Philips SC28C94A1A, SC28C94A1N, SC28C94C1A, SC28C94C1N Datasheet

 

SC28C94

Quad universal asynchronous receiver/transmitter (QUART)

Product specification Replaces SC26C94 of 1995 May 01 and SC68C94 of 1995 May 01 IC17 Data Handbook

1998 Aug 19

INTEGRATED CIRCUITS

Philips Semiconductors Product specification

SC28C94Quad universal asynchronous receiver/transmitter (QUART)

1998 Aug 19 853–21 11 19896

DESCRIPTION

The 28C94 quad universal asynchronous receiver/transmitter (QUART) combines four enhanced Philips Semiconductors industry-standard UARTs with an innovative interrupt scheme that can vastly minimize host processor overhead. It is implemented using Philips Semiconductors’ high-speed CMOS process that combines small die size and cost with low power consumption.

The operating speed of each receiver and transmitter can be selected independently at one of eighteen fixed baud rates, a 16X clock derived from a programmable counter/timer, or an external 1X or 16X clock. The baud rate generator and counter/timer can operate directly from a crystal or from external clock inputs. The ability to independently program the operating speed of the receiver and transmitter make the QUART particularly attractive for dual-speed channel applications such as clustered terminal systems.

Each receiver is buffered with eight character FIFOs (first-in-first-out memories) and one shift register to minimize the potential for receiver overrun and to reduce interrupt overhead in interrupt driven systems. In addition, a handshaking capability is provided to disable a remote UART transmitter when the receiver buffer is full. (RTS control)

The 28C94 provides a power-down mode in which the oscillator is stopped and the register contents are stored. This results in reduced power consumption on the order of several magnitudes. The QUART is fully TTL compatible and operates from a single +5V power supply.

FEA TURES

•New low overhead interrupt control

•All commercial parts meet industrial timing and temperature

parameters

•Four Philips Semiconductors industry-standard UARTs

•Eight byte receive FIFO and eight byte transmit FIFO for each

UART

•Programmable data format:

– 5 to 8 data bits plus parity – Odd, even, no parity or force parity – 1, 1.5 or 2 stop bits programmable in 1/16-bit increments

•Baud rate for the receiver and transmitter selectable from:

– 23 fixed rates: 50 to 230.4K baud Non-standard rates to 1.0M

baud

– User-defined rates from the programmable counter/timer

associated with each of two blocks

– External 1x or 16x clock

•Parity, framing, and overrun error detection

•False start bit detection

•Line break detection and generation

PIN CONFIGURATIONS

A5:0 CEN RDN

WRN

IACKN

RESET

X1/CLK

RDa-d

D7-0

DACKN

RQN

I/O0a–d I/O1a–d I/O2a–d I/O3a–d TDa-d

SD00158

Figure 1. Pin Configuration

•Programmable channel mode

– Normal (full-duplex), automatic echo, local loop back, remote

loopback

•Programmable interrupt priorities

•Identification of highest priority interrupt

•Global interrupt register set provides data from interrupting

channel

•Vectored interrupts with programmable vector format

•IACKN and programmable DTACKN signals

•Built-in baud rate generator with choice of 18 rates

•Four I/O pins per UART for modem controls, clocks, etc.

•Power down mode

•High-speed CMOS technology

•52-pin PLCC and 48-pin DIP

•Commercial and industrial temperature ranges available

•On-chip crystal oscillator

•TTL compatible

•Single +5V power supply with low power mode

•Two multifunction programmable 16-bit counter/timers

•1MHz 16x mode operation

•10ns data bus release time

•“Watch Dog” timer for each receiver

ORDERING INFORMATION

INDUSTRIAL

PACKAGES

VCC = +5V +10%,

TA = –40oC to +85oC

DWG #

48-Pin Plastic Dual In-Line Package (DIP) SC28C94A1N SOT240-1 52-Pin Plastic Leaded Chip Carrier (PLCC) Package SC28C94A1A SOT238-3

Philips Semiconductors Product specification

SC28C94Quad universal asynchronous receiver/transmitter (QUART)

1998 Aug 19

PIN CONFIGURATIONS

46 45

48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 32 31 30 29

1 2 3 4 5 6 7 8

9 10 11 12 13 14 15 16 17 18 19 20

28 27 26 25

21 22 23 24

X1/CLK

TXDD RXDD

IRQN

A5 A4 A3 A2 A1 A0

WRN

CEN RDN

DACKN

IACKN

TXDB RXDB

D7 D6 D5 D4 D3

RXDA

TXDA

I/O2B

I/O1B

I/O0B

I/O2A

I/O1A

I/O0A

I/O0C

I/O1C

I/O2C

I/O0D

I/O1D

I/O2D

RXDC

TXDC

RESET

CEN

RDN

DACKN

IACKN

TXDB

RXDB

D7 D6

D5 D4

I/O3B

D0 RXDA TXDA

I/O2B

I/O1B

I/O0B

I/O3A

I/O2A

I/O1A

A2A3A4

A5 IRQN RXDD

TXDD

X1/CLK X2

TXDC RXDC

I/O2D I/O1D

I/O0D

I/O3C

I/O2C

I/O1C

I/O0C

RESET

I/O3D

I/O0A

WRN

48-Pin Dual-In-Line Package

52-Pin PLCC Package

1234

567

9 10 11

18 19

21 22 23 24 31 32 33

505152

25 26 27 28 29 30

12 13 14 15 16 17

474849

VSSV

SD00159

Figure 2. Pin Configurations (cont.)

ABSOLUTE MAXIMUM RATINGS

1, 2

SYMBOL

PARAMETER RATING UNIT

Operating ambient temperature range

Note 4

C

STG

Storage temperature range –65 to +150

C

Voltage from V

to GND

–0.5 to +7.0 V

Voltage from any pin to ground

–0.5 to V

+0.5 V

Package power dissipation (PLCC52) 2.8 W Package power dissipation (PDIP48) 3.1 W Derating factor above 25C (PLCC52)

mW/C

Derating factor above 25C (PDIP48)

mW/C

NOTES:

1. Stresses above those listed under Absolute Maximum Ratings 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 operation section of this specification is not implied.

2. For operating at elevated temperatures, the device must be derated based on +150°C maximum junction temperature.

3. This product includes circuitry specifically designed for the protection of its internal devices from damaging effects of excessive static charge. Nonetheless, it is suggested that conventional precautions be taken to avoid applying any voltages larger than the rated maxima.

4. Parameters are valid over specified temperature range. See ordering information table for applicable temperature range and operating supply range.

Philips Semiconductors Product specification

SC28C94Quad universal asynchronous receiver/transmitter (QUART)

1998 Aug 19

BLOCK DIAGRAM

D0–D7

RDN

WRN

CEN

A0–A5

RESET

X1/CLK

BUS BUFFER

OPERATION CONTROL

ADDRESS

DECODE

R/W CONTROL

TIMING

CHANNEL A

MR 0, 1, 2

CR SR

INPUT PORT

OUTPUT PORT

OPCR

CSR Rx CSR Tx

CRYSTAL

OSCILLATOR

POWER UP-DOWN

LOGIC

SAME AS

DUART AB

8 BYTE TRANSMIT

FIFO

TRANSMIT SHIFT

8 BYTE

RECEIVE FIFO

RECEIVE SHIFT

CHANGE-OF-

STATE

DETECTORS (4)

IPCR

ACR

FUNCTION SELECT

LOGIC

CHANNEL B (AS ABOVE)

TIMING CLOCK

SELECTORS

COUNTER/

TIMER

ACR CTUR CTLR

INTERRUPT CONTROL

IMR ISR

INTERNAL DATA

BUS

TxDA

RxDA

TxDB RxDB

1:0

TIMING

CONTROL

DACKN

DUART CD

TXDC TXDD

RXDC

RXDD

I/O[3:0]C I/O[3:0]D

1:0

I/O[3:0]B I/O[3:0]A

•V

SS1

•V

SS2

•V

SS3

•V

SS4

BAUD RATE

GENERATOR

DUART AB

LOGIC

GLOBAL

REGISTERS

IRQN

IACKN

INTERRUPT ARBITRATION

DUART

COMMON

4 4

÷ 2

SD00160

Figure 3. Block Diagram

Philips Semiconductors Product specification

SC28C94Quad universal asynchronous receiver/transmitter (QUART)

1998 Aug 19

PIN DESCRIPTION

MNEMONIC TYPE NAME AND FUNCTION

CEN I Chip Select: Active low input that, in conjunction with RDN or WRN, indicates that the host MPU is trying to

access a QUART register. CEN must be inactive when IACKN is asserted. A5:0 I Address Lines: These inputs select a 28C94 register to be read or written by the host MPU. D7:0 I/O 8-bit Bidirectional Data Bus: Used by the host MPU to read and write 28C94 registers. RDN I Read Strobe: Active low input. When this line is asserted simultaneously with CEN, the 28C94 places the

contents of the register selected by A5:0 on the D7:0 lines. WRN I Write Strobe: Active low input. When this line is asserted simultaneously with CEN, the 28C94 writes the data

on D7:0 into the register selected by A5:0. DACKN O Data ACKnowledge: Active low, open-drain output to the host MPU, which is asserted subsequent to a read or

write operation. For a read operation, assertion of DACKN indicates that register data is valid on D7:0. For a

write operation, it indicates that the data on D7:0 has been captured into the indicated register. This signal

corresponds to READYN on 80x86 processors and DTACKN on 680x0 processors. IRQN O Interrupt Request: This active low open-drain output to the host MPU indicating that one or more of the

enabled UART interrupt sources has reached an interrupt value which exceeds that pre-programmed by host

software. The IRQN can be used directly as a 680x0 processor input; it must be inverted for use as an 80x86

interrupt input. This signal requires an external pull-up resistor. IACKN I Interrupt ACKnowledge: Active low input indicates host MPU is acknowledging an interrupt requested. The

28C94 responds by placing an interrupt vector or interrupt vector modified on D7-D0 and asserting DACKN. This

signal updates the CIR register in the interrupt logic. CEN must be high during this cycle. TDa-d O Transmit Data: Serial outputs from the four UARTs. RDa-d I Receive Data: Serial inputs to the four UARTs/ I/O0a-d I/O Input/Output 0: A multi-use input or output signal for each UART. These pins can be used as general purpose

inputs, Clear to Send inputs, 1X or 16X Transmit Clock outputs or general purpose outputs. Change-of-state

detection is provided for these pins. I/O pins have approximately 1.5 Mohm pull–up device. I/O1a-d I/O Input/Output 1: A multi-use input or output signal for each UART. These pins can be used as general purpose

or 1X or 16X transmit clock inputs, or general purpose 1X or 16X receive clock outputs. Change-of-state

detection is provided for these pins. In addition, I/O1a and I/O1c can be used as Counter/Timer inputs and I/O1b

and I/O1d can be used as Counter/Timer outputs. I/O pins have approximately 1.5 Mohm pull–up device. I/O2a-d I/O Input/Output 2: A multi-use input or output signal for each UART. These pins can be used as general purpose

inputs, 1X or 16X receive clock inputs, general purpose outputs, RTS output or 1X or 16X receive clock outputs.

I/O pins have approximately 1.5 Mohm pull–up device. I/O3a-d I/O Input/Output 3: A multi-use input or output signal for each UART. These pins can be used as general purpose

inputs, 1X or 16X transmit clock inputs, general purpose outputs, or 1X or 16X transmit clock outputs. I/O pins

have approximately 1.5 Mohm pull–up device. RESET I Master Reset: Active high reset for the 28C94 logic. Must be asserted at power-up, may be asserted at other

times that the system is to be reset and restarted. OSC set to divide by 1, MR pointer set to 1, DACKN enabled,

I/O pins to input. Registers reset: MR0, OPR, CIR. IRQN, DTACKN, IVR Interrupt Vector, Power Down, Test

registers, FIFO pointers, Baud rate generator, Error Status, Watch Dog T imers, Change of State detectors,

counter/timer to timer, Transmitter and Receiver controllers and all interrupt bits. If reset pin is not used, then

first chip access should be to celar ‘power-down’ mode. X1/CLK I Crystal 1 or Communication Clock: This pin is normally connected to one side of a 3.6864MHz or a

7.3728MHz crystal, or can be connected to an external clock up to 8MHz.

X2 O Crystal 2: If a crystal is used, this pin should be connected to its other terminal. If an external clock is applied to

X1, this pin should be left unconnected. VCC, V

Power and grounds: respectively.

COUNTER/TIMER

I/O PORT CONTROL

UARTS A/B

INTERRUPT CONTROL

BLOCK B

UARTS C/D

I/O CONTROL

I/O PORT CONTROL

A0-A5

D (7:0)

DTACKN

IACKN

BAUD RATE

GENERATOR

BUS

INTERFACE

BLOCK A

SD00161

Figure 4. Channel Architecture

Philips Semiconductors Product specification

SC28C94Quad universal asynchronous receiver/transmitter (QUART)

1998 Aug 19

Table 1. QUART Registers

A5:0

READ (RDN = Low) WRITE (WRN = Low)

000000 Mode Register a (MR0a, MR1a, MR2a) Mode Register a (MR0a, MR1a, MR2a) 000001 Status Register a (SRa) Clock Select Register a (CSRa) 000010 Reserved Command Register a (CRa)

00001 1 Receive Holding Register a (RxFIFOa) Transmit Holding Register a (TxFIFOa) 000100 Input Port Change Reg ab (IPCRab) Auxiliary Control Reg ab (ACRab) 000101 Interrupt Status Reg ab (ISRab) Interrupt Mask Reg ab (IMRab)

000110 Counter/T imer Upper ab (CTUab) Counter/Timer Upper Reg ab (CTURab)

000111 Counter/Timer Lower ab (CTLab) Counter/Timer Lower Reg ab (CTLRab) 001000 Mode Register b (MR0b, MR1b, MR2b) Mode Register b (MR0b, MR1b, MR2b) 001001 Status Register b (SRb) Clock Select Register b (CSRb) 001010 Reserved Command Register b (CRb)

00101 1 Receive Holding Register b (RxFIFOb) Transmit Holding Register b (TxFIFOb)

001100 Output Port Register ab (OPRab) Output Port Register ab (OPRab)

001101 Input Port Register ab (IPRab) I/OPCRa (I/O Port Control Reg a)

001110 Start Counter ab I/OPCRb (I/O Port Control Reg b)

001111 Stop Counter ab Reserved 010000 Mode Register c (MR0c, MR1c, MR2c) Mode Register c (MR0c, MR1c, MR2c) 010001 Status Register c (SRc) Clock Select Register c (CSRc) 010010 Reserved Command Register c (CRc)

01001 1 Receive Holding Register c (RxFIFOc) Transmit Holding Register c (TxFIFOc) 010100 Input Port Change Reg cd (IPCRcd) Auxiliary Control Reg cd (ACRcd) 010101 Interrupt Status Reg cd (ISRcd) Interrupt Mask Reg cd (IMRcd)

010110 Counter/T imer Upper cd (CTUcd) Counter/Timer Upper Reg cd (CTURcd)

010111 Counter/Timer Lower cd (CTLcd) Counter/Timer Lower Reg cd (CTLRcd)

011000 Mode Register d (MR0d, MR1d, MR2d) Mode Register d (MR0d, MR1d, MR2d)

011001 Status Register d (SRd) Clock Select Register d (CSRd)

011010 Reserved Command Register d (CRd)

011011 Receive Holding Register d (RxFIFOd) Transmit Holding Register d (TxFIFOd)

011100 Output Port Register cd (OPRcd) Output Port Register cd (OPRcd)

011101 Input Port Register cd (IPRcd) I/OPCRc (I/O Port Control Reg c)

011110 Start Counter cd I/OPCRd (I/O Port Control Reg d)

011111 Stop Counter cd Reserved 100000 Bidding Control Register a (BCRa) Bidding Control Register a (BCRa) 100001 Bidding Control Register b (BCRb) Bidding Control Register b (BCRb) 100010 Bidding Control Register c (BCRc) Bidding Control Register c (BCRc)

10001 1 Bidding Control Register d (BCRd) Bidding Control Register d (BCRd) 100100 Reserved Power Down 100101 Reserved Power Up

100110 Reserved Disable DACKN

100111 Reserved Enable DACKN 101000 Current Interrupt Register (CIR) Reserved 101001 Global Interrupting Channel Reg (GICR) Interrupt Vector Register (IVR) 101010 Global Int Byte Count Reg (GIBCR) Update CIR

10101 1 Global Receive Holding Reg (GRxFIFO) Global Transmit Holding Reg (GTxFIFO)

101100 Interrupt Control Register (ICR) Interrupt Control Register (ICR)

101101 Reserved BRG Rate. 00 = low; 01 = high

101110 Reserved Set X1/CLK divide by two2 (use when X1 is  4 Mhz)

101111 Reserved Set X1/CLK Normal

110000–111000 Reserved Reserved

111001 Reserved Test Mode

111010–111111 Reserved Reserved

Philips Semiconductors Product specification

SC28C94Quad universal asynchronous receiver/transmitter (QUART)

1998 Aug 19

NOTES:

1. Registers not explicitly reset by hardware reset power up randomly .

2. In X1/CLK divide by 2 all circuits receive the divided clock except the BRG and change-of-state detectors.

FUNCTIONAL BLOCKS

The QUART is composed of four Philips Semiconductors industry–standard UARTs, each having a separate transmit and receive channel.

The Basic UART cells in the QUART are configured with 8-byte Receive FIFOs and 8-byte Transmit FIFOs. Hardware supports interrupt priority arbitration based on the number of bytes available in the transmit and receive FIFOs, counter/timers, change of state detectors, break detect or receiver error. Attempts to push a full FIFO or pop an empty FIFO do not affect the count.

Baud Rate Generator

The baud rate generator used in the QUART is the same as that used in other Philips Semiconductors industry standard UARTs. It provides 18 basic Baud rates from 50 baud to 38,400 baud. It has been enhanced to provide to provide other baud rates up to 230,400 baud based on a 3.6364MHz clock; with an 8.0MHz clock rates to 500K baud. Other rates are available by setting the BRG rate to high at address 2D hex or setting Test 1 on at address 39 hex. See Table 6. These two modes are controlled by writing 00 or 01 to the addresses above. They are both set to 00 on reset. External Rx and Tx clocks yield rates to 1MHz in the 16X mode.

BLOCK DIAGRAM

As shown in the block diagram, the QUART consists of: data bus buffer , interrupt control, operation control, timing, and four receiver and transmitter channels. The four channels are divided into two different blocks, each block independent of the other.

Channel Blocks

There are two blocks (Block Diagram), each containing two sets of receiver/transmitters. In the following discussion, the description applies to Block A which contains channels a and b. However, the same information applies to all channel blocks.

Data Bus Buffer

The data bus buffer provides the interface between the external and internal data buses. It is controlled by the operation control block to

allow read and write operations to take place between the controlling

CPU and the QUART.

Operation Control

The operation control logic receives operation commands from the CPU and generates appropriate signals to internal sections to control device operation. It contains address decoding and read and write circuits to permit communications with the microprocessor via the data bus buffer. The functions performed by the CPU read and write operations are shown in Table 1.

Mode registers (MR) 0, 1 and 2 are accessed via an address counter. This counter is set to one (1) by reset or a command 1x to the Command Register for compatibility with other Philips Semiconductors software. It is set to 0 via a command Bx to the Command Register (CR). The address counter is incremented with each access to the MR until it reaches 2 at which time it remains at

2. All subsequent accesses to the MR will be to MR2 until the MR

counter is changed by a reset or an MR counter command. The Mode Registers control the basic configuration of the UART

channels. There is one for each UART. (Transmitter/receiver pair)

Timing Circuits

The timing block consists of a crystal oscillator, a baud rate generator, power up/down logic and a divide by 2 selector. Closely associated with the timing block are two 16-bit counter/timers; one for each DUART.

Oscillator

The crystal oscillator operates directly from a 3.6864MHz crystal connected across the X1/CLK and X2 inputs with a minimum of external components. If an external clock of the appropriate

frequency is available, it may be connected to X1/CLK. If an external

clock is used instead of a crystal, X1 must be driven and X2 left floating as shown in Figure 15. The clock serves as the basic timing reference for the baud rate generator (BRG), the counter/timer, and

other internal circuits. A clock frequency, within the limits specified in

the electrical specifications, must be supplied even if the internal BRG is not used.

The X1 pin always supplies the clock for the baud rate generator. The X1 pin also has a feature such that it may be divided by 2. The divide by two mode must always be used whenever the X1 pin is above 4MHz. The baud rate generator supplies the standard rates when X1 is at 3.6864MHz. In the divide by 2 mode, all circuits receive the divide by two clock except baud rate generator and I/O pin change-of-state detectors. The use of a 7.3738MHz X1 clock doubles standard baud rates.

Baud Rate Generator

The baud rate generator operates from the oscillator or external clock input and is capable of generating 18 commonly used data communications baud rates ranging from 50 to 38.4K baud. The eighteen BRG rates are grouped in two groups. Eight of the 18 are

common to each group. The group selection is controlled by ACR[7].

See the Baud Rate Table 6. The clock outputs from the BRG are at 16X the actual baud rate. The counter/timer can be used as a timer

to produce a 16X clock for any other baud rate by counting down the

crystal clock or an external clock. The clock selectors allow the independent selection, by the receiver and transmitter, of any of these baud rates or an external timing signal.

Counter/Timer

The counter timer is a 16-bit programmable divider that operates in one of three modes: counter, timer, time out. In the timer mode it generates a square wave. In the counter mode it generates a time delay. In the time out mode it monitors the time between received characters. The C/T uses the numbers loaded into the Counter/Timer Lower Register (CTLR) and the Counter/T imer Upper Register (CTUR) as its divisor.

There are two counter/timers in the QUART; one for each block. The counter/timer clock source and mode of operation (counter or timer) is selected by the Auxiliary Control Register bits 6 to 4 (ACR[6:4]). The output of the counter/timer may be used for a baud rate and/or may be output to the I/O pins for some external function that may be totally unrelated to data transmission. The counter/timer also sets the counter/timer ready bit in the Interrupt Status Register (ISR) when its output transitions from 1 to 0.

A register read address (see Table 1) is reserved to issue a start counter/timer command and a second register read address is reserved to issue a stop command. The value of D(7:0) is ignored. The STAR T command always loads the contents of CTUR, CTLR to the counting registers. The STOP command always resets the ISR(3) bit in the interrupt status register.

Philips Semiconductors Product specification

SC28C94Quad universal asynchronous receiver/transmitter (QUART)

1998 Aug 19

Timer Mode

In the timer mode a symmetrical square wave is generated whose half period

is equal in time to division of the selected counter/timer clock frequency by the 16-bit number loaded in the CTLR CTUR. Thus, the frequency of the counter/timer output will be equal to the counter/timer clock frequency divided by twice the value of the CTUR CTLR. While in the timer mode the ISR bit 3 (ISR[3]) will be set each time the counter/timer transitions from 1 to 0. (High to low)

This continues regardless of issuance of the stop counter command.

ISR[3] is reset by the stop counter command. NOTE: Reading of the CTU and CTL registers in the timer mode is not meaningful.

When the C/T is used to generate a baud rate

and

the C/T is selected through the CSR then the receivers and/or transmitter will be operating in the 16x mode. Calculation for the number ‘n’ to

program the counter timer upper and lower registers is shown below.

n=2 x 16 x Baud rate desired/(C/T Clock Frequency Often this division will result in a non-integer number; 26.3 for

example. One can only program integer numbers to a digital divider. Therefore 26 would be chosen. This gives a baud rate error of

0.3/26.3 which is 1.14%; well within the ability of the asynchronous mode of operation.

Counter Mode

In the counter mode the counter/timer counts the value of the CTLR CTUR down to zero and then sets the ISR[3] bit and sets the counter/timer output from 1 to 0. It then rolls over to 65,365 and continues counting with no further observable effect.

Reading the C/T in the counter mode outputs the present state of the C/T. If the C/T is not stopped, a read of the C/T may result in changing data on the data bus.

Timeout Mode

The timeout mode uses the received data stream to control the counter. The time-out mode forces the C/T into the timer mode. Each time a received character is transferred from the shift register to the RxFIFO, the counter is restarted. If a new character is not received before the counter reaches zero count, the counter ready

bit is set, and an interrupt can be generated. This mode can be used

to indicate when data has been left in the Rx FIFO for more than the programmed time limit. If the receiver has been programmed to interrupt the CPU when the receive FIFO is full, and the message ends before the FIFO is full, the CPU will not be interrupted for the remaining characters in the RxFIFO.

By programming the C/T such that it would time out in just over one character time, the above situation could be avoided. The processor would be interrupted any time the data stream had stopped for more than one character time. NOTE: This is very similar to the watch dog time of MR0. The difference is in the programmability of the delay time and that the watchdog timer is restarted by either a receiver load to the RxFIFO or a system read from it.

This mode is enabled by writing the appropriate command to the command register. Writing an ‘Ax’ to CRA or CRB will invoke the timeout mode for that channel. Writing a ‘Cx’ to CRA or CRB will disable the timeout mode. Only one receiver should use this mode at a time. However, if both are on, the timeout occurs after both receivers have been inactive for the timeout period. The start of the C/T will be on the logical or of the two receivers.

The timeout mode disables the regular STAR T/STOP counter commands and puts the C/T into counter mode under the control of the received data stream. Each time a received character is transferred from the shift register to the RxFIFO, the C/T is stopped after one C/T clock, reloaded with the value in CTUR and CTLR and then restarted on the next C/T clock. If the C/T is allowed to end the count before a new character has been received, the counter ready

bit, ISR[3], will be set. If IMR[3] is set, this will generate an interrupt. Since receiving a character restarts the C/T, the receipt of a character after the C/T has timed out will clear the counter ready bit, ISR[3], and the interrupt. Invoking the ‘Set Timeout Mode On’ command, CRx=‘Ax’, will also clear the counter ready bit and stop the counter until the next character is received.

The counter timer is controlled with six commands: Start/Stop C/T, Read/Write Counter/Timer lower register and Read/Write Counter/Timer upper register. These commands have slight differences depending on the mode of operation. Please see the detail of the commands under the CTLR CTUR Register descriptions.

Time Out Mode Caution

When operating in the special time out mode, it is possible to generate what appears to be a “false interrupt”, i.e., an interrupt without a cause. This may result when a time-out interrupt occurs and then, BEFORE the interrupt is serviced, another character is received, i.e., the data stream has started again. (The interrupt latency is longer than the pause in the data strea.) In this case, when a new character has been receiver, the counter/timer will be restarted by the receiver, thereby withdrawing its interrupt. If, at this time, the interrupt service begins for the previously seen interrupt, a read of the ISR will show the “Counter Ready” bit not set. If nothing else is interrupting, this read of the ISR will return a x’00 character. This action may present the appearance of a spurious interrupt.

Receiver and Transmitter

The QUART has four full-duplex asynchronous receiver/transmitters. The operating frequency for the receiver and transmitter can be selected independently from the baud rate generator, the counter/timer, or from an external input.

Registers associated with the communications channel are the mode registers (MR0, MR1 and MR2) Clock Select Register (CSR), Command Register (CR), Status Register (SR), Transmit FIFO (TxFIFO), and the Receive FIFO (RxFIFO). The transmit and receive FIFOs are each eight characters deep. The receive FIFO also stores three status bits with each character.

Transmitter

The transmitter accepts parallel data from the CPU and converts it to a serial bit stream on the TxD output pin. It automatically sends a start bit followed by the programmed number of data bits, an optional parity bit, and the programmed number of stop bits. The least significant bit is sent first. Following the transmission of the stop bits, if a new character is not available in the TxFIFO, the TxD output remains high and the TxEMT bit in the SR will be set to 1. Transmission resumes and the TxEMT bit is cleared when the CPU loads a new character in the TxFIFO. In the 16X clock mode, this also re-synchronizes the internal 1X transmitter clock so that transmission of the new character begins with minimum delay.

If the transmitter is disabled it continues operating until the character

currently being transmitted and any characters in the TxFIFO, including parity and stop bits, have been transmitted. New data cannot be loaded to the TxFIFO when the transmitter is disabled.

The transmitter can be forced to send a break (a continuous low condition) by issuing a STAR T BREAK command via the CR register. The break is terminated by a ST OP BREAK command or a transmitter reset..

TxFIFO

The TxFIFO empty positions are encoded as a three bit number for presentation to the bidding logic. The coding will equal the number of bytes that remain to be filled. That is, a binary number of 101 will

Philips Semiconductors Product specification

SC28C94Quad universal asynchronous receiver/transmitter (QUART)

1998 Aug 19

mean five bytes may be loaded; 111 means 7, etc. Eight positions will be indicated by a binary 111

and

the FIFO empty bit will be set.

Receiver

The receiver accepts serial data on the RxD pin, converts the serial

input to parallel format, checks for start bit, stop bit, parity bit (if any),

or break condition, and presents the assembled character to the CPU via the receiver FIFO.

The receiver operates in two modes: the 1X and 16X. The 16X mode is the more robust of the two. It allows the receiver to establish a phase relation to the remote transmitter clock within 1/16 of a bit time and also allows validation of the start bit. The 1X mode does not validate the start bit and assumes that the receiver clock rising edge is centered in the data bit cell. The use of the 1X mode implies that the transmitter clock is available to the receiver.

When operating in the 16X mode and after the receiver has been enabled the receiver state machine will look for a high to low

transition on the RxD input. The detection of this transition will cause

the divider being driven by the 16X clock to be reset to zero and continue counting. When the counter reaches 7 the RxD input is sampled again and if still low a valid STAR T BIT will be detected. If the RxD input is high at count 7 then an invalid start bit will have been sensed and the receiver will then look for another high to low transition and begin validating again.

When a valid start bit is detected the receiver state machine allows the 16X divider circuit to continue counting 0 to 15. Each time the receiver passes count 7 (the theoretical center of the bit time) another data bit is clocked into the receiver shift register until the proper number of bits have been received including the parity bit, if used, and 1/2 stop bit. After the STOP BIT is detected the receiver state machine will wait until the next falling edge of the 1X clock and then clock the assembled character and its status bits into the receiver FIFO on the next rising edge of the 1X clock. The delay

from the detection of the STOP BIT to the loading of the character to

the RxFIFO will be from one half to one and one half X1 crystal clock periods, or twice that if X1/2 is used. Receiver Status Register bits for FIFO READY, FIFO FULL, parity error, framing error, break detect will also set at this time. The most significant bits for data characters less than eight bits will be set to zero.

After the stop bit is detected, the receiver will immediately look for the next start bit. However, if a non-zero character was received without a stop bit (i.e. framing error) and RxD remains low for one-half of the bit period after the stop bit was sampled, then the

receiver operates as if a new start bit transition had been detected at

that point (one-half bit time after the stop bit was sampled). The parity error, framing error and overrun error (if any) are strobed into the SR at the received character boundary, before the RxRDY status bit is set.

If a break condition is detected (RxD is low for the entire character including the stop bit), only one character consisting of all zeros will be loaded in the FIFO and the received break bit in the SR is set to

1. The “Change of Break” bit in the ISR at position 2 or 6 is also set at this time. Note that the “Change of Break” bit will set again when the break condition terminates. The RxD input must return to high for two (2) clock edges of the X1 crystal clock for the receiver to recognize the end of the break condition and begin the search for a start bit. This will usually require a high time of one X1 clock

period or 3 X1 edges since the clock of the controller is not synchronous to the X1 clock.

NOTE: If the RxD input is low when the receiver is enabled and remains low for at least 9/16 of a bit time a valid start bit will be seen and data (probably random) will be clocked into the receiver FIFO. If the line remains low for a full character time plus a stop bit then a break will be detected.

RECEIVER FIFO

The RxFIFO consists of a first-in-first-out (FIFO) with a capacity of eight characters. Data is loaded from the receive shift register into the top-most empty position of the FIFO. The RxRDY bit in the status register (SR) is set whenever one or more characters are available to be read; a FFULL status bit is set if all eight stack

positions are filled with data. The number of filled positions is encoded into a 3-bit value. This value is sent to the interrupt bidding logic where it is used to generate an interrupt. A read of the RxFIFO, outputs the data at the top of the FIFO. After the read cycle, the data

FIFO and its associated status bits are ‘popped’ thus emptying a

FIFO position for new data.

NOTE: The number of filled positions in the RxFIFO is coded

as actual number filled positions. Seven filled will be coded as

7. Eight filled positions will be coded as 7

and

the RxFIFO full

status bit will be set.

Status

In addition to the data word, three status bits (parity error, framing

error, and received break) are appended to each data character in

the FIFO. Status can be provided in two ways, as programmed by

the error mode control bit in the mode register. In the ‘character’

mode, status is provided on a character-by-character basis: the

status applies only to the character at the top of the FIFO. In the ‘block’ mode, the status provided in the SR for these three bits is the

logical OR of the status for all characters coming to the top of the

FIFO since the last reset error command was issued. In either

mode, reading the SR does not affect the FIFO. The FIFO is

‘popped’ only when the RxFIFO is read. Therefore, the SR should

be read prior to reading the corresponding data character.

If the FIFO is full when a new character is received, that character is

held in the receive shift register until a FIFO position is available. If

an additional character is received while this state exists, the contents of the FIFO are not affected: the character previously in the shift register is lost and the overrun error status bit, SR[4], will be set

upon receipt of the start bit of the new (overrunning) character.

Watchdog Timer

A “watchdog” timer is associated with each receiver. Its interrupt is

enabled by MR0[7]. The purpose of this timer is alerting the control

processor that characters are in the RxFIFO which have not been

read and/or the datastream has stopped. This situation may occur

at the end of a transmission when the last few characters received

are not sufficient to cause an interrupt.

This counter times out after 64 bit times. It is reset each time a

character is transferred from the Receive shift register to the

RxFIFO or a read of the RxFIFO is executed.

Each receiver is equipped with a watchdog timer. This timer is

enabled by MR0[7] and counts 64 RxC1X clocks. Its purpose is to alert the controlling CPU that data is in the FIFO which has not been

read. This situation may occur at the end of a message when the

last group of characters was not long enough to cause an interrupt.

WAKE-UP MODE (MULTI-DROP OR 9-BIT)

In addition to the normal transmitter and receiver operation described

above, the QUART incorporates a special mode which provides

automatic “wake up” of a receiver through address frame (or character)

recognition for multi-processor or multi-station communications. This

mode is selected by programming MR1[4:3] to ‘11’.

In this mode of operation a ‘master’ station transmits an address

character to the several ‘slave’ stations on the line. The address character is identified by setting its parity bit to 1. The slave stations

will usually have their receivers partially enabled as a result of

Philips Semiconductors Product specification

SC28C94Quad universal asynchronous receiver/transmitter (QUART)

1998 Aug 19

setting MR1[4:3] to 1 1. When the receiver sees a one in the parity

position, it considers it an address bit and loads that character to the

RxFIFO and set the RxRDY bit in the status register. The user would usually set the receiver interrupt to occur on RxRDY as well. (All characters whose parity bits are set to 0 will be ignored). The local processor at the slave station will read the ‘address’ character just received. The local processor will test for an address match for this station and if match occurs it will enable the local receiver and receive the following data characters. The master will normally follow an address character(s) with data characters. Since the data characters transmitted by the master will have their parity bits set to zero, stations other than the addressed one(s) will ignore the data.

NOTE: The time between address and data fields must be enough for the local processor to test the address character

and enable the receiver. At bit times approaching 10µs this may

begin to be a point of concern.

The parity (Address/Data) bit should not be changed until the last stop

bit of an address has been sent. Similarly the A/D bit should not be changed to address until the last stop bit has been sent. Either of these conditions will be indicated by an active TxEMT bit in the SR.

The parity bit is not part of the TxFIFO. It is in the transmitter state machine. However, it could be controlled in the FIFO if 5, 6 or 7 bit data was transmitted by using a 6, 7 or 8 bit character. The most significant bit would then be in the ‘parity’ position and represent the A/D bit. The design of the UART is based, however, on the A/D bit being controlled from the MR register.

Parity should be changed immediately before the data bytes will be loaded to the transmitter.

A transmitted character consists of a start bit, the programmed number of data and stop bits and an “address/data” bit. The parity bit is used as the address or data indicator. The polarity of the A/D bit is selected by setting MR1[2] to zero or one; zero indicates that the current byte is data, while one indicates that the current byte is addressed. The desired polarity of the A/D bit (parity) should be programmed before

the TxFIFO is loaded.

The receiver should be enabled before the beginning of the first data

bit. The time required is dependent on the interrupt latency of the slave receivers. The transmitter is able to start data immediately after the address byte has been sent.

While in this mode, the receiver continuously looks at the received data stream, whether it is enabled or disabled. If disabled, it sets the RxRDY status bit and loads the character in the RxFIFO if the received A/D bit is a one, but discards the received character if the received A/D bit is a zero. If enabled, all received characters are then transferred to the CPU via the RxFIFO. In either case, the data bits are loaded in the data FIFO while the A/D bit is loaded in the status FIFO position normally used for parity error (SR[5]). Framing error, overrun error, and break detect operate normally whether or not the receiver is enabled.

INPUT OUTPUT (I/O) PINS

There are 16 multi-use pins; four for each UART. These pins are accessed and controlled via the Input Port Register (IPR), I/O Port Control Register (I/OPCR), Input Port Change Register (IPCR), and Output Port Register (OPR). They may be individually programmed to be inputs or outputs. See Table 5.

I/O0x and I/O1x pins have change of state detectors. The change of

state detectors sample the input ports every 26.04µs (with the X1 clock at 3.686400MHz) and set the change bit in the IPCR if the pin has changed since it was last read. Whether the pins are programmed as inputs or outputs the change detectors still operate and report changes accordingly . See the register descriptions of the I/O ports for the detailed use of these features.

A read of the IPCR resets the I/O COS (Change Of State) detectors.

Interrupt Priority System

The interrupt control for the QUART has been designed to provide

very low interrupt service overhead for the controlling processor

while maintaining a high degree of flexibility in setting the

importance of interrupts generated in different functional blocks of

the device.

This is accomplished by allowing each function of the QUART (18

total) which may cause an interrupt to generate a variable numeric

code which contains the identity of the source, channel number and

severity level. This code is compared (at the X1 clock rate or the X1 clock rate divided by 2) to an interrupt threshold. When the interrupt-

ing source generates a code that is numerically greater than the

interrupt threshold the IRQN is asserted

This is referred to as the bidding process. The winning bid contains,

in different fields, all the characteristics of the winning bidder. This

data may be used in several ways to steer the controlling processor

to the proper type and amount of service required (usually the

amount of service refers to the number of bytes written to the trans-

mitter or read from the receiver). Access to the winning bidder is

provided via the CIR (Current Interrupt Register), interrupt vectors,

modified interrupt vectors and Global registers.

NOTE: IRQN is essentially a level output. It will go active on an

interrupt condition and stays active until all interrupting sources are

serviced.

IRQN is designed to be an open drain active low level output. It will

go low under the control of the arbitration system and remain low

until the arbitration has determined that no more sources require

service.

When only one Rx or Tx is interrupting, it is possible to see the

IRQN assert more than once if, during an access to the FIFO, the

CEN input is inactive for more than two cycles of the X1 clock or X1

divide by 2 if that feature is enabled.

IACKN may be thought of as a special read input. Driving IACKN

low will update the CIR and then read the Interrupt Vector Register

or the Interrupt Vector Register modified by the CIR.

Functional Description of the Interrupt Arbitration

For the purpose of this description, a ‘source’ is any one of the 18

QUART circuits that may generate an interrupt. The QUART

contains eighteen sources which may cause an interrupt:

1. Four receiver data FIFO filled functions.

2. Four receiver BREAK detect functions.

3. Four transmitter FIFO space available functions.

4. Four “Change of State” detectors.

5. Two counter/timers.

The interrupt logic at each source produces a numeric code that

identifies its interrupt priority condition currently pending. This code

is compared to a programmable Interrupt Threshold via the

arbitration logic which determines if the IRQN should be asserted.

The arbitration logic only judges those possible interrupt sources

which have been allowed to bid via the IMR (Interrupt Mask

The arbitration logic produces a value which is the concatenation of

the channel number, interrupt type, FIFO fill level and user-defined

fields. The channel number and interrupt type fields are hardwired.

During the “bid arbitration” process all bids from enabled sources

are presented, simultaneously, to an internal interrupt bus. The

bidding system and formats are discussed in more detail in

following sections.

Philips Semiconductors Product specification

SC28C94Quad universal asynchronous receiver/transmitter (QUART)

1998 Aug 19

The interrupt arbitration logic insures that the interrupt with the numerically largest bid value will be the only source driving the interrupt bus at the end of the arbitration period. The arbitration period follows the period of the X1 clock. The maximum speed is

4.0MHz. If a higher speed X1 clock is used then the X1 clock “divide by 2” feature must be used.

The value of the winning bid determined during the arbitration cycle is compared to the “Interrupt Threshold” contained in the ICR (Interrupt Control Register). If the winning bid exceeds the value of the ICR the IRQN is asserted.

Priority Arbitration and Bidding

Each of the five “types” of interrupts has slightly different “bid” value, as follows:

Receivers

Transmitters

Break Detect

Change of State

Counter/Timer

# rcv’d rEr 1 1 Chan #

31112

0 # avail 1 0 Chan # 13 112

Programmable 1 0 0 Chan #

31112

Programmable 0 0 1 Chan #

31112

Programmable 1 0 1 Chan #

21112

0 1

SD00162

Please see “Interrupt Notes” at the end of this specification. Bits shown above as ‘0’ or ‘1’ are hard-wired inputs to the arbitration

logic. Their presence allows determination of the interrupt type and they insure that no bid will have a value of all zeros (a condition that is indistinguishable from not bidding at all). They also serve to set a default priority among the non-receive/transmit types when the programmable fields are all zeros.

The channel number always occupies the two LSBs. Inclusion of the channel number insures that a bid value generated will be unique and that a single “winner” will drive the Interrupt Bus at the end of the arbitration interval. The channel number portion of each UARTs bid is hard-wired with UARTa being channel number 0 and so forth.

As can be seen above, bits 4:2 of the winning bid value can be used to identify the type of interrupt, including whether data was received correctly or not. Like the Channel number field, these bits are hard-wired for each interrupt source.

The “# rcv’d” and “# avail” fields indicate the number of bytes present in the receiver FIFO and the number of empty bytes in the transmitter FIFO, respectively.

NOTE: When there are zero bytes in the receiver’s FIFO, it does NOT bid. Similarly, a full transmitter FIFO makes NO bid. In the case where all bids have been disabled by the Interrupt Mask Register or as a result of their byte counts, the active-low Interrupt Bus will return FFh. This value always indicates no interrupt source is active and IRQN will be negated.

The high order bit of the transmitter “bid” is always zero. An empty transmit FIFO is, therefore, fixed at a lower interrupt priority than a 1/2 full receive FIFO. Bit 4 of a receiver bid is the Receiver Error Bit (RER). The RER is the OR of the parity, framing and overrun error conditions. The RER does little to modify the priority of receiver

interrupts vs. transmitter interrupts. It is output to the Interrupt Bus

to allow inclusion of good data vs. problem data information in the

Current Interrupt Register.

The high order bits of bids for received break, CoS (Change of

State) and Counter/Timer events are all programmable. By

programming ones in these fields, the associated interrupt source

can be made more significant than most receiver and all transmitter

interrupts. Values near zero in these fields makes them lower

priority classes of interrupt.

The channel address for C/T ab will be encoded as channel B (01)

The channel address for C/T cd will be encoded as channel D (11)

As shown in Figure 8, the bid arbitration process is controlled by the

EVAL/HOLDN signal derived from the oscillator clock.

Receipt of an IACKN signal from the host MPU latches the latest “winning bid” from the latched Interrupt Bus into the Current Interrupt

If the IACKN falling edge of Figure 8 occurs during EVAL time, the result from the last arbitration (captured by the Interrupt Bus latches) is stored in CIR. Otherwise, the next EVAL pulse is inhibited and the

value in the Interrupt Bus Latches is stored in CIR.

Clearing the Interrupt

Activities which change the state of the ISR will cause the IRQN to

assert or negate. In addition, the accessing of a global or local RxFIFO or TxFIFO reduces the associated byte count for transmitter

and receiver data interrupts. If the byte count falls below the

threshold value, the interrupt request is withdrawn. Other interrupt

conditions are cleared when the interrupting source is cleared.

Once the interrupt is cleared, the programmable value lowered or its

byte count value reduced by one of the methods listed above, a

different bidder (or no bidder at all) will win the on-going arbitration.

When the winning bid drops below the Interrupt Threshold

Arbitration - Aftermath

At the end of the arbitration, i.e., the falling edge of EVAL, the

winning interrupt source is driving its Channel number, number of bytes (if applicable) and interrupt type onto the Interrupt Bus. These

values are captured into a latch by the trailing edge of EVAL. The

output of this latch is used by the Interrupt Threshold comparator;

the winning value is captured into another set of latches called the

Current Interrupt Register (CIR) at the time of an Interrupt

Acknowledge cycle or execution of the “Update CIR” command. The Current Interrupt Register and associated read logic is shown in

Figure 9. Interrupting channel number and the three bit interrupt

type code and FIFO fill level are readable via the Internal Data Bus.

The contents of the appropriate receiver or transmitter byte

“counter”, as captured at the time of IACKN assertion, make up bits

7:5 of the CIR. If the interrupt type stored in the Current Interrupt

CIR7:5 field will read as the programmable fields of their respective

bid formats.

The buffers driving the CIR to the DBUS also provide the means of

implementing the Global Interrupting Channel and Global Byte

Count Registers, described in a later section.

The winning bid channel number and interrupt type fields can also

be used to generate part of the Interrupt Vector, as defined by the

Interrupt Control Register.

Philips Semiconductors Product specification

SC28C94Quad universal asynchronous receiver/transmitter (QUART)

1998 Aug 19

Interrupt Context

The channel number of the winning “bid” is used by the address decoders to provide data from the interrupting UART channel via a set of Global pseudo-registers. The interrupt Global pseudo-registers are:

1. Global Interrupting Byte Count

2. Global Interrupting Channel

3. Global Receive Holding Register

4. Global Transmit Holding Register The first two Global “registers” are provided by Current Interrupt

Register fields as shown in Figure 9. The interrupting channel number latched in CIR modifies address decoding so that the Receive or Transmit Holding Register for the interrupting channel is accessed during I/O involving the Global Receive and Transmit Holding Registers. Similarly, for data interrupts from the transmitter and receiver, the number of characters available for transfer to the CPU or the number of transmit FIFO positions open is available by reading the Global Interrupt Byte Count Register. For non-data interrupts, a read of the Global Interrupt Byte Count Register yields a value equal to the highest programmable filed.

In effect, once latched by an IACK or the Update CIR command, the winning interrupt channel number determines the contents of the global registers. All Global registers will provide data from the interrupting UART channel.

Interrupt Threshold Calculation

The state of IRQN is determined by comparison of the winning “bid” value to the Interrupt Threshold field of the Interrupt Control Register.

The logic of the bidding circuit is such that when no interrupt source has a value greater than the interrupt threshold then the interrupt is not asserted and the CIR (Current Interrupt Register) is set to all ones. When one or more of the 18 interrupt sources which are enabled via the IMR (Interrupt Mask Register) exceed the threshold then the interrupt threshold is effectively disconnected from the bidding operation while the 18 sources now bid against each other. The final result is that the highest bidding source will disable all others and its value will be loaded to the CIR and the IRQN pin asserted low. This all occurs during each cycle of the X1, X2 crystal clock.

Table 2. Receiver FIFO Interrupt Fill Level

MR0[6] MR1[6] Interrupt Condition

0 0 1 1

0 1 0 1

1 or more bytes in FIFO (Rx RDY) default* 3 or more bytes in FIFO 6 or more bytes in FIFO 8 bytes in FIFO (Rx FULL)

For the receiver these bits control the number of FIFO positions empty when the receiver will attempt to interrupt. After the reset the receiver FIFO is empty. The default setting of these bits cause the receiver to attempt to interrupt when it has one or more bytes in it.

Table 3. Transmitter FIFO Interrupt Fill Level

MR0[5] MR0[4] Interrupt Condition

0 0 1 1

0 1 0 1

8 bytes empty (Tx EMPTY) default* 4 or more bytes empty 6 or more bytes empty 1 or more bytes empty (Tx RDY)

For the transmitter these bits control the number of FIFO positions empty when the receiver will attempt to interrupt. After the reset the transmit FIFO has 8 bytes empty. It will then attempt to interrupt as soon as the transmitter is enabled. The default setting of the MR0 bits (00) condition the transmitter to attempt to interrupt only when it

is competely empty. As soon as one byte is loaded, it is no longer

empty and hence will withdraw its interrupt request.

*These conditions, for interrupt purposes, make the RxFIFO look

like a 3 byte FIFO; the TxFIFO a 1 byte FIFO. This is to allow

software compatibility with previous Philips UART devices. Both

FIFOs accept 8 bytes of data regardless of this bit setting. Only the

interrupt is affected.

INTERRUPT NOTE ON 28C94: For the receivers and transmitters, the bidding of any particular

unit may be held off unless one of four FIFO fill levels is

attained. This is done by setting the RxINT and TxINT bits in

MR0 and MR1 to non-zero values. This may be used to prevent

a receiver or transmitter from generating an interrupt even

though it is filed above the bid threshold. Although this is not

in agreement with the idea that each enabled interrupt source

bid with equal authority, it does allow the flexibility of giving

particular receiver or transmitters more interrupt importance

than others.

This may be used when the Interrupt Threshold is set at or

above 100000. Note than in this case the transmitter cannot

generate an interrupt. If the interrupt threshold MSBs were set

to 011

and

the ‘Receiver Interrupt Bits’ on the MR registers set to a value other than 00 then the RxFIFO could not generate and interrupt until it had 4, 6 or 8 bytes. This in effect partially defeats the hardwired characteristic that the receiver interrupts should have more importance than the transmitter. This characteristic has been implemented by setting the MSB of the transmitter bid to zero.

Vectored Interrupts

The QUART responds to an Interrupt Acknowledge (IACK) initiated by the host by providing an Interrupt Acknowledge Vector on D7:0. The interrupt acknowledge cycle is terminated with a DACKN pulse. The vector provided by the QUART can have one of the three forms under control of the IVC control field (bits 1:0 of the Interrupt Control Register):

With IVC = 00 (IVR only)

IVR7:0

With IVC = 01 (channel number)

IVR7:2

Chan #

With IVC = 10 (type & channel number)

IVR7:5

Chan #

Type

SD00163

A code of 11 in the Interrupt Vector Control Field of the ICR results in NO interrupt vector being generated. The external data bus is driven to a high impedance throughout the IACK cycle. A DACKN will be generated normally for the IACK cycle, however.

NOTE: If IACKN is not being used then the command “UPDATE CIR” must be issued for the global and interrupt registers to be updated.

PROGRAMMING UART CONTROL REGISTERS

The operation of the QUART is programmed by writing control words into the appropriate registers. Operational feedback is provided via status registers which can be read by the CPU. Addressing of the registers is described in Table 1.

The bit formats of the QUART registers are depicted in Table 2.

+ 26 hidden pages