/receivers for increased
range and common mode noise rejection, for example, would require the signals to be independent of one another.
When the device is in Backplane Mode, the clock provided by the nPULSE2 signal may be used for encoding the
data into a different encoding scheme or other synchronous operations needed on the serial data stream.

Differential Driver Configuration

The Differential Driver Configuration is a special case of the Backplane Mode. It is a DC coupled configuration
recommended for applications like car-area networks (CAN) or other cost-sensitive applications which do not require
direct compatibility with existing ARCNET nodes and do not require isolation.

Figure 7 illustrates this configuration.

The Differential Driver Configuration cannot communicate directly with nodes utilizing the Traditional Hybrid
Configuration. Like the Backplane Configuration, the Differential Driver Configuration does not isolate the node from

physical medium.

the
The Differential Driver interface includes a RS485 Driver/Receiver to transfer the data between the cable and the

COM20051I. The nPULSE1 signal transmits the data, provided the nTXEN signal is active. The nPULSE1 signal
issues a 200nS negative pulse to transmit a logic "1". The RXIN pin receives the data. A negative pulse on this
input indicates a logic "1". Lack of pulse indicates a logic "0". The transmitter portion of the COM20051I is disabled
during reset and the nPULSE1, nPULSE2 and nTXEN pins are inactive.

CABLE TYPE

Table 2 - Typical Media

NOMINAL

IMPEDANCE

ATTENUATION PER 1000 FT.

AT 5MHZ

RG-62 Belden #86262 93 5.5dB
RG-59/U Belden #89108 75 7.0dB
RG-11/U Belden #89108 75 5.5dB
IBM Type 1* Belden #89688 150 7.0dB
IBM Type 3* Telephone Twisted Pair Belden #1155A 100 17.9dB

COMCODE 26 AWG Twisted Pair Part #105-064-703 105 16.0dB

*Non-plenum-rated cables of this type are also available.

Note:

For more detailed information on Cabling options including RS-485, transformer-coupled RS-485 and Fiber

Optic interfaces, please refer to TN7-5 -

Cabling Guidelines for the COM20020 ULANC

available from Standard

,

Microsystems Corporation.

SMSC DS – COM20051I Page 21 Rev. 03/27/2000

ALE

AD0-AD7

nINTR

nRESET IN

nRD

nW R

nCS

ADDRESS
DECODING
CIRCUITRY

1K x 8

RAM

ADDITIONAL
REGISTERS

STATUS/
COMMAND
REGISTER

MICRO-

SEQUENCER

AND

TX/RX
LOG IC

WORKING

RESET

REGISTERS

LOG IC

BUS

ARBITRATION

CIRCUITRY

FIGURE 9 – ARCNET CORE BLOCK DIAGRAM

RECONFIGURATION

TIMER

NODE ID

LOG IC

nPULSE1
nPULSE2
nTXEN
RXIN

20 MH z
or 40 MHz
CORE CLOCK

SMSC DS – COM20051I Page 22 Rev. 03/27/2000

ARCNET CORE FUNCTIONAL DESCRIPTION

MICROSEQUENCER

The ARCNET core contains an internal microsequencer which performs all of the control operations necessary to
carry out the ARCNET protocol. It consists of a clock generator, a 544 x 8 ROM, a program counter, two instruction
registers, an instruction decoder, a no-op generator, jump logic, and reconfiguration logic.

The ARCNET core derives The Program Counter Clock and Instruction Ex ecution Clock from the SYSTEM CLOCK.
If the system clock is 40 MHz the Program Counter Clock runs at 10 MHz and the Instruction Execution Clock runs at
5 MHz. If the System Clock is 20 MHz the above clocks run at 5 MHz and 2.5 MHz respectively.

is stored in the ROM and the instructions are fetched and then placed into the instruction registers. One register
holds the op code, while the other holds the immediate data. Once the instruction is fetched, it is decoded by the
internal instruction decoder, at which point the ARCNET core proceeds to execute the instruction. When a no-op
instruction is encountered, the microsequencer enters a timed loop and the program counter is temporarily stopped
until the loop is complete. When a jump instruction is encountered, the program counter is loaded with the jump
address from the ROM. The ARCNET core contains an internal reconfiguration timer which interrupts the
microsequencer if it has timed out. At this point the program counter is cleared and the MYRECON bit of the
Diagnostic Status Register is set.

INTERNAL REGISTERS

The ARCNET core contains eight internal registers. Tables 4 and 5 illustrate the ARCNET core register map.
Reserved locations should not be accessed. All undefined bits are read as undefined and must be written as logic
"0".

Interrupt Mask Register (IMR)

The ARCNET core is capable of generating an interrupt signal when certain status bits become true. A write to the
IMR specifies which status bits will be enabled to generate an interrupt. The bit positions in the IMR are in the same
position as their corresponding status bits in the Status Register and Diagnostic Status Register. A logic "1" in a
particular position enables the corresponding interrupt. The Status bits capable of generating an interrupt include the
Receiver Inhibited bit, New Next ID bit, Excessive NAK bit, Reconfiguration Timer bit, and Transmitter Available bit.
No other Status or Diagnostic Status bits can generate an interrupt.

The five maskable status bits are ANDed with their respective mask bits, and the results are ORed to produce the
interrupt signal. An RI or TA interrupt is masked when the corresponding mask bit is reset to logic "0", but will
reappear when the corresponding mask bit is set to logic "1" again, unless the interrupt status condition has been
cleared by this time. A RECON interrupt is cleared when the "Clear Flags" command is issued. An EXCNAK
interrupt is cleared when the "POR Clear Flags" command is issued. A New Next ID interrupt is cleared by reading
the New Next ID Register. The Interrupt Mask Register defaults to the value 0000 0000 upon hardware reset only.
Refer to Table 3.

(Location +00Hex)

Table 3 - Cleaning Interrupt Bit

INTERRUPT TYPE CLEANING INTERRUPT BIT

RI Issue "Enable Receive to Page Fnn" command

EXCNAK Issue "Clean Flags" Command with "p" bit set

RECON Issue "Clear Flags" Command with "r" bit set

New Next ID Read New Next ID Register

TA Issue "Enable Transmit From Page Fnn"

Command

The microprogram

SMSC DS – COM20051I Page 23 Rev. 03/27/2000

Table 4 – Read Register Summary

REGISTER

STATUS

DIAG.
STATUS

ADD RE SS
PTR
HIGH
ADD RE SS
PTR
LO W

DATA

RESERVED

CONFIG­URATION

TENTID
NODEID

SETUP

NEXT ID

MSB LS B

RI

MY-

RECON

RDDATA

A7

D7

X

RESET

TID7
NID7

P1MODE

NXTID7

X

DUPID

AUT O -

IN C

A6

D6

X

CCHEN

TID6

NID6

FOUR

NAKS

NXTID6

X

RCVACT

X

A5

D5

X

TXEN

TID5

NID5

ET3

NXTID5

READ

POR

TOKEN

X

A4

D4

X

ET1

TID4

NID4
RCV_

ALL

NXTID4

TEST

EXCNAK

X

A3

D3

X

ET2

TID3

NID3

CKP3

NXTID3

RECON

TENTID

X

A2

D2

X

BACK-

PLANE

TID2

NID2

CKP2

NXTID2

TMA

NEW

NEXTID

A9

A1

D1

X

SUB-

AD1

TID1
NID1

CKP1

NXTID1

NXTID0

TA

X

A8

A0

D0

X

SUB-

AD0

TID0
NID0

SLOW

ARB

OFFSET

ADD RE SS

00

01

02

03

04

05

06

07

RESERVED

IRR

N O TE : The S LO W AR B bit m ust be set for 5 Mbps operation.

XXXXXXXX

X

5MBS DEC3 DEC2 DEC1 EXT INT1 INT0

08

09

SMSC DS – COM20051I Page 24 Rev. 03/27/2000

Table 5 – Write Register Summary

OFFSET

ADDRESS

00

01

02

03

04

05

06

WRITE

MSB LSB

RI

D7

RDDATA

A7

D7

0

RESET

TID7

0

D6

AUTO-

INC

A6

D6

0

CCHEN

TID6

0

D5

0

A5

D5

0

TXEN

TID5

0

D4

0

A4

D4

0

ET1

TID4

EXCNAK

D3

0

A3

D3

0

ET2

TID3

RECON

D2

0

A2

D2

0

BACK-

PLANE

TID2

NEW

NEXTID

D1

A9

A1

D1

0

SUB-

AD1

TID1

TA

D0

A8

A0

D0

0

SUB­AD0

TID0

REGISTER

INTERRUPT
MASK

COMMAND

ADDRESS
PTR HIGH

ADDRESS
PTR LOW

DATA

RESERVED

CONFIG­URATION

TENTID

07

08

09

NID7

P1MODE

NID6

FOUR

NAKS

00

XXXXXX

XDEC35MBS EXTDEC2 DEC1 INT1 INT0

NID5

ET3

0

NID4
RCV_

ALL

NID3

CKP3 CKP2 CKP1

0

0

NID2

0

NID1

0

XX

N O TE: Th e SLOWARB b it m us t be s e t f or 5 Mbps oper ation.

NID0

SLOW

ARB

0

NODEID

SETUP

NEXT ID

RESERVED

IRR

SMSC DS – COM20051I Page 25 Rev. 03/27/2000

Interrupt Routing Register

(Location +09HEX)

The Interrupt Routing Register (IRR) routes the interrupt generated by the ARCNET core to the appropriate 80C32
interrupt input (INT0 or INT1) or to one of the eight general purpose digital I/O ports (P1.0-1.7) of the 80C32. The
interrupt routing operates on a priority driven scheme where if two bits are enabled the highest priority always wins.
INT0 has highest priority followed by INT1 then EXT. The nINT0 and nINT1 bits route the interrupt signal to either the
nINT0 or nINT1 pin of the 80C32. The 80C32 nINT1 and nINT0 inputs are wire ANDed with the routed interrupt. This
allows the 80C32's interrupts to be used for more than one source. If many interrupts are being used in the system,
the COM20051I supports the use of an external interrupt controller to arbitrate simultaneous interrupts. External
interrupt controllers are supported by programming the EXT bit of the IRR. This will cause the interrupt signal to be
present on one of the Port 1 pins as programmed by Bits 3 - 5.

The 5 Mbps bit programs the ARCNET core to operate at a 5 Mbps data rate. The 5 Mbps bit causes the clock to the
ARCNET core to double its frequency from 20MHz to 40MHz. 5 Mbps operation requires the SLOWARB bit of the
SETUP register to be set. Failure to set the SLOWARB bit may result in errors when accessing the ARCNET buffer
RAM.

Table 6 - Interrupt Routing Register

BIT BIT NAME SYMBOL DESCRIPTION

6 5 Mbps Enable 5MBPS Causes the ARCNET core to operate at a 5 Mbps

data rate. Defaults to 0.

3-5 Port 1 Bit Assignment DEC1 - 3

Selects one of the eight Port 1 bits to output the
interrupt on. 000 - P1.0
001 - P1.1
010 - P1.2
011 - P1.3
100 - P1.4
101 - P1.5
110 - P1.6
111 - P1.7
Defaults to 000 (P1.0).

2 External Interrupt Enable EXT Enables routing of the ARCNET interrupt onto on the

Port 1 pins. Defaults to 0.

1 Interrupt 1 Enable. INT1 Enables wire Oring of the ARCNET interrupt with the

INT1 pin. Defaults to 0.

0 Interrupt 0

Enable.

INT0 Enables wire ORing if the ARCNET interrupt with the

INT0 pin. Defaults to 0.

Data Register

(Location +04Hex)

This read/write 8-bit register is used as the channel through which the data to and from the RAM passes. The data is
placed in or retrieved from the address location presently specified by the address pointer. The contents of the Data
Register are undefined upon hardware reset.

contents of ARCNET Core Internal RAM upon writing Address Pointer Low-only once.

In the case of READ operation, the Data Register is loaded with the

Tentative ID Register

(Location +07Hex)

The Tentative ID Register is a read/write 8-bit register accessed when the Sub Address Bits are set up accordingly
(please refer to the Configuration Register description). The Tentative ID Register can be used while the node is on­line to build a network map of those nodes existing on the network. It minimizes the need for operator interaction with
the network. The node determines the existence of other nodes by placing a Node ID value in the Tentative ID
Register and waiting to see if the Tentative ID bit of the Diagnostic Status Register gets set.

The network maps

developed by this method has only historical value, since nodes may join or depart from the network at any time.

When using the Tentative ID feature, a node cannot detect the existence of the next logical node to which it passes
the token. The Next ID Register will hold the ID value of that node. The Tentative ID Register defaults to the value
0000 0000 upon hardware reset only.

Node ID Register

(Location +07Hex)

The Node ID Register is a read/write 8-bit register accessed when the Sub Address Bits are set up accordingly
(please refer to the Configuration Register). The Node ID Register contains the unique value which identifies this

SMSC DS – COM20051I Page 26 Rev. 03/27/2000

particular node. Each node on the network must occupy a unique Node ID value at all times. The Duplicate ID bit of
the Diagnostic Status Register helps the user find a unique Node ID. Refer to the Initialization Sequence section for
further detail on the use of the DUPID bit. The microsequencer of the ARCNET core does not wake up until a Node
ID other than zero is written into the Node ID Register. During this time, no microcode is executed, no tokens are
passed by this node, and no reconfigurations are caused by this node. Once a non-zero Node ID is placed into the
Node ID Register, the core wakes up but will not join the network until the TXEN bit of the Configuration Register is
set. W hile the Transmitter is disabled, the Receiver portion of the device is still functional and will provide the user
with useful information about the network. The Node ID Register defaults to the value 0000 0000 upon hardware
reset only.

Next ID Register

(Location +07Hex)

The Next ID Register is an 8-bit, read-only register, accessed when the sub-address bits are set up accordingly
(please refer to the Configuration Register). The Next ID Register holds the value of the Node ID to which the
COM20051I will pass the token. When used in conjunction with the Tentative ID Register, the Next ID Register can
provide a complete network map. The Next ID Register is updated each time a node enters/leaves the network or
when a network reconfiguration occurs. Each time the microsequencer updates the Next ID Register, a New Next ID
interrupt is generated. This bit is cleared by reading the Next ID Register. Default value is 0000 0000 upon hardware
or software reset.

Status Register

(Location +00Hex)

The ARCNET Status Register is an 8-bit read-only register. All of the bits, except for bits 5 and 6, are software
compatible with previous SMSC ARCNET devices. In previous SMSC ARCNET devices the Extended Timeout
status was provided in bits 5 and 6 of the Status Register. In the COM20020, the COM20020-5, COM20010,
COM90C66, and the COM90C165, these bits exist in and are controlled by the Configuration Register. The Status
Register contents are defined as in Table 6, but are defined differently during the Command Chaining operation.
Please refer to the Command Chaining section for the definition of the Status Register during Command Chaining
operation. The Status Register defaults to the value 1XX1 0001 upon either hardware or software reset.

Diagnostic Status Register

(Location +01Hex)

The Diagnostic Status Register contains seven read-only bits which help the user troubleshoot the network or node
operation. Various combinations of these bits and the TXEN bit of the Configuration Register represent different
situations. All of these bits, except the Excessive NAK bit and the New Next ID bit, are reset to logic "0" upon
reading the Diagnostic Status Register or upon software or hardware reset. The EXCNAK bit is reset by the "POR
Clear Flags" command upon software or hardware reset. The Diagnostic Status Register defaults to the value 0000
000X upon either hardware or software reset.

Command Register

(Location +01Hex)

Execution of commands are initiated by performing

write operations

to this register. Any combinations of written

data other than those listed in Table 8 are not permitted and may result in incorrect chip and/or network operation.

Address Pointer Registers

(Location +02Hex, +03Hex)

These read/write registers are each 8-bits wide and are used for addressing the internal ARCNET RAM. New pointer
addresses should be written by first writing to the High Register and then writing to the Low Register because writing
to the Low Register loads the address. The contents of the Address Pointer High and Low Registers are undefined
upon hardware reset.

Configuration Register

(Location +06Hex)

The Configuration Register is a read/write register which is used to configure the different modes of the ARCNET
core. The Configuration Register defaults to the value 0001 1000 upon hardware reset only.

Setup Register

(Location +07Hex)

The Setup Register is a read/write 8-bit register accessed when the Sub Address Bits are set up accordingly (see the
bit definitions of the Configuration Register). The Setup Register allows the user to change the network speed (data
rate) or the arbitration speed independently, invoke the Receive All feature, change the nPULSE1 driver type, and
reduce protocol timeouts by a factor of 3. The data rate may be slowed to 156.25 Kbps and/or the arbitration speed
may be slowed by a factor of two.

The SLOWARB must be set to a 1 for the 5 Mbps operation.

The Setup Register

defaults to the value 0000 0000 upon hardware reset only.

SMSC DS – COM20051I Page 27 Rev. 03/27/2000

Table 7 - Status Register

BIT BIT NAME SYMBOL

7 Receiver

Inhibited

RI This bit, if high, indicates that the receiver is not enabled

because either an "Enable Receive to Page fnn"
command was never issued, or a packet has been
deposited into the RAM buffer page fnn as specified by
the last "Enable Receive to Page fnn" command. No
messages will be received until this command is issued,
and once the message has been received, the RI bit is
set, thereby inhibiting the receiver. The RI bit is cleared
by issuing an "Enable Receive to Page fnn" command.
This bit, when set, will cause an interrupt if the
corresponding bit of the Interrupt Mask Register (IMR) is
also set. When this bit is set and another station attempts
to send a packet to this station, this station will send a
NAK.

6,5 (Reserved) These bits are undefined.

4 Power On Reset POR This bit, if hig h, indicates that the ARCNET core has bee n

reset by either a software reset, a hardware reset, or
writing 00H to the Node ID Register. The POR bit is
cleared by the "Clear Flags" command.

3 Test TEST This bit is intended for test and diagnostic purposes. It is

a logic "0" under normal operating conditions.

2 Reconfiguration RECON This bit, if high, indicates that the Line Idle Timer has

timed out because the RXIN pin was idle for 82 S. The
RECON bit is cleared during a "Clear Flags" command.
This bit, when set, will cause an interrupt if the
corresponding bit in the IMR is also set. The interrupt
service routine should consist of examing the MYRECON
bit of the Diagnostic Status Register to determine whether
there are consecutive reconfigurations caused by this
node.

1 Transmitter

Message
Acknowledged

TMA This bit, if high, indicates that the packet transmitted as a

result of an "Enable Transmit from Page fnn" command
has been acknowledged. This bit should only be
considered valid after the TA bit (bit 0) is set. Broadcast
messages are never acknowledged. The TMA bit is
cleared by issuing the "Enable Transmit from Page fnn"
command.

0 Transmitter

Available

TA This bit, if high, indicates that the transmitter is available

for transmitting. This bit is set when the last byte of
scheduled packet has been transmitted out, or upon
execution of a "Disable Transmitter" command. The TA
bit is cleared by issuing the "Enable Transmit from Page
fnn" command after the node next receives the token.
This bit, when set, will cause an interrupt if the
corresponding bit in the IMR is also set.

DESCRIPTION

SMSC DS – COM20051I Page 28 Rev. 03/27/2000

Table 8 - Diagnostic Status Register

BIT

7 My

6 Duplicate ID DUPID This bit, if high, indicates that the value in the Node ID

5 Receive

4 Token Seen TOKEN This bit, if high, indicates that a token has been seen on

3 Excessive NAK EXCNAK This bit, if high, indicates that either 128 or 4 Negative

2 Tentative ID TENTID This bit, if high, indicates that a response to a token whose

1 New Next ID NEW

1,0 (Reserved) These bits are undefined.

BIT NAME SYMBOL

Reconfiguration

Activity

DESCRIPTION

MY­RECON

This bit, if high, indicates that a past reconfiguration was
caused by this node. It is set when the Lost Token Timer
times out, and is typically read following an interrupt
caused by RECON. Refer to the Improved Diagnostics
section for further detail.

Register matches both Destination ID characters of the
token and a response to this token has occurred. The
EOT character and the trailing zero's are also verified. A
logic "1" on this bit indicates a duplicate Node ID, thus the
user should write a new value into the Node ID Register.

This bit is only useful for duplicate ID detection when
the device is off line, that is, when the transmitter is
off.

When the device is on line it will be set every time the
device gets the token. This bit is reset automatically upon
reading the Diagnostic Status Register. Refer to the
Improved Diagnostics section for further detail.

RCVACT This bit, if high, indicates that data activity (logic "1") was

detected on the RXIN pin of the device. Refer to the
Improved Diagnostics section for further detail.

the network, sent by a node other than this one. Refer to
the Improved Diagnostic section for further detail.

Acknowledgements have occurred in response to the Free
Buffer Enquiry. This bit is cleared upon the "POR Clear
Flags" command. Reading the Diagnostic Status Register
does not clear this bit. This bit, when set, will cause an
interrupt if the corresponding bit in the IMR is also set.
Refer to the Improved Diagnostics section for further detail.

DID matches the value in the Tentative ID Register has
occurred. In addition, the EOT character is checked. The
second DID and the trailing zero's are not checked. Since
each node sees every token passed around the network,
this feature can be used with the device on-line in order to
build and update a network map. Refer to the Improved
Diagnostics section for further detail.
This bit, if high, indicates that the Next ID Register has

NXTID

been updated and that a node has either joined or left the
network. Reading the Diagnostic Status Register does not
clear this bit. This bit, when set, will cause an interrupt if
the corresponding bit in the IMR is also set. The bit is
cleared by reading the Next ID Register.

SMSC DS – COM20051I Page 29 Rev. 03/27/2000

DATA

0000
0000

0000
0001

0000
0010

b0fn
n100

00fn
n011

0000
c101

000r
p110

0000
1000

Clear
Transmit
Interrupt

Disable
Transmitter

COMMAND

Table 9 - Command Register

DESCRIPTION

This command is used only in the Command Chaining
operation. Please refer to the Command Chaining section for
definition of this command.

This command will cancel any pending transmit command
(transmission that has not yet started) and will set the TA
(Transmitter Available) status bit to logic "1" when the ARCNET
core next receives the token.

Disable
Receiver

This command will cancel any pending receive command. If the
COM20051I is not yet receiving a packet, the RI (Receiver
Inhibited) bit will be set to logic "1" the next time the token is
received. If packet reception is already underway, reception will

run to its normal conclusion.
Enable
Receive to
Page fnn

This command allows the ARCNET core to receive data packets

into RAM buffer page fnn and resets the RI status bit to logic

"0". The values placed in the "nn" bits indicate the page that the

data will be received into (page 00 or 01). If the value of "f" is a

logic "1", an offset of 256 bytes will be added to that page

specified in "nn", allowing a finer resolution of the buffer. Refer

to the Selecting RAM Page Size section for further detail. If the

value of "b" is logic "1", the device will also receive broadcasts

(transmissions to ID zero). The RI status bit is set to logic "1"

upon successful reception of a message.
Enable
Transmit from
Page fnn

This command prepares the ARCNET core to begin a transmit

sequence from RAM buffer page fnn the next time it receives

the token. The values of the "nn" bits indicate which page to

transmit from (0 or 1). If "f" is logic "1", an offset of 256 bytes is

added to that page specified in "nn", allowing a finer resolution

of the buffer. Refer to the Selecting RAM Page Size section for

further detail. When this command is loaded, the TA and TMA

bits are reset to logic "0". The TA bit is set to logic "1" upon

completion of the transmit sequence. The TMA bit will have

been set by this time if the device has received an ACK from the

destination node. The ACK is strictly hardware level, sent by

the receiving node before its microcontroller is even aware of

message reception. Refer to Figure 3 for details of the transmit

sequence and its relation to the TA and TMA status bits.
Define
Configuration

This command defines the maximum length of packets that may

be handled by the device. If "c" is a logic "1", the device

handles both long and short packets. If "c" is a logic "0", the

device handles only short packets.
Clear Flags This command resets certain status bits of the COM20051I. A

logic "1" on "p" resets the POR status bit and the EXCNAK

Diagnostic status bit. A logic "1" on "r" resets the RECON

status bit.
Clear
Receive
Interrupt

This command is used only in the Command Chaining

operation. Please refer to the Command Chaining section for

definition of this command.

SMSC DS – COM20051I Page 30 Rev. 03/27/2000

Table 10 - Address Pointer High Register

BIT

7 Read Data RDDATA This bit tells the ARCNET core whether the following

6 Auto Increment AUTOINC This bit controls whether the address pointer will

5-2 (reserved) These bits are undefined.
1-0 Address 9-8 A9-A8 These bits hold the upper two address bits which provide

BIT BIT NAME

7-0 Address 7-0 A7-A0 These bits hold the lower 8 address bits which provide

BIT NAME

SYMBOL

DESCRIPTION

access will be a read or write. A logic "1" prepares the
device for a read, a logic "0" prepares it for a write.

increment automatically. A logic "1" on this bit allows
automatic increment of the pointer after each access,
while a logic "0" disables this function. Please refer to
the Sequential Access Memory section for further detail.

addresses to RAM.

Table 11 - Address Pointer Low Register

SYMBOL

DESCRIPTION

the addresses to RAM.

Table 12 - Configuration Register

BIT BIT NAME

SYMBOL

DESCRIPTION

7 Reset RESET A software reset of the ARCNET core is executed by

writing a logic "1" to this bit.

A software reset does not
reset the microcontroller interface mode, nor does it
affect the Configuration Register.

The only registers that
the software reset affect are the Status Register, the
Next ID Register, and the Diagnostic Status Register.
This bit must be brought back to logic "0" to release the

The Software Reset applies only to th e ARCNET

reset.

core. It does not apply to the 8051 microcontroller of the

6 Command

Chaining
Enable

CCHEN

COM20051I.

This bit, if high, enables the Command Chaining
operation of the device. Please refer to the Command
Chaining section for further details. A low level on this
bit ensures software compatibility with previous SMSC
ARCNET devices.

5 Transmit

Enable

TXEN When low, this bit disables transmissions by keeping

nPULSE1, nPULSE2 if in non-Backplane Mode, and
nTXENABLE inactive. When high, it enables the above
signals to be activated during transmissions. This bit
defaults low upon reset. This bit is typically enabled
once the Node ID is determined, and never disabled
during normal operation. Please refer to the Improved
Diagnostics section for details on evaluating network
activity.

SMSC DS – COM20051I Page 31 Rev. 03/27/2000

BIT BIT NAME

4,3 Extended

Timeout 1,2

Table 12 - Configuration Register

SYMBOL

ET1, ET2

These bits allow the network to operate over longer
distances than the default

DESCRIPTION

maximum

4 miles by
controlling the Response, Idle, and Reconfiguration
Times. All nodes should be configured with the same
timeout values for proper network operation. The bit
combinations follow.

2.5 Mbps
operation

Response Idle Time Reconfig
ET2 ET1 Time ("S) ("S) Time (mS)
0 0 1193.6 1312 1680
0 1 596.8 656 1680
1 0 298.4 328 1680
1 1 74.7 82 840

5 Mbps
operation

Response Idle Time Reconfig
ET2 ET1 Time ("S) ("S) Time (mS)
0 0 596.8 656 840
0 1 298.4 328 840
1 0 74.7 82 840
1 1 37.35 41 420

2

Backplane

BACK-

PLANE

A logic "1" on this bit puts the device into Backplane
Mode signalling which is used for Open Drain and
Differential Driver interfaces.

1,0

Sub

Address 1,0

SUBAD 1,0 These bits determine which register at address 07 may

be accessed. The combinations are as follows:
SUBAD1 SUBAD0 Register
0 0 Tentative ID

0 1 Node ID
1 0 Setup
1 1 Next ID

SMSC DS – COM20051I Page 32 Rev. 03/27/2000

Table 13 - Setup Register

BIT BIT NAME

SYMBOL

7 Pulse1 Mode P1MODE This bit determines the type of PULSE1 output driver

used in Backplane Mode. When high, a push/pull output
is used. When low, an open drain output is used. The
default is open drain.

6 Four NACKS FOUR NACKS This bit, when set, will cause the EXNACK bit in the

Diagnostic Status Register to set after four NACKs to
Free Buffer Enquiry are detected by the ARCNET core.
This bit, when reset, will set the EXNACK bit after 128
NACKs to Free Buffer Enquiry. The default is 128.

5 Reserve Reserve This bit, when set, scales down protocol timeout values

of Response Time and Idle Time but not Reconfiguration
Time

to optimize network performance in short
topologies. Provides a scaling factor of ÷ 3. Defaults to
a zero. Must be reset to be ARCNET compliant.

4 Receive All RCVALL This bit, when set, allows the COM20051I to receive all

valid data packets on the network, regardless of their
destination ID. This mode can be used to implement a
network monitor with the transmitter on- or off-line. Note
that ACKs are only sent for packets received with a
destination ID equal to the COM20051I's programmed
node ID. This feature can be used to put the
COM20051I in a 'listen-only' mode, where the transmitter
is disabled and the COM20051I is not passing tokens.
Defaults low.

3,2,1 Clock

Prescaler Bits

3,2,1

CKP2,1,0 These bits are used to determine the data rate of the

COM20051I. The following table is for a 40MHz crystal:
Response
CKP3 CKP2 CKP1 DIVISOR CLOCK
0 0 0 8 2.5 Mbps
0 0 1 16 1.25 Mbps
0 1 0 32 625 Kbps
0 1 1 64 312.5 Kbps
1 0 0 128 156.25 Kbps
1 0 1 256 Reserved
1 1 0 Reserved
1 1 1 Reserved

NOTE: The lowest data rate achievable by the
COM20051I is 156.25 Kbps. A divide by 256 is provided
for those systems that use faster clock speeds. Defaults
to 000 or 2.5 Mbps.

0 Slow

Arbitration
Select

SLW-ARB This bit, when set, will divide the arbitration clock by 2.

Memory cycle times will increase when slow arbitration
is selected.

Note: For 5 Mbps operation, SLOWARB must be set.

Defaults to low.

DESCRIPTION

SMSC DS – COM20051I Page 33 Rev. 03/27/2000

Data Register

I/O Address 04H

D0-D7

Address Pointer Register

I/O Address 02H

High

I/O A ddress 03H

Low

Memory

D a ta B u s

8

1K x 8

IN T E RN A L

RAM

Memory

Address Bus

10-Bit Counter

10

FIGURE 10 – SEQUENTIAL ACCESS OPERATION

SMSC DS – COM20051I Page 34 Rev. 03/27/2000

INTERNAL RAM

The integration of the 1K x 8 RAM in the ARCNET core rep resents sig nificant real e state savings . The PC board is
now free of the cumbersome external RAM, external latch, and multiplexed address/data bus and control
functions which were necessary to interface to the RAM.

The integration of RAM represents significant cost savings because it isolates the system designer from the changing
costs of external RAM and it minimizes reliability problems, assembly time and costs, and layout complexity.

Sequential Access Memory

The internal RAM is accessed via a pointer-based scheme. Rather than interfering with system memory, the internal
RAM is indirectly accessed through the Address High and Low Pointer Registers. The data is channeled to and from
the microcontroller via the 8-bit Data Register. For example: a packet in the internal RAM buffer is read by the
microcontroller by writing the corresponding address into the Address Pointer High and Low Registers (offsets 02H
and 03H). Note that the High Register should be written first, followed by the Low Register, because writing to the
Low Register loads the address. At this point the device accesses that location and places the corresponding data
into the Data Register. The microcontroller then reads the Data Register (offset 04H) to obtain the data at the
specified location. If the Auto Increment bit is set to logic "1", the device will automatically increment the address and
place the next byte of data into the Data Register, again to be read by the microcontroller. This process is
continued until the entire packet is read out of RAM. Refer to Figure 10 for an illustration of the Sequential Access
operation.

When switching between reads and writes, the pointer must first be written with the starting address. At least one
cycle time should separate the pointer being loaded and the first read (see timing parameters).

Access Speed

The ARCNET core is able to accommodate very fast access cycles to its registers and buffers. Arbitration to the
buffer does not slow down the cycle because the pointer based access method allows data to be prefetched from
memory and stored in a temporary register. Likewise, data to be written is stored in the temporary register and
then written to memory.

A Slow Arbitration Bit is provided in the Setup Register to slow down the arbitration clock for buffer accesses at 5
Mbps. The SLOWARB bit must be set to a "1" for 5 Mbps operation.

SOFTWARE INTERFACE

The 80C32 core in terfaces to the ARCNET c ore via software by acce ssing the vari ous registe rs. These action s are
described in the Internal Registers section. The software flow for accessing the data buffer is based on the
Sequential Access scheme. The basic sequence is as follows:

!"

Disable Interrupts

!"

Write to Pointer Register High (specifying Auto-Increment mode.)

!"

Write to Pointer Register Low (this loads the address.)

!"

Enable Interrupts

!"

Read or write the Data Register (repeat as many times as necessary to empty or fill the buffer.

!"

The pointer may now be read to determine how many transfers were completed.

The software flow for controlling the Configuration, Node ID, Tentative ID, and Next ID registers is generally limited to
the initialization sequence and the maintenance of the network map.

Additionally, it is necessary to understand the details of how the other Internal Registers are used in the transmit and
receive sequences and to know how the internal RAM buffer is properly set up. The sequence of events that tie
these actions together is discussed as follows.

Selecting RAM Page Size

During normal operation, the 1K x 8 of RAM is divided into two pages of 512 bytes each. The page to be used is
specified in the "Enable Transmit (Receive) from (to) Page fnn" command, where "nn" specifies page 0 or 1. This
allows the user to have constant control over the allocation of RAM.

When the Offset bit "f" (bit 5 of the "Enable Transmit (Receive) from (to) Page fnn" command word) is set to logic "1",
an offset of 256 bytes is added to the page specified. For example: to transmit from the second half of page 0, the
command "Enable Transmit from Page fnn" (fnn=100 in this case) is issued by writing 0010 0011 to the Command

SMSC DS – COM20051I Page 35 Rev. 03/27/2000

Register. This allows a finer resolution of the buffer pages without affecting software compatibility. This scheme is

A

useful for applications which frequently use packet sizes of 256 bytes or less, especially for microcontroller systems
with limited memory capacity. The remaining portions of the buffer pages which are not allocated for current transmit
or receive packets may be used as temporary storage for previous network data, packets to be sent later, or as extra
memory for the system, which may be indirectly accessed.
If the device is configured to handle both long and short packets (see "Define Configuration" command), then the
receive page should always be 512 bytes long because the user never knows what the length of the receive packet
will be. In this case, the transmit page may be made 256 bytes long, leaving at least 256 bytes free at any given
time. Please note that it is the responsibility of software to reserve 512 bytes for the receive page if the device is
configured to handle long packets. The ARCNET core does not check page boundaries during reception.

If the device is configured to handle only short packets, then both transmit and receive pages may be allocated as
256 bytes long, allowing two receive and two transmit packets.

The general rule which may be applied to determine where in RAM a page begins is as follows:
Address = (nn x 512) + (f x 256).

Transmit Sequence

During a transmit sequence, the microcontroller selects a 256 or 512 byte segment of the RAM buffer and writes into
it. The appropriate buffer size is specified in the "Define Configuration" command. W hen long packets are enabled,
the ARCNET core interprets the packet as either a long or short packet, depending on whether the buffer address 2
contains a zero or non-zero value. The format of the buffer is shown in Figure 11. Address 0 contains the Source
Identifier (SID); Address 1 contains the Destination Identifier (DID); Address 2 (COUNT) contains, for short packets,
the value 256-N, where
0, indicating that it is indeed a long packet. In the latter case, Address 3 (COUNT) would contain the value 512-N,

N represents the number of information bytes in the message.

where

N represents the number of information bytes in the message,

The SID in Address 0 is used by

or for long packets, the value

SHORT PACK ET

DDRESS ADDRESS

0
1
2

FORM AT

SID
DID

COUN T = 256- N

NOT USED

0
1
2
3

COUNT

DA TA BYTE 1
DA TA BYTE 2

COUNT

511

255

511

DA TA BYTE N-1

DA TA BYTE N

NOT USED

N = DA TA P ACKET LENGTH
SID = SOURCE ID
DID = DESTINATION ID
(DID = 0 FOR BROADCASTS)

FIGURE 11 – RAM BUFFER PACKET CONFIGURATION

LONG PACK ET

FORM AT

SID
DID

0

COUN T = 5 12-N

NOT USED

DA TA BYTE 1
DA TA BYTE 2

DA TA BYTE N-1

DA TA BYTE N

SMSC DS – COM20051I Page 36 Rev. 03/27/2000

the receiving node to reply to the transmitting node. The ARCNET core puts the local ID in this location, therefore
it is not necessary to write into this location.

Please note that a short packet may contain between 1 and 253 data bytes, while a long packet may contain between
257 and 507 data bytes. A minimum value of 257 exists on a long packet so that the COUNT is expressable in eight
bits. This leaves three exception packet lengths which do not fit into either a short or long packet; packet lengths of
254, 255, or 256 bytes. If packets of these lengths must be sent, the user must add dummy bytes to the packet in
order to make the packet fit into a long packet. Note that only the number of bytes specified in the byte count plus the
three-byte header are transmitted. For example, if the byte count is equal to 253, only three bytes of data will be
transmitted plus the header (SID, DID, Byte Count) for a total of six bytes.

Once the

packet is written into the buffer,

the microcontroller awaits a logic "1" on the TA bit, indicating that a
previous transmit command has concluded and another may be issued. Each time the message is loaded and a
transmit command issued, it will take a variable amount of time before the message is transmitted, depending on the
traffic on the network and the location of the token at the time the transmit command was issued.

the Transmit command will generate an interrupt if the Interrupt Mask allows it.

If the device is configured for the

The conclusion of

Command Chaining operation, please see the Command Chaining section for further detail on the transmit sequence.
Once the TA bit becomes a logic "1", the microcontroller

may issue

the "Enable Transmit from Page fnn" command,
which resets the TA and TMA bits to logic "0". If the message is not a BROADCAST, the ARCNET core
automatically

sends

a FREE BUFFER ENQUIRY to the destination node in order to send the message. At this point,
one of four possibilities may occur.
The first possibility is if a free buffer is available at the destination node, in which case it responds with an
ACKnowledgement. At this point, the ARCNET core fetches the data from the Transmit Buffer and performs the
transmit sequence. If a successful transmit sequence is completed, the TMA bit and the TA bit are set to logic "1". If
the packet was not transmitted successfully, TMA will not be set. A successful transmission occurs when the
receiving node responds to the packet with an ACK. An unsuccessful transmission occurs when the receiving node
does not respond to the packet.

The second possibility is if the destination node responds to the Free Buffer Enquiry with a Negative
AcKnowledgement. A NAK occurs when the RI bit of the destination node is a logic "1". In this case, the token is
passed on from the transmitting node to the next node. The next time the transmitter receives the token, it will again
transmit a FREE BUFFER ENQUIRY. If a NAK is again received, the token is again passed onto the next node. The
Excessive NAK bit of the Diagnostic Status Register is used to prevent an endless

limit of FBE-NAK sequences existed,

the transmitting node would continue issuing a Free Buffer Enquiry, even

sending

of FBE's and NAK's. If no

though it would continuously receive a NAK as a response. The EXCNAK bit generates an interrupt (if enabled) in
order to tell the microcontroller to disable the transmitter via the "Disable Transmitter" command. This causes the
transmission to be abandoned and the TA bit to be set to a logic "1" when the node next receives the token, while the
TMA bit remains at a logic "0". Please refer to the Improved Diagnostics section for further detail on the EXCNAK bit.

The third possibility which may occur after a FREE BUFFER ENQUIRY is issued is if the destination node does not
respond at all. In this case, the TA bit is set to a logic "1", while the TMA bit remains at a logic "0". The user should
determine whether the node should try to reissue the transmit command.
The fourth possibility is if a non-traditional response is received (some pattern other than ACK or NAK, such as
noise). In this case, the token is not passed onto the next node, which causes the Lost Token Timer of the next node
to time out, thus generating a network reconfiguration.

The "Disable Transmitter" command may be used to cancel any pending transmit command when the ARCNET core
next receives the token. Normally, in an active network, this command will set the TA status bit to a logic "1" when the
token is received. If the "Disable Transmitter" command does not cause the TA bit to be set in the time it takes the
token to make a round trip through the network, one of three situations exists. Either the node is disconnected from
the network, or there are no other nodes on the network, or the external receive circuitry has failed. These situations
can be determined by either using the improved diagnostic features of the ARCNET core or using another software
timeout which is greater than the worst case time for a round trip token pass, which occurs when all nodes transmit a
maximum length message.

Receive Sequence

A receive sequence begins with the RI status bit becoming a logic "1", which indicates that a previous reception has
concluded. The microcontroller will be interrupted if the corresponding bit in the Interrupt Mask Register is set to logic
"1". Otherwise, the microcontroller must periodically check the Status Register. O nce the microcontroller is alerted to
the fact that the previous reception has concluded, it may issue the "Enable Receive to Page fnn" command, which
resets the RI bit to logic "0" and selects a new page in the RAM buffer. Again, the appropriate buffer size is specified
in the "Define Configuration" command. Typically, the page which just received the data packet will be read by the
microcontroller at this point.

SMSC DS – COM20051I Page 37 Rev. 03/27/2000

Once the "Enable Receive to Page fnn" command is issued, the microcontroller attends to other duties. There is no
way of knowing how long the new reception will take, since another node may transmit a packet at any time. When
another node does transmit a packet to this node, and if the "Define Configuration" command has enabled the
reception of long packets, the ARCNET core interprets the packet as either a long or short packet, depending on
whether the content of the buffer location 2 is zero or non-zero. The format of the buffer is shown in Figure 12.
Address 0 contains the Source Identifier (SID), Address 1 contains the Destination Identifier (DID), and Address 2
contains, for short packets, the value 256-N, where N represents the message length, or for long packets, the value
0, indicating that it is indeed a long packet. In the latter case, Address 3 contains the value 512-N, where N
represents the message length. Note that on reception, the ARCNET core deposits packets into the RAM buffer in
the same format that the transmitting node arranges them, which allows for a message to be received and then
retransmitted without rearranging any bytes in the RAM buffer other than the SID and DID. Once the packet is
received and stored correctly in the selected buffer, the ARCNET core sets the RI bit to logic "1" to signal the
microcontroller that the reception is complete.

COMMAND CHAINING

The Command Chaining operation allows consecutive transmissions and receptions to occur without

on-chip 80C32

intervention. Through the use of a dual two-level FIFO, commands to be transmitted and received, as well as the
status bits, are pipelined.

In order for the COM20051I to be compatible with previous SMSC ARCNET device

drivers,

the device defaults to the
non-chaining mode. In order to take advantage of the Command Chaining operation, the Command Chaining Mode
must be enabled via a logic "1" on bit 6 of the Configuration Register.

In Command Chaining, the Status Register appears as in Figure 12.
The following is a list of Command Chaining guidelines for the software programmer. Further detail can be found in

the Transmit Command Chaining and Receive Command Chaining sections.

!"

The device is designed such that the interrupt service routine latency does not affect performance.

!"

Up to two outstanding transmissions and two outstanding receptions can be pending at any given time.
The commands may be given in any order.

!"

Up to two outstanding transmit interrupts and two outstanding receive interrupts are stored by the
device, along with their respective status bits.

!"

The Interrupt Mask bits act on TTA (Rising Transition on Transmitter Available) for transmit operations
and TRI (Rising Transition of Receiver Inhibited) for receive operations. TTA is set upon completion of
a packet transmission only. TRI is set upon completion of a packet reception only. Typically there is no
need to mask the TTA and TRI bits after clearing the interrupt.

!"

The traditional TA and RI bits are still available to reflect the present status of the device.

SMSC DS – COM20051I Page 38 Rev. 03/27/2000

Transmit Command Chaining

When the microcontroller issues the first "Enable Transmit to Page fnn" command, the ARCNET core responds in the
usual manner by resetting the TA and TMA bits to prepare for the transmission from the specified page. The TA bit
can be used to see if there is currently a transmission pending, but the TA bit is really meant to be used in the non­chaining mode only. The TTA bits provide the relevant information for the device in the Command Chaining mode.

In the Command Chaining Mode, at any time after the first command is issued, the processor can issue a second
"Enable Transmit from Page fnn" command. The ARCNET core stores the fact that the second transmit command
was issued, along with the page number.

After the first transmission is completed, the ARCNET core updates the Status Register by setting the TTA bit, which
generates an interrupt. The interrupt service routine should read the Status Register. At this point, the TTA bit will be
found to be a logic "1" and the TMA (Transmit Message Acknowledge) bit will tell the processor whether the
transmission was successful. After reading the Status Register, the "Clear Transmit Interrupt" command is issued,

MSB LSB

TRI RI TA POR TEST RECON

TMA TTA

TRI

TMA TTA

FIGURE 12 – COMMAND CHAINING STATUS REGISTER QUEUE

thus resetting the TTA bit and clearing the interrupt. Note that only the "Clear Transmit Interrupt" command will
clear the TTA bit and the interrupt. It is not necessary, however, to clear the bit or the interrupt right away because
the status of the transmit operation is double buffered in order to retain the results of the first transmission for analysis
by the processor. This information will remain in the Status Register until the "Clear Transmit Interrupt" command is
issued. Note that the interrupt will remain active until the command is issued, and the second interrupt will not occur
until the first interrupt is ac knowledged. The ARCNET core guarantees a minimum of 200nS interrupt inactive time
interval between interrupts. The TMA bit is also double buffered to reflect whether the appropriate transmission was
a success. The TMA bit should only be considered valid after the corresponding TTA bit has been set to a logic "1".
The TMA bit never causes an interrupt.

When the token is received again, the second transmission will be automatically initiated after the first is completed
by using the stored "Enable Transmit from Page fnn" command. The operation is as if a new "Enable Transmit from
Page fnn" command has just been issued. After the first Transmit status bits are cleared, the Status Register will
again be updated with the results of the second transmission and a second interrupt resulting from the second
transmission will occur. The ARCNET core guarantees a minimum of 200ns interrupt inactive time interval before the
following edge.

The Transmitter Available (TA) bit of the Interrupt Mask Register now masks only the TTA bit of the Status Register,
not the TA bit as in the non-chaining mode. Since the TTA bit is only set upon transmission of a packet (not by
RESET), and since the TTA bit may easily be reset by issuing a "Clear Transmit Interrupt" command, there is no
need to use the TA bit of the Interrupt Mask Register to mask interrupts generated by the TTA bit of the Status
Register. In Command Chaining mode, the "Disable Transmitter" command will cancel the oldest transmission. This
permits canceling a packet destined for a node not ready to receive. If both packets should be canceled, two
"Disable Transmitter" commands should be issued.

Receive Command Chaining

Like the Transmit Command Chaining operation, the processor can issue two consecutive "Enable Receive from
Page fnn" commands.

After the first packet is received into the first specified page, the TRI bit of the Status Register will be set to logic "1",
causing an interrupt. Again, the interrupt need not be serviced immediately. Typically, the interrupt service routine

SMSC DS – COM20051I Page 39 Rev. 03/27/2000

will read the Status Register. At this point, the RI bit will be found to be a logic "1". After reading the Status Register,
the "Clear Receive Interrupt" command should be issued, thus resetting the TRI bit and clearing the interrupt. Note
that only the "Clear Receive Interrupt" command will clear the TRI bit and the interrupt. It is not necessary, however,
to clear the bit or the interrupt right away because the status of the receive operation is double buffered in order to
retain the results of the first reception for analysis by the processor, therefore the information will remain in the Status
Register until the "Clear Receive Interrupt" command is issued. Note that the interrupt will remain active until the
"Clear Receive Interrupt" command is issued, and the second interrupt will be stored until the first interrupt is
acknowledged. A minimum of 200nS interrupt inactive time interval between interrupts is guaranteed.

The second reception will occur as soon as a second packet is sent to the node, as long as the second "Enable
Receive to Page fnn" command was issued. The operation is as if a new "Enable Receive to Page fnn" command
has just been issued. After the first Receive status bits are cleared, the Status Register will again be updated with
the results of the second reception and a second interrupt resulting from the second reception will occur.

In the ARCNET core, the Receive Inhibit (RI) bit of the Interrupt Mask Register now masks only the TRI bit of the
Status Register, not the RI bit as in the non-chaining mode. Since the TRI bit is only set upon reception of a packet
(not by RESET), and since the TRI bit may easily be reset by issuing a "Clear Receive Interrupt" command, there is
no need to use the RI bit of the Interrupt Mask Register to mask interrupts generated by the TRI bit of the Status
Register.

In Command Chaining mode, the "Disable Receiver" command will cancel the oldest reception, unless the reception
has already begun. If both receptions should be canceled, two "Disable Receiver" commands should be issued.

RESET DETAILS
Internal Reset Logic

The ARCNET core supports two reset options; software and hardware reset. A software reset is generated when a
logic "1" is written to bit 7 of the Configuration Register. The device remains in reset as long as this bit is set.

The
software reset does not affect the contents of the Address Pointer Registers, the Configuration Register, the IMR, or
the Setup Register. A hardware reset occurs when a high signal is asserted on the nRESET input. The minimum
reset pulse width is 3.2 s

(for 20 MHz core clock, 1.6 s for 40 MHz core clock)

. This pulse width is used by the

internal digital filter, which filters short glitches to allow only valid resets to occur.
Upon reset, the transmitter portion of the device is disabled and the internal registers assume those states outlined in

the Internal Registers section.
After the nRESET signal is removed the user may write to the internal registers. Since writing a non-zero v alue to the

Node ID Register wakes up the ARCNET core, the Setup Register should be written before the Node ID
Register. Once the Node ID Register is written to, the ARCNET core reads the value and executes two write cycles
to the RAM buffer. Address 0 is written with the data D1H and address 1 is written with the Node ID. The data
pattern D1H was chosen arbitrarily, and is meant to provide assurance of proper microsequencer operation.

INITIALIZATION SEQUENCE

When the ARCNET core is powered on the internal registers may be written to. Since writing a non-zero value to the
Node ID Register wakes up the core, the Setup Register should be written to before the Node ID Register. Until a
non-zero value is placed into the NID Register, no microcode is executed, no tokens are passed by this node, and no
reconfigurations are generated by this node. Once a non-zero value is placed in the register, the core wakes up, but
the node will not attempt to join the network until the TX Enable bit of the Configuration Register is set.

Before setting the TX Enable bit, the software may make some determinations. The software may first observe the
Receive Activity and the Token Seen bits of the Diagnostic Status Register to verify the health of the receiver and the
network.

Next, the uniqueness of the Node ID value placed in the Node ID Register is determined. The TX Enable bit should
still be a logic "0" until it is ensured that the Node ID is unique. If this node ID already exists, the Duplicate ID bit of
the Diagnostic Status Register is set after a maximum of 840mS (or 1680mS if the ET1 and ET2 bits are other than
1,1). To determine if another node on the network already has this ID, the ARCNET core compares the value in the
Node ID Register with the DID's of the token, and determines whether there is a response to it. Once the Diagnostic
Status Register is read, the DUPID bit is cleared. The user may then attempt a new ID value, wait 840mS before
checking the Duplicate ID bit, and repeat the process until a unique Node ID is found. At this point, the TX Enable bit
may be set to allow the node to join the network. Once the node joins the network, a reconfiguration occurs, as
usual, thus setting the MYRECON bit of the Diagnostic Status Register.

SMSC DS – COM20051I Page 40 Rev. 03/27/2000

The Tentative ID Register may be used to build a network map of all the nodes on the network, even once the
COM20051I has joined the network. Once a value is plac ed in the Tentative ID Register, the ARCNET core looks for
a response to a token whose DID matches the Tentative ID Register. The software can record this information and
continue placing Tentative ID values into the register to continue building the network map. A complete network map
is only valid until nodes are added to or deleted from the network. Note that a node cannot detect the existence of
the next logical node on the network when using the Tentative ID. To determine the next logical node, the software
should read the Next ID Register.

IMPROVED DIAGNOSTICS

The COM20051I allows the user to better manage the operation of the network through the use of the internal
Diagnostic Status Register.

A high level on the My Reconfiguration (MYRECON) bit indicates that the Token Reception Timer of this node
expired, causing a reconfiguration by this node. After the Reconfiguration (RECON) bit of the Status Register
interrupts the microcontroller, the interrupt service routine will typically read the MYRECON bit of the Diagnostic
Status Register. Reading the Diagnostic Status Register resets the MYRECON bit. Successive occurrences of a
logic "1" on the MYRECON bit indicates that a problem exists with this node. At that point, the transmitter should be
disabled so that the entire network is not held down while the node is being evaluated.

The Duplicate ID (DUPID) bit is used before the node joins the network to ensure that another node with the same ID
does not exist on the network. Once it is determined that the ID in the Node ID Register is unique, the software
should write a logic "1" to bit 5 of the Configuration Register to enable the basic transmit function. This allows the
node to join the network.

The Receive Activity (RCVACT) bit of the Diagnostic Status Register will be set to a logic "1" whenever activity (logic
"1") is detected on the RXIN pin.

The Token Seen (TOKEN) bit is set to a logic "1" whenever any token has been seen on the network (except those
tokens transmitted by this node).

The RCVACT and TOKEN bits may help the user to troubleshoot the network or the node. If unusual events are
occurring on the network, the user may find it valuable to use the TXEN bit of the Configuration Register to qualify
events. Different combinations of the RCVACT, TOKEN, and TXEN bits, as shown indicate different situations:

Normal Results:

RCVACT=1, TOKEN=1, TXEN=0:

The node is not part of the network. The network is operating properly without this

node.
RCVACT=1, TOKEN=1, TXEN=1:

The node sees receive activity and sees the token. The basic transmit function is

enabled. Network and node are operating properly.
MYRECON=0, DUPID=0, RCVACT=1, TXEN=0, TOKEN=1:

Single node network.

SMSC DS – COM20051I Page 41 Rev. 03/27/2000

Abnormal Results:

RCVACT=1, TOKEN=0, TXEN=X: The node sees receive activity, but does not see the token. Either no other nodes
exist on the network, some type of data corruption exists, the media driver is malfunctioning, the topology is set up
incorrectly, there is noise on the network, or a reconfiguration is occurring.

RCVACT=0, TOKEN=0, TXEN=1:

No receive activity is seen and the basic transmit function is enabled. The

transmitter and/or receiver are not functioning properly.
RCVACT=0, TOKEN=0, TXEN=0:

No receive activity and basic transmit function disabled. This node is not

connected to the network.
The Excessive NAK (EXCNAK) bit is used to replace a timeout function traditionally implemented in software. This

function is necessary to limit the number of times a sender issues a FBE to a node with no available buffer. When
the destination node replies to 128 FBEs with 128 NAKs or 4 FBEs with 4 NAKs, the EXCNAK bit of the sender is set,
generating an interrupt. At this point the software may abandon the transmission via the "Disable Transmitter"
command. This sets the TA bit to logic "1" when the node next receives the token, to allow a different
transmission to occur. The timeout value for the EXNACK bit (128 or 4) is determined by the FOUR-NAKS bit on the
Setup Register.

The user may choose to wait for more NAK's before disabling the transmitter by taking advantage of the wraparound
counter of the EXCNAK bit. When the EXCNAK bit goes high, indicating 128 or 4 NAKs, the "POR Clear Flags"
command may be issued to reset the bit so that it will go high again after another count of 128 or 4. The software
may count the number of times the EXCNAK bit goes high, and once the final count is reached, the "Disable
Transmitter" command may be issued.

The New Next ID bit permits the software to detect the withdrawal or addition of nodes to the network.
The Tentative ID bit allows the user to build a network map of those nodes existing on the network. This feature is

useful because it minimizes the need for human intervention. When a value placed in the Tentative ID Register
matches the Node ID of another node on the network, the TENTID bit is set, telling the software that this NODE ID

already exists on the network.

The software should periodically place values in the Tentative ID Register and

monitor the New Next ID bit to maintain an updated network map.

SMSC DS – COM20051I Page 42 Rev. 03/27/2000

COM20051I APPLICATIONS INFORMATION

PROGRAMMING THE COM20051I NETWORK CORE

The COM20051I ARCNET core is relatively simple to program. The ARCNET core was designed to allow the
processor to remain in control of the node and to perform network flow control with a minimum of processor
intervention. There are two methods of operating the ARCNET core: in Command Chaining and Non-Command
Chaining mode. Command chaining mode permits the processor to pipeline up to two transmit and two receive
commands. In normal Non-Command Chaining mode, the microcontroller must access the ARCNET core for each
individual transmission and reception since the ARCNET core can only handle one transmit and one receive
command at any given time. The Command Chaining mode permits the ARCNET core to handle up to two transmit
and two receive commands at any given time. Receive and Transmit status bits are buffered for each command in
Command Chaining mode.

The basic software approach can be divided into three parts: ARCNET initialization, Transmit Interrupt servicing, and
Receive Interrupt servicing. Both Command and Non-Command Chaining examples will be illustrated. It is strongly
suggested that an interrupt driven approach be used since polling methods can result in lost transmit opportunities
and in lost receptions.

ARCNET Initialization

Basic COM20051I ARCNET core initialization consists of the following:

1) Programming of the Address Decode Register

2) Configuring the Operating Mode (Command or Non-Command Chaining, Backplane or Dipulse operation)

3) Programming the Interrupt Routing Register (IRR)

4) Programming the Data Rate (optional)

5) Enabling the Receive All mode (optional)

6) Obtaining a Node ID and programming in its value

7) Entering the Network

8) Issuing initial Receive commands
Figure 14 is a flowchart of a typical initialization procedure. Steps 1 through 5 are straightforward and do not require

much explanation. Step 6, Selection of a Node ID can be somewhat more involved depending on the application.
There are many methods of obtaining a unique node id for a particular node ranging from simply reading a switch to
sophisticated software algorithms. Several methods will be discussed but the best method depends on the particular
application.

Node ID Selection

The ARCNET Protocol is a token passing protocol that relies on each station to have a unique Node ID in order to
permit proper network operation. Many methods will be discussed but it is up to the user to choose the right method
for his application.

Method 1 - Hardware Switch Read
This is the simplest and the most common form of obtaining a Node ID for an individual node. It consists of adding

an eight bit DIP switch and address decoder as shown in Figure 15. A simple read from the microcontroller will
obtain the ID value.

SMSC DS – COM20051I Page 43 Rev. 03/27/2000

START

W RITE ARCNET CORE

BASE ADDRESS TO

DECODE REGISTER

ADDR ESS FFFFh

O B TA IN A N D

PROGRAM UNIQUE

ID

WRITE TO

CONFIGURATION

REG ISTER. PRO GRAM

CHAINING M ODE AND

ENCODING METHOD.

CHEC K STATUS

AND JO IN NETWORK

PROGRAM IRR

SET IM R AND

ENABLE RECEIVER

PROG RAM DATA
R ATE TH RO UG H

SETUP REGISTER

END

ENABLE RECEIVE

ALL M ODE

THROUGH SETUP

REGISTER

FIGURE 13 – TYPICAL INITIALIZATION PROCEDURE

SMSC DS – COM20051I Page 44 Rev. 03/27/2000

COM20051I

ADDRES

A15-

DECODE

NID

D7-

'LS24

EN

DIP

SWITC

FIGURE 14 – NODE ID SELECTION USING A HARDWARE SWITCH READ

Method 2 - Non-Volatile Memory Storage
This method involves the storage of the Node ID in either a PROM, EPROM, or EEPROM. The microcontroller

simply reads the proper location and programs the ID value. This method is well suited for closed networks that are
not expected to expand or are maintained solely by the OEM. Such examples might include machine control and
automobiles.

Method 3 - Hardwire Daisy Chain
The Daisy Chain method uses an additional wire running from node to node in a daisy chained fashion. This type of

setup relies on a master node that uses a fixed ID value that is stored in the master node's non-volatile memory or by
reading a switch. Once the master programs its ID value it brings the additional line low. The next node that is in line
physically detects the low level on the line and uses a duplicate ID search algorithm to detect the first available ID on
the network. Once this node finds an ID and joins the network, it brings its ID line low to allow the next node to join.
This process continues until all nodes have joined the network. Figure 16 shows an example of how this is
accomplished.

Method 4 - Duplicate ID Software Search
The Duplicate ID software search is a software only method of locating available node ID values on a given network.

The basic algorithm involves a sequential search of ID values and checking their availability by using a combination
of diagnostic bits located within the ARCNET core of the COM20051I. This method is not well suited for the initial
power-up sequence of a network since there are instances in which the algorithm will assign the same ID to two
nodes when they power-up simultaneously. This occurs due to basic constraints imposed by the ARCNET protocol
itself. The Duplicate ID algorthim is very useful for adding nodes to already existing networks that use a mix of node
ID selection methods. Please refer to the section on Duplicate ID Detection for details regarding the algorithm.

SMSC DS – COM20051I Page 45 Rev. 03/27/2000

Entering the Network

After establishing a unique Node ID, the COM20051I can join the network. Prior to actually joining the network,
several diagnostic bits should be checked to ensure that the node is properly functioning and that the network is
functioning properly. The DUPID, RCVACT, and TOKEN bits should all be checked to ensure that the network is
operating properly. A read of the Diagnostic Status register should be done to clear previous data prior to checking
the status bits. This will ensure that the Diagnostic Status bits are reflecting the latest status of the network. A period
of time lasting 840ms should elapse prior to checking the status bits. The 840ms period is a worst case situation
based on a full 255 node network with every node transmitting a 512 byte packet. T he actual w ait time will be less for
most systems and should be calculated based on the number of nodes and maximum packet size. Once the wait
period is over, the Diagnostic Status bits should read as follows: DUPID = 0, RCVACT = 1, and TOKEN = 1. Any
other combination indicates that a problem exists on the network. Once a valid status condition is established, the
TXEN bit of the Configuration Register should be programmed to a '1' to allow the transmitter output to drive the
network. The node is now a member of the network.

T

TIME

LINE 1

LINE 2

LINE 3

SLAVE #3

OUT

LINE 1

COM20051I

MASTE SLAVE SLAVE SLAVE

0

NETWORK

COM20051I

IN OUT

LINE 2

LINE 3

COM20051I

IN OUT

COM20051I

IN

FIGURE 15 – DAISY CHAIN METHOD OF NODE ID SELECTION

T

OUT

SMSC DS – COM20051I Page 46 Rev. 03/27/2000

Initializing Receptions and Interrupt Masking

Normally the receiver is enabled immediately after joining the network. Since a node never will know when a packet
will be transmitted to it, it is advisable to have the receiver enabled at the earliest time possible. Generally, Enable to
Receive commands are given at the end of the initialization. For systems using the Command Chaining feature, two
receive commands should be given. For Non-Command Chaining systems a single Enable to Receive command
should be issued.

Interrupt Masking is a critical function in Non-Command Chaining systems. The interrupt mask is critical because it
provides the only method of releasing interrupts in Non-Command Chaining systems. For Non-Command Chaining
systems, the RI interrupt should be unmasked right after the Enable to Receive commands to ensure that an interrupt
will be generated. Note that the interrupt must be masked to release the interrupt once the packet has been
received. For Command Chaining systems, the Receive Interrupt mask should be enabled and left enabled prior to
the first Enable to Receive Commands. The Command Chaining mode utilizes a software Clear Receive Interrupt
command to release the interrupt thus the interrupt can be left unmasked.

DUPLICATE ID DETECTION (AUTO NODE ID SELECTION)

The Duplicate ID algorithm was introduced previously as a software only method of obtaining unique ID values to join
the network. The algorithm uses many of the diagnostic features found in the COM20051I including the RECON,
RCVACT, TOKEN, DUPID, and MYRECON diagnostic bits to locate a unique ID value. Caution should be used
when using this algorithm in that it cannot isolate a unique value when two nodes power-on simulatneously or within
close proximity of each other. Two nodes using the same ID value will cause the network to fail.

The basic algorithm operates as follows: The COM20051I's Node ID is temporaily initialized to FEh. A 52us waiting
period is then entered to allow the core to read the new node ID value. This is necessary to allow time for the
ARCNET microsequencer to read the value from the ARCNET Node register. A one second timing loop is then
entered in which several diagnostic bits are sampled in order to determine if the selected node ID is being used or
not. The first determination made is if the RCVACT (RECEIVER ACTIVITY) and the TOKEN bits are set. If both are
reset, this node is most likely to be the first node to join the network. If one of these bits is reset, then a timing loop is
entered and the bits are sampled again. Once both bits are found to be set then the DUPID bit is sampled. If this bit
is set then the current node ID is decremented and the timing loop is restarted. If the DUPID bit is reset then the
program continues in the timing loop and the RECON bit tested after the loop is exited to determine if a RECON had
occurred during the sampling of the diagnostic status bits. If the RECON bit is reset then another loop is entered to
synchronize the COM20051I with its Token Rotation timer in order to prevent multiple nodes from using the same
Node ID. This loop samples the MYRECON bit which only gets set when the node has not seen a token to itself for
840ms. While the program is sampling the MYRECON bit, the DUPID bit is checked to prevent two nodes which
have reconfigured close together from using the same ID. If the DUPID bit is set before the MYR ECO N bit is set then
another node ID must be found. Once the MYRECON bit is found to be set then the TOKEN bit is checked again to
make sure that another node has not RECONed just prior to this node. If all these conditions are satisfied then the
TXEN bit of the Configuration Register is set and a transmission is sent out to the same Node ID. The TA and TMA
bits are polled for 840ms. If the TMA bit is sampled as set within 840ms then the node ID is not valid and the
selection process starts again. In the event that the MYRECON bit is not found to be set, an 840ms timer has been
incorporated into the polling loop to account for this. The only time the MYRECON bit will not get set is when there is
a single node network. Once a valid node ID has been located then the transmitter is enabled and the initialization
process is completed.

SMSC DS – COM20051I Page 47 Rev. 03/27/2000

D

DEC NODE ID

N

Y

C

START

INIT NODE ID TO FEh

SET RECON COUNT

TO ZE R O

WAIT 52µS FO R T HE

CORE TO USE NEW ID

ISSUE CLEAR FLAGS

COMMAND

READ DIAGNOSTI C
STATUS REGISTER

INC. RECON COUNT

N

IS RECON

COUNT = 3?

Y

CLEAR THE

DIAGNOSTIC STATUS

REGISTER

WAIT 82.2µS MIN. FOR

RCVACT

INITI A L I ZE THE 840 mS

TIMER

IS RCVACT &

TOKEN RESET?

Y

Y

IS DUPID SET?

N

IS RCVACT &
TOKEN SET?

IS

MYRECON

SET?

Y

IS DUPID SET?

N

DID THE 840mS

TIMER EXPIRE?

N

Y

DID THE 840mS

TIMER EXPIRE?

Y

IS THE RECON

BIT SET?

Y

N

N

A

N

HAS A TOKEN

BEEN SEEN?

Y

B

FIGURE 16 – DUPLICATE ID DETECTION ALGORITHM

Y

N

SMSC DS – COM20051I Page 48 Rev. 03/27/2000

A

B

SET NODE ID = FFh

SET TXEN BIT

INIT IALIZE THE 840mS

TIMER

ISSUE A ENABLE T O TX

CMD TO YOUR OWN ID

VALUE

WAIT 82.2µ S FOR RCV

ACTIVITY

INIT. THE 840mS TIMER

READ THE DIAGNOSTIC

STATUS REGISTER

Y

IS RCVACT SET?

IS MYRECON SET?

DID THE 840mS

TIMER EXPIRE?

N

Y

N

N

Y

IS TA SET?

Y

N

IS TMA SET?

N

Y

N

DID THE 840mS

TIMER EXPIRE?

Y

GOOD ID VALUE

C

SET THE NODE ID TO FEh

D

FIGURE 16 - DUPLICATE ID DETECTION ALGORITHM (continued)

SMSC DS – COM20051I Page 49 Rev. 03/27/2000

DUPID Algorithm Exception Handling
There are two cases that require special processing that the normal Duplicate ID algorithm does not handle. The first

case is when the node is the first node to join the network. The second case is when the node is the second node to
join the network.

When a node is the first to join a network it will detect no receive activity and detect no tokens (i.e. RCVACT = 0 and
TOKEN = 0). If this is the case, a special routine is entered that monitors the MYRECON bits and constantly polls the
RCVACT, TOKEN, and DUPID bits. If any of the RCVACT, TOKEN, or DUPID bits gets set prior to the MYRECON
bit, then another node has entered the network just prior to this one. If the MYRECON bit sets and the RCVACT,
TOKEN, and DUPID bits remain reset then the node takes the ID value of FFh. Node ID FFh is reserved for the first
node to join the network.

The second case involves a node that is the second node to join the network. In this case the node will detect the
presence of tokens and receive activity (RCVACT & TOKEN = 1), and no duplicate IDs (DUPID = 0). The algorithm
will detect that RECON bit has set. This is because the only existing node on the network is undergoing constant
reconfigurations because it cannot pass tokens to another node. In order to prevent an endless loop from occuring
due to the RECON test failure, a count of RECON bit tests is kept. If three consecutive polls of the RECON bit are
positive it can be safely assumed that this node is the second node to join the network and takes the ID value of FEh.

Basic Transmit and Receive Service Routines

Transmit and Receive Service routines are often found together as part of an single interrupt service routine used to
service the ARCNET interrupt. The COM20051I provides a 1Kx8 RAM for buffering of both transmit and receive
packets and accommodates two types of packet formats: A short format which takes a maximum of 253 bytes of data
and a long format which takes a maximum of 507 bytes of data. Most industrial/embedded applications use the short
(256 byte) format, thus allowing the RAM to buffer four packets. The following examples will use a two packet buffer
for receptions and a two packet buffer for transmissions. Examples are shown for both Command and Non­Command Chaining systems. Figure 18 shows how a typical program will run.

Upon entry into the service routine all relevant 8051 registers are pushed onto the stack including the PSW and
DPTR. The state of the COM20051I address registers is also saved so that any procedures that were interrupted in
the process of accessing the COM20051I RAM are not corrupted. The COM20051I status register is then read and
stored in a variable. A series of case statements then check the bit settings and branch to the appropriate service
routine.

Service Priority
The ARCNET core of the COM20051I can generate a single interrupt from multiple sources including reception of

packets, transmission of packets, EXcessive NACK counts, RECONfigurations, and New Next ID generation. When
testing status bits to determine the source of the interrupt it is best to use the following priority:

1) RI - Receiver Inhibited (packet received)

2) TA - Transmitter Available (packet transmitted)

3) EXNACK - Excessive Number of NACK's to FBE's

4) RECON - either a full or mini RECON was detected

5) New Next ID - the node to which this node is passing the token to has not responded to the token pass and this

node has found a new id to pass the token to.

SMSC DS – COM20051I Page 50 Rev. 03/27/2000

START

IN IT IA L IZ E 8 0 5 1

PERIPHERALS

EXTERNAL RAM

TEST

PROGRAM

ARCNET

DECODE

ARCNET

RAM TEST

IN IT IA L IZ E

ARCNET

CORE

FIND

NODE ID

TX ISR

R X IS R

EXECUTE M AIN

TASK

END

FIGURE 17 – TYPICAL PROGRAM EXECUTION

Receive Interrupt

A basic Receive service routine is shown in the flowchart of Figure 19. The basic concept of the service routine is not
to process the packet data but to remove the packet from the ARCNET buffer and free the buffer for another
reception. This method will minimize the number of missed packets due to unavailable buffers.

The Receiver Interrupt Service Routine (RX_ISR) will handle one reception at a time and enable another receive
command upon exiting the service routine. Received data is stored in one of four buffers in external RAM. Several
variables are kept in order to facilitate processing. RAM_BUF_REG is a byte wide, bit mapped software register
which shows which external buffers are empty so that new data can be copied. RX_PAGE_REG is a byte wide
register in which the lower nibble contains the ARCNET buffer RAM page address in which the latest reception can
be found. The upper nibble contains the page address in which the next received is to be stored. T his register allows

SMSC DS – COM20051I Page 51 Rev. 03/27/2000

the service routine to track from which page the most current data can be found when back to back Enable to
Receive commands are issued. MIN_RX_PAGE and MAX_RX_PAGE are two constants that contain the page
address boundaries for the ARCNET memory. If it is desired to allow more RAM for receptions and less RAM for
transmissions then these constants must be changed accordingly.

Upon entering the RX_ISR the RX_PAGE_REG is read. The lower three bits are masked off and stored as the page
address. RAM_BUF_REG is then read and the first free buffer found is used to store the packet. Once the data has
been copied, the RAM_BUF_REG is checked to make sure that at least one buffer is empty before issuing another
receive command. If no buffers are available, the interrupt is cleared and a new command is issued, thus discarding
the received packet. This is done so that all receptions are handled. If it is found that receptions are being missed
then more buffering on the CPU side is necessary. If a free buffer exists, the second receive command page is then
checked to see if the maximum page address has been reached. If the page address is at its maximum value, then
the page address is set to the minimum page address, otherwise the page address is incremented to next page. The
Enable to Receive command string is then generated from the page address. RX_PAGE_REG is updated to make
the second receive command the first command and the new command will be the second command. The actual
command is then issued. The Clear Receive Interrupt command is then issued and the routine is exited.

Transmit Interrupt Servicing

The Transmit ISR (TX_ISR) services interrupts caused by the TA bit being set. The routine will first check the TMA
bit to see if the last transmission has been received error free. If TMA is good then another transmission occurs. If
TMA is bad then the last transmission is sent again and the ISR is exited. Limits can be imposed on the number of
re-tries before aborting.

Several variables are kept in order to simplify servicing of the Transmit interrupt. TX_PEND_REG is a byte wide, bit
mapped register that conveys information about whether a packet needs to be transmitted and where it is located in
the ARCNET RAM. The lower nibble of the register tells the ISR from which external RAM page the transmit data is
to come. Bits 5 and 6 are status bits telling the ISR how many transmit commands are in the Command Chaining
pipeline. To identify from which ARCNET buffer page the last transmission originated, a variable called LAST_TX is
used that contains the page number used in the last Enable to Transmit command. LAST_TX is used to re-transmit
data when a bad TMA bit is found.

Upon entering the ISR, the status register is read and the TMA bit is checked. If the TMA bit is not set, then the
action as described above is taken. If TMA is good then the interrupt is cleared and either bit 6 or bit 5 of the
TX_PEND_REG is cleared depending on the number of transmissions pending. The configuration register is then
read and the Command Chaining enable bit is checked. If Command Chaining is enabled, then bit 4 of TX_PEND is
set, otherwise it is cleared. A series of case statements are then executed which read the TX_PEND_REG and look
for a set bit. If a bit is set in the lower nibble, it signifies that the ARCNET RAM has been loaded with data and that
page is ready to be transmitted. Software should be written so that an external routine will copy transmit data into the
ARCNET RAM. This will speed up the ISR. Bits 5 and 6 are checked to see that the pipeline is not completely
filled. If they are both set, then the routine is exited. If either one of the bits is reset, then the pending bit in
TX_PEND_REG is reset, bit 5 or 6 is set, and an Enable to Transmit command is given to the page corresponding to
the bit position in TX_PEND_REG.

Up to two transmit commands can be given within the ISR at any given time. If no transmissions are pending, then
the routine is exited without any new transmissions.

SMSC DS – COM20051I Page 52 Rev. 03/27/2000

Initiating Transmissions

The Transmit Service routine can only execute after a transmission occurs. Therefore, initial transmit commands
must originate from routines external to the Transmit Interrupt service routine. The Transmit ISR can only
transmit packets if and only if packets are pending transmission upon entry into the routine. Normally this is not the
case, thus the programmer must take care to monitor and update the TX_PEND_REG for each transmission.

TX_PEND_REG

BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0

NOT

USED

TX CMD #2

ISSUED

TX CMD #1

ISSUED

NOT

USED

TX PEND.

IN

BUFFER 4

TX PEND.

IN

BUFFER 3

RAM_BUF_REG

BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0

NOT

USED

NOT

USED

NOT

USED

NOT

USED

NOT

USED

NOT

USED

RX_PAGE_REG

BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0

RECEIVE COMMAND PAGE #2 RECEIVE COMMAND PAGE #1

TX PEND.

IN

BUFFER 2

RX BUF2RX BUF

TX PEND.

IN

BUFFER 1

1

SMSC DS – COM20051I Page 53 Rev. 03/27/2000

RX_ISR

PUSH DPTR, PSW

SAVE A, HI_ADDR

& LO_ADDR

DISABLE INTRPTS

READ STATUS

REGISTER;

SAVE AS

STAT _REG

READ

RX_PAGE REG

MASK OUT ALL

BITS EXCEPT 3

LSB'S TO GET

ADDR OFFSET

SWAP LOWER &
UPPER NIBBLES

TO MAKE 2ND

RX_CMD THE IS T

READ

RAM_BUF_REG

A

FIGURE 18 – RECEIVE INTERRUPT SERVICE ROUTINE

SMSC DS – COM20051I Page 54 Rev. 03/27/2000

SET DPTR TO

START OF

BUF_1

SET BIT 0

IN RA M_B U F

SET DPTR TO

START OF

BUF_2

SET BIT 1

IN RA M_B U F

SET DPTR TO

START OF

BUF_3

SET BIT 2

IN RA M_B U F

SET RX_PAGE =

MIN_RX_PAGE

CLEAR

INT

EXIT

Y

RAM BUFFER

1 FRE E?

Y

RAM BUFFER

2 FREE?

Y

RAM BUFFER

3 FREE?

SET BIT 3

IN RA M_B U F

SET DPTR

TO S TART O F

COPY PACKET
TO EXTERNAL RAM

Y

MAX RX_PAGE

REACHED?

Y

EXTERNAL

BUFFERS

A

EXT.

EXT.

EXT.

BUF_4

ALL

FULL?

N

N

N

N

CURRENT
RX_PAGE

SET RX_PAGE =

N

TO RECEIVE

COMM AND

ISSUE ENABLE

FIGURE 18 – RECEIVE INTERRUPT SERVICE ROUTINE (continued)

SMSC DS – COM20051I Page 55 Rev. 03/27/2000

CLEAR INT. (MASK

OFF FO R

NON-CHAINING)

3

FIGURE 19 – TRANSMIT INTERRUPT SERVICE ROUTINE

START

READ ARCNET

STATUS REG.

YN

TMA SET?

CLEAR BIT 5

CLEAR TX INT

(MASK OFF FOR

NON-CHAINING)

READ

TX_PEND_REG

N

BIT 6 SET?

CLEAR BIT 6

STORE TX_PEND

Y

SET LAST_TX =
TX_OFFSET

SET BIT 6 AND

STORE TX_PEND

BIT 5 OF

TX_PEND

SET?

Y

SEND ENABLE TO

TRANSMIIT CMD.

N

CMD CHAIN

ENABLED?

EXIT

N

UNMASK

INT E R R U P T

SMSC DS – COM20051I Page 56 Rev. 03/27/2000

3

BIT 0 OF

TX_PEND

SET?

Y

N

BIT 4 OF

TX_PEND

SET?

Y

BITS 5&6

SET?

*CCHEN CHECK TO

BE IMPLEMENTED

N

Y

N

EXIT

SET OFFSET =

#013H

CALL

SEND_TX_CMD

CLEAR BIT 0 OF

TX_PEND

YN

BIT 4 OF

TX_PEND

SET?

EXIT

BIT 1 OF

TX_PEND

SET?

Y

N

SAME PROCEDURE AS

FOR BIT 0 BUT WITH

OFFSET = 33H

BIT 2 OF

TX_PEND

SET?

BIT 3 OF

TX_PEND

SET?

EXIT

N

N

Y

SAME PROCEDURE AS

FOR BIT 0 BUT WITH

OFFSET = 1BH

Y

SAME PROCEDURE AS

FOR BIT 0 BUT WITH

OFFSET = 3BH

FIGURE 19 – TRANSMIT INTERRUPT SERVICE ROUTINE (continued)

SMSC DS – COM20051I Page 57 Rev. 03/27/2000

CMD . CHAIN

SET?

UNMASK

TA

INTERRUPT

RET

TRANSMIT

REGISTER

READ STATUS

TA BI T

SET?

YN

CHK TTA BIT

(FOR CMD. CHAIN

ONL Y)

SET BIT 6 OF

TX_PEND

AND STORE

ENABLE TO

TX COMMA ND

N

SET OFFSET OF TX

PAGE

SET OFFSET OF TX

PAGE

SET OFFSET OF TX

PAGE

SET OFFSET OF TX

PAGE

FIGURE 20 – EXTERNAL TRANSMIT ROUTINE

READ TX_PEND_REG

BIT6 & BIT 5
SE T?

N

BIT 5
SET?

Y

SET BIT 6 OF

TX_PEND_REG

AND STORE

BIT 0 SET?

BIT 1 SET?

BIT 2 SET?

SMSC DS – COM20051I Page 58 Rev. 03/27/2000

N

PACKET COPY

SET HI_ADDR F OR

WRITE AT TX PAGE

WITH AU TOINC SET

& LO_ADDR = 00H

SET DPTR = START

OF TX BUFFER

READ DA TA,

INCREMENT

DPTR & SAVE DPTR

WRITE DATA TO

ARCNET RAM

READ

LO_ADDR_REG

LO_ADDR =

00H

RET

FIGURE 20 – EXTERNAL TRANSMIT ROUTINE (continued)

SMSC DS – COM20051I Page 59 Rev. 03/27/2000

USING ARCNET DIAGNOSTICS TO OPTIMIZE YOUR SYSTEM

EXcessive Negative ACKnowledgement (EXNACK) Loops

Under certain conditions (node failure for example), a particular node's receiver may not become available. This will
result in a NACK to FBE response each time the transmitter attempts to transmit a message. This can continue
endlessly without CPU intervention and degrade network performance. In order to handle such failures, the
COM20051I incorporates an EXNACK interrupt that will signal the CPU that it has made a specified number of Free
Buffer requests which have not been acknowledged. The limit is specified by the 4NACKS bit of the Setup register of
the ARCNET core. The default value is 128 re-tries but can be reduced to four by setting the 4NACKS bit. Once the
limit is reached, an interrupt is generated by the ARCNET core. Note that FBEs will be issued until the CPU takes
appropriate action by either aborting the transmission or continuing to retry.

The EXNACK (EXcessive Negative ACKnowledge) Interrupt handler is a simple procedure. For most applications,
the Interrupt handler will abort the transmission by issuing a Disable Transmitter command. A Clear Flags command
must be issued to release the interrupt and clear the EXNACK flag. Note that housekeeping should be done to
signify to external procedures that the transmission was aborted and is still not pending. Figure 22 shows a typical
flowchart of a EXNACK ISR.

Generating Network Maps

Most applications will require a table of existing nodes to be stored somewhere in the system for system
administration and maintainence (it is usually the master node). The DUPID, TENTID, and New Next ID features are
used to generate the Network

Map. The map is generated by synchronizing the node to the token rotation time using the D UPID feature. Whenever
a node is part of the network the DUPID bit will set when the node's receiver detects its own transmit activity in
response to receipt of the token. Reading the Diagnostic Status register resets the DUPID bit. Resetting the bit and
polling until the DUPID is set will synchronize the node the token rotation time. At this time a node ID value is
programmed into the TENTID register.

wait again for the Tent-ID bit to be set during one token rotation time.

values have been accounted for. There is one case in which the TENTID does not function. The TENTID detector
cannot detect the node ID to which the node is passing the token. This is because the ARCNET core operates in a
half-duplex mode thus blocking the recevier while it is transmitting the token. After compiling a node ID table using
the TENTID feature, the Next ID register can be read to find out the missing ID value.

In order to keep the map updated, the New Next ID feature can be used to indicate changes in the network. The New
Next ID feature will interrupt the processor any time the Next ID register is updated. This occurs whenever a node
drops (causing a mini-recon) or when a node joins the network (causing a full recon). Whenever the Next ID interrupt
is generated, a message should be sent to the Master node identifying a change in the network. The Master node
can then update the network map accordingly.

Network Mapping While Off-Line
The previous mapping algorithm described a fast method of building a network map while on-line (being an active

part of the network). Often it is desirable to build a map while off-line. A similar algorithm is used, but instead of using
the TENTID feature the DUPID feature is used and the new node IDs are programmed into the node ID register.
Since the node is not an active part of the network, there is no reliable method of initially timing the token rotation. In
order to find an existing ID to time the rotation, an 840ms timer is used. At the end of the 840ms, the DUPID is
checked. If it is set then the node ID value exists and can be used as a fake ID (remember the node is not on the
network). That first ID value remains in the node ID register and the above algorithm is used to determine the
remainder of the ID values.

Wait for the Token or the Recon bits to be set and clear Tent-ID bit. Then

This process should continure until all node ID

SMSC DS – COM20051I Page 60 Rev. 03/27/2000

END

EXNACK ISR

ISSUE DISABLE

TRANSMITTER

COMMAND

ISSUE CLEAR

FLAGS CMD.

FREE UP PAGE

IN TX_PEND REG

FIGURE 21 – EXCESSIVE NACK INTERRUPT SERVICE ROUTINE

SMSC DS – COM20051I Page 61 Rev. 03/27/2000

A

START

READ DIAG.

STATUS REG.

TO CLEAR

DUPID

POLL DUPID

BIT

TENTID

SET?

Y

LOG VALUE IN

NODE ID

TABLE

N

END OF

NODE IDS

N

DECREMENT &

PROG. TEN T ID

Y

N

DUPID

SET?

END

PROGRAM

TENTID

VALUE

POLL D UPID

BIT

N

DUPID

SET?

READ TENTID

BIT

FIGURE 22A – ONLINE NETWORK MAPPING ROUTINE

SMSC DS – COM20051I Page 62 Rev. 03/27/2000

START

WRITE ID FFH

INTO NODE ID

REGISTER

READ DIAG.

STATUS REG.

TO CLEAR DUPID

POLL DUPID

BIT

Y

DUPID

SET?

A

N

DECREMENT

840 MS TIMER

TIMER

EXPIRE?

DECREMENT
& PROGRAM

NODE ID

FIGURE 22B – ADDITIONAL CODE - OFFLINE NETWORK MAPPING ROUTINE

SMSC DS – COM20051I Page 63 Rev. 03/27/2000

CABLING THE COM20051I

The COM20051I supports several types of cables and cable interfaces in either bus or star topologies. The following
is a list of media transceivers, supported media, and supported topologies:

TRANSCEIVER

ALLOWED

TOPOLOGIES

MAX.
DIST.

MAX.

NODE

ISOLATION

VOLTAGE

SUPPORTED

MEDIA

HYC9068 Star only 2000ft. N/A <500V Coax
HYC9088 Bus or Star 1000ft.

(Coax)

400ft.

(TP)

8

10

<500V Coax

Twisted

Pair (TP)

RS-485 Bus 900ft. 32 15V

Twisted Pair

D.C.

Coupled

Isolated
RS-485

Bus 700ft. 12 2.5KV Twisted

Pair

Fiber-Optic Star 3Km N/A >2.5KV Fiber

Note: The above figures are for a 2.5 Mbps data rate.

Changing the data rate can significantly increase or

decrease the line length and node count.
For more information please refer to TN7-5 RS-485 Cabling Guidelines for the COM20020 Universal Local Area

Network Controller (ULANC) and Experimental Procedure for Verifcation of RS-485 Cabling Guidelines or Cabling
Alternatives for the COM20020, COM90C66, and COM90C165.

Compatability Considerations

Compatability with existing ARCNET installations is of importance to some designers. The COM20051I is compatible
with older ARCNET installations when it is operated in

dipulse mode at 2.5 Mbps using normal timeout values

. It
is suggested that the HYC9068 or HYC9088 transceivers be used to maintain compatability. The COM20051I is not
compatible with standard ARCNET systems when the device is used in backplane mode, when the reduced timeout
features are enabled, or at data rates other than 2.5 Mbps.

SMSC DS – COM20051I Page 64 Rev. 03/27/2000

USING THE COM20051I'S EMULATION MODE

The COM20051I is unique among special purpose 8051 based devices in that it can be used in conjunction with a
standard 16MHz 80C32 ICE. The COM20051I incorporates an Emulate mode that puts the internal microcontroller in
a high impedance state so an external processor (like an ICE) can access the internal ARCNET core through the
COM20051I's pins. The ICE should be connected in parallel to the COM20051I as shown in Figure 24. The ICE
should operate off of its internal oscillator.

EMULATOR POD

U3

31

C2

33pf

C3

33pf

P1.0 A15
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7

Y1
16Mhz

EA/VP

19

X1

18

X2

9

RESET

12

INT0

13

INT1

14

T0

15

T1

1

P1.0/T2

2

P1.1/T2x

3

P1.2

4

P1.3

5

P1.4

6

P1.5

7

P1.6

8

P1.7

8032

P0.0
P0.1
P0.2
P0.3
P0.4
P0.5
P0.6
P0.7

P2.0
P2.1
P2.2
P2.3
P2.4
P2.5
P2.6
P2.7

PSEN
ALE/P

RXD

RD

WR

TXD

AD0

39
38

AD1

37

AD2

36

AD3

35

AD4

34

AD5

33

AD6

32

AD7

21
22
23
24
25
26
27
28

17
16
29
30
11
10

A8
A9
A10
A11
A12
A13
A14

C23 33pf

1.5K

R19

C22

33pf

P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7

SMSC COM20051I

U1

35

EA/VP

21

X1

Y3

20

X2

10

RESET

14

INT0

15

INT1

16

T0

17

T1

2

P1.0

3

P1.1

4

P1.2

5

P1.3

6

P1.4

7

P1.5

8

P1.6

9

P1.7

COM20051I

PSEN
ALE/P

PULSE1
PULSE2

RXIN
TXEN

P0.0
P0.1
P0.2
P0.3
P0.4
P0.5
P0.6
P0.7

P2.0
P2.1
P2.2
P2.3
P2.4
P2.5
P2.6
P2.7

TXD
RXD

43

AD0

42

AD1

41

AD2

40

AD3

39

AD4

38

AD5

37

AD6

36

AD7

24
25
26
27
28
29
30
31

19

RD

18

WR

32
33
13
11
12
23
1
34

A8
A9
A10
A11
A12
A13
A14
A15

FIGURE 23 – CONNECTIONS FOR EMULATION MODE

C0
C1
C2
C3
C4
C5

SMSC DS – COM20051I Page 65 Rev. 03/27/2000

OPERATIONAL DESCRIPTION

MAXIMUM GUARANTEED RATINGS

*

Operating Temperature Range..............................................................................................................-40oC to +85oC

Storage Temperature Range...............................................................................................................-55oC to +150oC

Lead Temperature (soldering, 10 seconds) ........................................................................................................+325 oC

Positive Voltage on any pin, with respect to ground ...................................................................................... VCC+0.3V

Negative Voltage on any pin, with respect to ground...............................................................................................-0.3V

Maximum VCC .........................................................................................................................................................+7V

*Stresses above those listed may cause permanent damage to the device. This is a stress rating only and functional
operation of the device at these or any other condition above those indicated in the operational sections of this
specification is not implied.

NOTE:

When powering this device from laboratory or system power supplies, it is important that the Absolute
Maximum Ratings not be exceeded or device failure can result. Some power supplies exhibit voltage spikes or
"glitches" on their outputs when the AC power is switched on or off. In addition, voltage transients on the AC power
line may appear on the DC output. If this possibility exists it is suggested that a clamp circuit be used.

DC ELECTRICAL CHARACTERISTICS

V

5.0V ± 10%

CC=

COM20051: (TA=0# C to +70# C, COM20051I: (TA =-40# C to +85# C

PARAMETER

Low Input Voltage 1 (All
inputs except A2,
XTAL1, nRESET, nRD,
nWR, and RXIN)
High Input Voltage 1
(All Inputs except A2,

SYMBOL

V

IL1

V

IH1

MIN

2.0

TYP

MAX UNIT COMMENT

0.8 V

V

TTL Levels

TTL Levels

XTAL1, nRESET, nRD,
nWR, and RXIN)
Low Input Voltage
(XTAL1)

V

1.0 V TTL Clock Input

IL2

High Input Voltage
(XTAL1)

Low to High Threshold
Input Voltage
(nRESET,nRD, nWR,
RXIN)
High to Low Threshold
Input Voltage
(nRESET, nRD,nWR,
RXIN)
Low Output Voltage
(Ports 1, 2 and 3,
ALE, PSEN)

Low Output Voltage
(Port 0)

High Output Voltage
(Ports 1, 2 and 3,
ALE, PSEN)

V

4.0 V TTL Clock Input

IH2

1.8 V Schmitt Trigger @ 5V

V

ILH

1.2 V Schmitt Trigger @ 5V

V

IHL

VOL 0.3

0.45

1.0

V

0.3

OL1

0.45

1.0

VOH VCC-0.3

V
VCC-0.7
V

-1.5

CC

V

IOL = 100 " A

V

IOL = 1.6 mA

V

IOL = 3.5 mA

V

IOL = 200 " A

V

IOL = 3.2 mA

V

IOL = 7.0 mA
IOH = -10 "A

V

IOH = -30 "A

V

IOH = -60 "A

SMSC DS – COM20051I Page 66 Rev. 03/27/2000

PARAMETER

High Output Voltage
(Port 0 in External Bus
Mode)

Low Output Voltage
(nPULSE1 in Normal

SYMBOL

VCC-0.3

V

OH1

MIN

TYP

MAX UNIT COMMENT

V
VCC-0.7
VCC-1.5

0.4 V I

V

OL3

V
V

Mode, nPULSE2,
TXEN)
High Output Voltage
(nPULSE1 in Normal

2.4 V I

V

OH3

Mode, nPULSE2,
TXEN)
Low Output Voltage
(nPULSE1 in

0.5 V I

V

OL4

Backplane
Mode)
Input Leakage Current ILI 0.02 ±10

RST Pulldown Resistor RRST 45.5K

"

$

Pin Capacitance CIO 10 pF
Power Supply Current ICC 50 mA
Output Capacitance

(nPULSE1 in

400 pF

C

OUT2

Backplane
Mode)

IOH = -200 "A
IOH = -3.2 mA
IOH = -3.0 mA

SINK

SOURCE

SINK

0<VIN<VCC

A

= 4mA

= 2mA

= 48mA

SMSC DS – COM20051I Page 67 Rev. 03/27/2000

TIMING DIAGRAMS

AC CHARACTERISTICS

100 pF Loading for ALE, PORT0, and nPSEN; 16 MHz Processor Clock

(0°C - 70°C, 5.0V ± 10%)

t7

t1

t3

t8

ALE

nPSEN

t2

t5

t4

t9

t6

PORT 0

A0 - A7 INSTR. IN

PORT 2

A8 - A15

t1
t2
t3
t4
t5
t6
t7
t8

t9
t10
t11

Parameter min typ max units

ALE Pulse Width
Address Valid to ALE Low
ALE Low to nP SEN Low
nPSEN Low to Valid Instruction Input
nPSEN Low to Address Float
Add ress to Valid Instructio n Input
Address Hold after ALE
ALE Low to Valid Instruction Input
nPSEN Pulse Width
Input Instruction Hold after nPSEN
Input Instruction Float after nPSEN

85

7

22.6

27

142.5
0

FIGURE 24 – EXTERNAL PROGRAM MEMORY READ CYCLE

t10

t11

82.5
10

207.5

150

37.5

nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS

SMSC DS – COM20051I Page 68 Rev. 03/27/2000

A

LE

nPSEN

nRD

PORT 0

PORT 2

t1
t2
t3
t4
t5
t6
t7
t8

t9
t10
t11
t12
t13

t1

t3

t6

t13

t12

A0 - A7

Parameter min typ max units

ALE Pulse Width
AL E L o w to Valid Data In
ALE Low to nRD or nWR Low
nRD or nWR High to ALE High
nRD Pulse Width
Address Valid to ALE Low
Addr ess to Va l id D ata In
nRD Low to Address Float
nRD Low to Va lid D ata In
Da ta Hold after n RD
Da ta Floa t a fter nRD
Address Valid to nRD or nWR Low
Address Hold after ALE Low

t2

t8

t7

A8 - A15 FROM DPH

FIGURE 25 – EXTERNAL DATA MEMORY READ CYCLE

t4

t5

t9

t11

t10

DATA IN

85

137.5

22.5
275

7

0

120

27

350

238.5

102.5

397.5
0

147.5

55

nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS

SMSC DS – COM20051I Page 69 Rev. 03/27/2000

t1 t2

t5

t8

t3

ALE

nPSEN

nWR

t9

t7

t4

t6

PORT 0

A0 - A7 DATA OUT

t10

PORT 2

A8 - A15 FROM DPH

Parameter min typ max units

t1

ALE Pulse Width

t2

ALE Low to nRD or nWR Low

t3

nRD or nWR High to ALE High

t4

WR Pulse Width

t5

Address Valid to ALE Low

t6

Data Ho ld a fter n WR

t7

Address Valid to nRD or nWR Low

t8

Address Hold after ALE Low

t9

Data Va lid to nWR Trans ition

t10

Data Va lid to nWR Hig h

85

137.5

22.5
275

7

12.5
120

27

2

287.5

238.5

102.5

nS
nS
nS
nS
nS
nS
nS
nS
nS
nS

FIGURE 26 – EXTERNAL DATA MEMORY WRITE CYCLE

SMSC DS – COM20051I Page 70 Rev. 03/27/2000

TXEN

t4

t1

nPUL SE 1

t3

nPUL SE 2

t6

RXIN

Parameter min max units

t1

PULSE1, PULSE2 Pulse Width

t2

PULSE1, PULSE2 Period

t3

PULSE1, PULSE2 Overlap

t4

TXEN Low to PULSE 1 Low **

t5

Beginning of Last Bit Time to TXEN High**

t6

RXIN Pulse Width

t7

RX IN Period

* t1, t6 = .25 x ( 1 ) for data rates other than 2.5 Mpbs.
d ata ra te

* t2,t7 = 8 x (crystal period) for clock frequencies other than 20
MHz.

This period applies to data of two consecutive one's.
** t4: For clock frequencies other than 20 MHz, t4 = 18 x (crystal period) ± 50 nsec.
** t5: For clock frequencies other than 20 MHz, t5 = 6 x (crystal period) ± 50 nsec.

Note: Clock frequency for 5 Mbps is 40 MHz.

t2

t1

t7

t2

typ

100*
400*

-10
850
250

10

100*
400*

t5

LAST BIT

(400 nS BIT TIME)

nS
nS

0

+10

nS

950

nS

350

nS
nS
nS

FIGURE 27 – NORMAL MODE TRANSMIT OR RECEIVE TIMING

(These signals are to and from the Hybrid)

SMSC DS – COM20051I Page 71 Rev. 03/27/2000

TXEN

nPULSE

nPULSE t5
(Internal

t1

t9

t4

t2

t3

t6

t7

LAST

(400 nS BIT

t8

RXIN

t10

t11

-25

650

10

0

0

Paramete min

nPULSE2 High to TXEN

t1

nPULSE1 Pulse

t2

nPULSE1

t3
t4

nPULSE2 Low to nPULSE1
nPULSE2 High

t5

nPULSE2 Low

t6
t7

nPULSE2
nPULSE2 High to TXEN

t8

(first rising edge on nPULSE 2 after Last Bit

t9

TXEN Low to First nPULSE1

t10

RXIN Pulse

t11

RXIN

* t5,t6 = 2 x (crystal period) for clock frequencies other than
* t2,t7,t10 = 4 x (crystal period) f or clock frequencies other than
* t3,t11 = 8 x (crystal period) for clock frequencies other than

** t9: For clock frequenci es other than 20 MHz, t9 = 14 x (clock period) ±

This period applies to data of two cons ecutive

Note: Clock frequency f or 5 Mbps i s 40

typ max units

50
200*
400*

25
100*
100*
200*

50

750
200*
400*

nS
nS
nS
nS
nS
nS
nS
nS

nS
nS
nS

FIGURE 28 – BACKPLANE MODE TRANSMIT OR RECEIVE TIMING

(These signals are to and from the differential driver or the cable)

SMSC DS – COM20051I Page 72 Rev. 03/27/2000

t1

XTAL1

Parameter

t1

Input Cloc k Hig h T i me
t2 Input Clock Low Time
t3

Input Cloc k Period
t4 Input Cloc k Frequency

FIGURE 29 – TTL INPUT TIMING ON XTAL1 PIN

nRESET IN

t1

nINTR

Parameter

t1

nRESET IN Pulse Width

t2 nINTR High to Next nINTR Low nS

Note: For 5 Mbps operation, t2 is 100 nS.

FIGURE 30 – RESET AND INTERRUPT TIMING

t2

t2

t3

min

10
10
25
10

min

3.2

*200

typ

typ

max units

nS
nS

125

nS

40 MHz

max units

µS

SMSC DS – COM20051I Page 73 Rev. 03/27/2000

INDEX CORNER

G x 45°

D
D1

SEE DETAIL A

F x 45°

E1E

-E-

E3/D3 E2/D2

DIM

A

A1

B
B1
B2

C

D/E
D1/E1
D2/E2
D3/E3

e
F

G

J

R

44 LEAD

.160-.188
.090-.120
.013-.021
.026-.032
.025 MIN
.020-.045
.685-.695
.650-.656
.600-.630
.500 REF

.050 B SC

.042-.060
.042-.048
.000-.028
.025-.045

-D-

J x 45°, 3 places

DETAIL A

-C-

B

SEATING
PLANE

Notes:

1. Dimensions are in inches.

2. Coplanarity is .004" maximum.

3. Tolerance on the position of the leads is .007" maximum.

4. Package body dimensions D1 and E1 do not include the mold protrusion.
Maximum mold protrusion is .010".

.007 E D

B1

B2

e

C

A1

A .004

R

-C-

SEATING PLANE

FIGURE 31 – 44 PIN PLCC PACKAGE DIMENSIONS

SMSC DS – COM20051I Page 74 Rev. 03/27/2000