MICROCHIP PIC16C505 User Manual
PIC16C505
14-Pin, 8-Bit CMOS Microcontroller
Device included in this Data Sheet:
PIC16C505
High-Performance RISC CPU:
• Only 33 instructions to learn
• Operating speed:
- DC - 20 MHz clock input
- DC - 200 ns instruction cycle
Memory
Device
Program Data
PIC16C505 1024 x 12 72 x 8
• Direct, indirect and relative addressing modes for
data and instructions
• 12-bit wide instructions
• 8-bit wide data path
• 2-level deep hardware stack
• Eight special function hardware registers
• Direct, indirect and relative addressing modes for
data and instructions
• All single cycle instructions (200 ns) except for
program branches which are two-cycle
Peripheral Features:
• 11 I/O pins with individual direction control
• 1 input pin
• High current sink/source for direct LED drive
• Timer0: 8-bit timer/counter with 8-bit
programmable prescaler
Pin Diagram:
PDIP, SOIC, Ceramic Side Brazed
RB5/OSC1/CLKIN
RB4/OSC2/CLKOUT
RB3/MCLR
VDD
/VPP
RC5/T0CKI
RC4
RC3
1
2
3
4
5
6
7
14
PIC16C505
13
12
11
10
9
8
V
SS
RB0
RB1
RB2
RC0
RC1
RC2
Special Microcontroller Features:
• In-Circ uit Serial Programming (ICSP™)
• Power-on Reset (POR)
• Device Reset Timer (DRT)
• Watchdog Timer (WDT) with dedicated on-chip
RC oscillator for reliable operation
• Programmable Code Protection
• Internal weak pull-ups on I/O pins
• Wake-up from Sleep on pin change
• Power-saving Sleep mode
• Selectable oscillator options:
- INTRC: Precision internal 4 MHz oscillator
- EXTRC: External low-cost RC oscillator
- XT: Standard crystal/resonator
- HS: High speed crystal/resonator
- LP: Power saving, low frequency
crystal
CMOS Technology:
• Low-power, high-speed CMOS EPROM
technology
• Fully static design
• Wide operating voltage range (2.5V to 5.5V)
• Wide temp erature ranges
- Commercial: 0°C to +70°C
- Industrial: -40°C to +85°C
- Extended: -40°C to +125°C
- < 1.0 µA typical standby curre nt @ 5V
• Low power consumption
- < 2.0 mA @ 5V, 4 MHz
-15µA typical @ 3.0V, 32 kHz for TMR0
running in SLEEP mode
- < 1.0 µA typical standby curre nt @ 5V
1999 Microchip Technology Inc. DS40192C-page 1
PIC16C505
TABLE OF CONTENTS
1.0 General Description................................................................................................ .. .... .. .............................................................3
2.0 PIC16C505 Device Varieties.......................................................................................................................................................5
3.0 Architectural Overview ................................................................................................................................................................7
4.0 Memory Organization................................................................................................................................................................11
5.0 I/O Port............ ......................... ...................................... ...................................... ........................... .......................................... 19
6.0 Timer0 Module and TMR0 Register ..........................................................................................................................................23
7.0 Special Features of the CPU.....................................................................................................................................................27
8.0 Instruction Set Summary........................................................................................................................................................... 39
9.0 Development Support................................................................................................................................................................ 51
10.0 Electrical Characteristics - PIC16C505 ..................................................................................................................................... 57
11.0 DC and AC Characteristics - PIC16C505.................................................................................................................................. 71
11.0 Packaging Informatio n...... ......................... .................................................. .......................... .................................................. ..75
Index .................................................................. ...... .... ....... ...... .... .... ......... .... .... ..................................................................................79
On-Line Support........................................................... ......... .. .... .... .. ......... .. .... .... .. ......... ..................................................................... 81
Reader Response................................................................................................................................................................................82
PIC16C505 Product Identification System ..........................................................................................................................................83
To Our Valued Customers
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An errata sheet may exist for current devices, describing minor operational differences (from the data sheet) and recommended
workarounds. As device/documentation issues become known to us, w e will pub lish an errata sheet. The errata will specify the re vision of silicon and revision of document to which it applies.
To determine if an errata sheet exists for a particular device, please check with one of the following:
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We constantly strive to improve the quality of all our products and documentation. We have spent a great deal of time to ensure
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DS40192C-page 2
1999 Microchip Technology Inc.
PIC16C505
1.0 GENERAL DESCRIPTION
The PIC16C505 from Microchip Technology is a lowcost, high-performance, 8-bit, fully static, EPROM/
ROM-based CM OS microc ontroller. It employs a RISC
architecture with only 33 single word/single cycle
instructions. All instructions are single cycle (200 µs)
except for program branches, which take two cycles.
The PIC16C505 del ivers pe rformanc e an order of magnitude higher than its com petitors in the sam e price category. The 12-bit wide instructions are highly
symmetrical resulti ng in a typical 2:1 code co mpression
over other 8-bit microcontrollers in its class. The easy
to use and easy to remember instruction set reduces
development time significantly.
The PIC16C505 product is equipped with special features that reduce system cost and pow er requirements .
The Power-On Reset (POR) and Device Reset Timer
(DRT) eliminate the need for external reset circuitry.
There are five oscillator configurations to choose from,
including INTRC internal oscillator mode and the
power-saving LP (L ow Power) oscilla tor mode. Power
saving SLEEP mode, Watchdog Timer and code
protection features improve system cost, power and
reliability.
The PIC16C505 is available in the cost-effective OneTime-Program mable (OTP) version, which is suitable
for production in any volume. The customer can take
full advantage of Microchip’s price leadership in OTP
microcontrollers, while benefiting from the OTP’s
flexibility.
The PIC16C505 produ ct is supported by a f ull-fea tured
macro assembler, a software simulator, an in-circuit
emulator, a ‘C’ compiler, a low-cost development programmer and a full featured programmer. All the tools
are supported on IBM
PC and compatible machines.
1.1 Applications
The PIC16C505 fits in applications ranging from personal care appliances and security systems to lowpower remote transmitters/receivers. The EPROM
technology makes customizing application programs
(transmitter codes, appliance settings, receiver frequencies, etc.) extremely fast and convenient. The
small footprint packages, for through hole or surface
mounting, make this microc ontroller pe rfe ct f or applic ations with space limitations . Lo w-cost, lo w-po we r, highperformance, ease of use and I/O flexibility make the
PIC16C505 very versatile even in areas where no
microcontroller use has been considered before (e.g.,
timer functions, replacement of “glue” logic and PLD’s
in larger systems, and coprocessor applications).
1999 Microchip Technology Inc. DS40192C-page 3
PIC16C505
TABLE 1-1: PIC16C505 DEVICE
PIC16C505
Clock
Memory
Peripherals
Features
The PIC16C505 device has Power-on Reset, selectable Watchdog Timer, selectable code protect, high I/O current capability and
precision internal oscillator.
The PIC16C505 device uses serial programming with data pin RB0 and clock pin RB1.
Maximum Frequency
of Operation (MHz)
EPROM Program Memory 1024
Data Memory (bytes) 72
Timer Module(s) TMR0
Wake-up from SLEEP on
pin change
I/O Pins 11
Input Pins 1
Internal Pull-ups Yes
In-Circuit Serial Programming Yes
Number of Instructions 33
Packages 14-pin DIP, SOIC, JW
20
Yes
DS40192C-page 4
1999 Microchip Technology Inc.
PIC16C505
2.0 PIC16C505 DEVICE VARIETIES
A variety of packaging options are available.
Depending on application and production
requirements, the proper device option can be
selected using the information in this section. When
placing orders, please use the PIC16C505 Product
Identification System at the back of this data sheet to
specify the correct part number.
2.1 UV Erasable Devices
The UV erasable version, offered in a ceramic windowed package, is optimal for prototype development
and pilot programs.
The UV erasable version can be erased and
reprogrammed to any of the configuration modes.
Note: Please note that erasing the device will
also erase the pre-programmed internal
calibration value for the internal oscillator.
The calibration value must be saved prior
to erasing the part.
Microchip’s PIC ST A RT
grammers all support programming of the PIC16C505.
Third party programmers also are a vailab le; ref er to the
Microchip Third Party Gu ide,
sources.
PLUS and PRO MATE II pro-
(DS00104), for a list of
2.3 Quick-Turnaround-Production (QTP)
Devices
Microchip offers a QTP Programming Service for
factory production orders. This service is made
2.2 One-Time-Programmable (OTP) Devices
The availability of OTP devices is especially useful for
customers who need the flexibility of frequent code
updates or small volume applications.
The OTP devices, packaged in plastic packages, permit the user to program them once. In addition to the
program memory, the configuration bits must also be
programmed.
1999 Microchip Technology Inc. DS40192C-page 5
PIC16C505
NOTES:
DS40192C-page 6
1999 Microchip Technology Inc.
PIC16C505
3.0 ARCHITECTURAL OVERVIEW
The high performance of the PIC16C505 can be
attributed to a number of architectural features
commonly found in RISC microprocessors. To begin
with, the PIC16C505 uses a Harvard architecture in
which program and data are accessed on separate
buses. This improves bandwidth over traditional von
Neumann architecture where program and data are
fetched on the same bus. Separating program and
data memory further allows instructions to be sized
differently than the 8-bit wide data word. Instruction
opcodes are 12 bits wide, making it possible to have
all single word instructions. A 12-bit wide program
memory access bus fetches a 12-bit instruction in a
single cycle. A two-stage pipeline overlaps fetch and
ex ecutio n of ins tructions . Co nsequ ently, all instructions
(33) execute in a single cycle (200ns @ 20MHz)
except for program branches.
The Table below lists program memory (EPROM) and
data memory (RAM) for the PIC16C505.
Memory
Device
Program Data
PIC16C505 1024 x 12 72 x 8
The PIC16C505 device contains an 8-bit ALU and
working register. The ALU is a general purpose
arithmetic unit. It performs arithmetic and Boolean
functions betwe en data in the wo rking register and an y
register file.
The ALU is 8-bits wide and capable of addition,
subtraction, shift and logical operations. Unless
otherwise mentioned, arithmetic operations are two's
complement in nature. In two-operand instructions,
one operand is typically the W (working) register. The
other operand is either a file register or an immediate
constant. In single o pera nd ins tructions , th e oper an d is
either the W register or a file register.
The W register is an 8-bit working register used for
ALU operations. It is not an addressable register.
Depending on the instruction executed, the ALU may
affect the values of the Carry (C), Digit Carry (DC),
and Zero (Z) bits in the STATUS register. The C and
DC bits operat e as a borrow
respectively, in subtracti on. See the SUBWF and ADDWF
instructions for examples.
A simplified block diagram is shown in Figure 3-1, with
the corresponding device pins described in Table 3-1.
and digit bor row out bit,
The PIC16C505 can directly or indirectly address its
register files and data memory. All special function
registers, including the program counter, are mapped
in the data memory. The PIC16C505 has a highly
orthogonal (symmetrical) instruction set that makes it
possible to carry out any operation on any register
using any addre ssing mode. This sy mmetrical nat ure
and lack of ‘special optimal situations’ make
programming with the PIC16C505 simple yet efficient.
In addition, the learning curve is reduced significantly.
1999 Microchip Technology Inc. DS40192C-page 7
PIC16C505
FIGURE 3-1: PIC16C505 BLOCK DIAGRAM
OSC1/CLKIN
OSC2
Program
Bus
EPROM
1K x 12
Program
Memory
12
Instruction reg
Instruction
Decode &
Control
Timing
Generation
12
Program Counter
STACK1
STACK2
Direct Addr
8
Device Reset
Timer
Power-on
Reset
Watchdog
Timer
Internal RC
OSC
RAM Addr
5
Data Bus
Addr MUX
3
8
RAM
e
t
y
b
2
7
File
Registers
9
5-7
FSR reg
STATUS reg
MUX
ALU
W reg
Timer0
s
Indirect
Addr
8
PORTB
RB0
RB1
RB2
RB3/MCLR/VPP
RB4/OSC2/CLKOUT
RB5/OSC1/CLKIN
PORTC
RC0
RC1
RC2
RC3
RC4
RC5/T0CKI
MCLR
VDD, VSS
DS40192C-page 8
1999 Microchip Technology Inc.
TABLE 3-1: PIC16C505 PINOUT DESCRIPTION
PIC16C505
Name
RB0 13 13 I/O TTL/ST Bi-directional I/O port/ serial programming data. Can
RB1 12 12 I/O TTL/ST Bi-directional I/O po rt/ serial programming clock. Can
RB2 11 11 I/O TTL Bi-directional I/O port.
RB3/MCLR
RB4/OSC2/CLKOUT 3 3 I/O TTL Bi-directional I/O port/oscillator crystal output. Con-
RB5/OSC1/CLKIN 2 2 I/O TTL/ST Bidirectional IO port/oscillator crystal input/external
RC0 10 10 I/O TTL Bi-directional I/O port.
RC1 9 9 I/O TTL Bi-directional I/O port.
RC2 8 8 I/O TTL Bi-directional I/O port.
RC3 7 7 I/O TTL Bi-directional I/O port.
RC4 6 6 I/O TTL Bi-directional I/O port.
RC5/T0CKI 5 5 I/O ST Bi-directional I/O port. Can be configured as T0CKI.
DD 11P— Positive supply for logic and I/O pins
V
VSS 14 14 P — Ground reference for logic and I/O pins
Legend: I = input, O = output, I/O = input/output, P = power, — = not used, TTL = TTL input,
/VPP 4 4 I TTL/ST Input port/master clear (reset) input/programming volt-
ST = Schmitt Trigger input
DIP
Pin #
SOIC
Pin #
I/O/P
Type
Buffer
Type
Description
be software progr am med f o r internal weak pull-up and
wake-up from SLEEP on pin change. This buffer is a
Schmitt Trigger input when used in serial progr amming
mode.
be software progr am med f o r internal weak pull-up and
wake-up from SLEEP on pin change. This buffer is a
Schmitt Trigger input when used in serial progr amming
mode.
age input. When configured as MCLR
active low reset to the device. Voltage on MCLR
must not exceed V
Can be software p rogr ammed f or internal weak pull-up
and wake-up from SLEEP on pin change. Weak pullup only when config ure d as R B3. ST w hen co nfi gured
as MCLR.
nections to crystal or resonator in crystal oscillator
mode (XT and LP modes only, RB4 in other modes).
Can be software p rogr ammed f or internal weak pull-up
and wake-up from SLEEP on pin change. In EXTRC
and INTRC modes, th e pin output can b e configured to
CLKOUT, which has 1/4 the frequency of OSC1 and
denotes the instruction cycle rate.
clock source input (RB5 in Internal RC mode only,
OSC1 in all other oscillator modes). TTL input when
RB5, ST input in external RC oscillator mode.
DD during normal device operation.
, this pin is an
/VPP
1999 Microchip Technology Inc. DS40192C-page 9
PIC16C505
3.1 Clocking Scheme/Instruction Cycle
The clock input (OSC1/CLKIN pin) is internally divided
by four to generate four non-overlapping quadrature
clocks namely Q1, Q2, Q3 and Q4. Internally, the
program counter is incremented every Q1, and the
instruction is fetched from program memory and
latched into the in st ruction register in Q4. It is d ec ode d
and executed during the following Q1 through Q4. The
clocks and instruction execution flow is shown in
Figure 3-2 and Example 3-1.
FIGURE 3-2: CLOCK/INSTRUCTION CYCLE
Q2 Q3 Q4
OSC1
Q1
Q2
Q3
Q4
PC
Q1
PC PC+1 PC+2
Q1
3.2 Instruction Flow/Pipelining
An Instruction cycle consists of four Q cycles (Q1, Q2,
Q3 and Q4). The instruction fetch and execute are
pipelined such that fetch takes one instruction cycle,
while decode and execute takes another instruction
cycle. However, due to the pipelining, each instruction
effectively executes in one cycle. If an instruction
causes the program counter to change (e.g., GOTO)
then two cycles are required to complete the
instruction (Example 3-1).
A fetch cycle begins with the program counter (PC)
incrementing in Q1.
In the execution cycle, the fetched instruction is
latched into the Instruction Register (IR) in cycle Q1.
This instruction is then decoded and executed during
the Q2, Q3 and Q4 cycles. Data memory is read
during Q2 (operand read) and written during Q4
(destination write).
Q2 Q3 Q4
Q1
Q2 Q3 Q4
Internal
phase
clock
Fetch INST (PC)
Execute INST (PC-1) Fetch INST (PC+1)
Execute INST (PC) Fetch INST (PC+2)
Execute INST (PC+1)
EXAMPLE 3-1: INSTRUCTION PIPELINE FLOW
1. MOVLW 03H
2. MOVWF PORTB
3. CALL SUB_1
4. BSF PORTB, BIT1
All instructions are single cycle, except for any program branches. These take two cycles, since the fetch
instruction is “flushed” from the pipeline, while the new instruction is being fetched and then executed.
Fetch 1 Execute 1
Fetch 2 Execute 2
Fetch 3 Execute 3
Fetch 4 Flush
Fetch SUB_1 Execute SUB_1
DS40192C-page 10
1999 Microchip Technology Inc.
PIC16C505
4.0 MEMORY ORGANIZATION
PIC16C505 memory is organized into program memory and data memory. For the PIC16C505, a paging
scheme is used. Program memory pages are
accessed using one STATUS register bit. Data memory banks are accessed using the File Select Register
(FSR).
4.1 Program Memory Organization
The PIC16C505 devices have a 12-bit Program
Counter (PC).
The 1K x 12 (0000h-03FFh) for the PIC16C505 are
physically implemented. Refer to Figure 4-1.
Accessing a location above this boundary will cause a
wrap-around within the first 1K x 12 space. The
effective reset vector is at 0000h, (see Figure 4-1).
Location 03FFh contains the internal clock oscillator
calibration value. This value should never be
overwritten.
FIGURE 4-1: PROGRAM MEMORY MAP
AND STACK FOR THE
PIC16C505
PC<11:0>
CALL, RETLW
Stack Level 1
Stack Level 2
Reset Vector (note 1)
Space
User Memory
On-chip Program
Memory
1024 Words
12
0000h
01FFh
0200h
03FFh
0400h
Note 1: Address 0000h becomes the
effective reset vector . Location 03FFh
contains the MOVLW XX INTERNAL RC
oscillator calibration value.
7FFh
1999 Microchip Technology Inc. DS40192C-page 11
PIC16C505
4.2 Data Memory Organization
Data memory is composed of registers or bytes of
RAM. Therefore, data memory for a device is specified
by its register file. The register file is divided into two
functional groups: Special Function Registers and
General Purpose Registers.
The Special Function Registers include the TMR0
register, the Program Counter (PCL), the Status
Register, the I/O registers (ports) and the File Select
Register (FSR). In addition, Special Function
Registers are used to control the I/O port configuration
and prescaler options.
The General Purpose Registers are used for data and
control inf ormation un der c om m and of the instructions.
FIGURE 4-2: PIC16C505 REGISTER FILE MAP
FSR<6:5> 00 01
File Address
00h
01h
02h
03h
04h
05h
06h
07h
08h
0Fh
10h
1Fh
(1)
INDF
TMR0
PCL
STATUS
FSR
OSCCAL
PORTB
PORTC
General
Purpose
Registers
General
Purpose
Registers
Bank 0
20h
2Fh
30h
General
Purpose
Registers
3Fh
Bank 1
For the PIC16C505, the register file is composed of 8
Special Function Registers, 24 General Purpose
Registers and 48 General Purpose Registers that may
be addressed using a banking scheme (Figure 4-2).
4.2.1 GENERAL PURPOSE REGISTER FILE The General Purpose Register file is accessed, either
directly or indirectly, through th e File Select Register
FSR (Section 4.8).
10
40h
Addresses map back to
addresses in Bank 0.
4Fh
50h
General
Purpose
Registers
5Fh
Bank 2
11
60h
6Fh
70h
General
Purpose
Registers
7Fh
Bank 3
DS40192C-page 12
Note 1: Not a physical register.
1999 Microchip Technology Inc.
PIC16C505
4.2.2 SPECIAL FUNCTION REGISTERS
The Special Function Registers (SFRs) are registers
used by the CPU and peripheral functions to control
the operation of the device (Table 4-1).
The Special Function Registers can be classified into
two sets. The Special Function Registers associated
with the “core” functions are described in this section.
Those related to the operation of the peripheral
features are described in the section for each
peripheral feature.
TABLE 4-1: SPECIAL FUNCTION REGISTER (SFR) SUMMARY
Value on
Power-On
Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
00h INDF Uses contents of FSR to address data memory (not a physical register)
01h TMR0 8-bit real-time clock/counter
(1)
02h
03h STATUS RBWUF —PAOTO PD ZDCC
04h
05h OSCCAL CAL5 CAL4 CAL3 CAL2 CAL1 CAL0 — —
N/A TRISB — — I/O contro l reg ister s
N/A TRISC — — I/O control registers
N/A OPTION RBWU RBPU TOCS TOSE PSA PS2 PS1 PS0
06h PORTB — — RB5 RB4 RB3 RB2 RB1 RB0
07h POR TC — — RC5 RC4 RC3 RC2 RC1 RC0
Legend: Shaded cells not used by Port Registers, read as ‘0’, — = unimplemented, read as ‘0’, x = unknown, u = unchanged,
Note 1: If reset was due to wake-up on pin change, then bit 7 = 1. All other rests will cause bit 7 = 0.
Note 2: Other (non-power-up) resets include external reset through MCLR
PCL Low order 8 bits of PC
FSR
q = depends on condition.
Indirect data memory address pointer
, watchdog timer and wake-up on pin change reset.
Reset
xxxx xxxx uuuu uuuu
xxxx xxxx uuuu uuuu
1111 1111 1111 1111
0001 1xxx q00q quuu
110x xxxx 11uu uuuu
1000 00-- uuuu uu--
--11 1111 --11 1111
--11 1111 --11 1111
1111 1111 1111 1111
--xx xxxx --uu uuuu
--xx xxxx --uu uuuu
Val ue on
All Other
Resets
(2)
(1)
1999 Microchip Technology Inc. DS40192C-page 13
PIC16C505
4.3 STATUS Register
For example, CLRF STATUS will clear the upper three
bits and set the Z bit. This leaves the STATUS register
This register contains the arithmetic status of the ALU,
the RESET status and the page preselect bit.
The STATUS register can be the destination for any
instruction, as with any other register. If the STATUS
register is the destination for an instruction that affe cts
the Z, DC or C bits, then the write to these three bits is
disabled. These bits are set or cleared according to
the device logic. Furthermore, the TO
and PD bits are
as 000u u1uu (where u = unchanged).
It is recommended, therefore, that only BCF, BSF and
MOVWF instructions be used to alter the STATUS
register, because these instructions do not affect the
Z, DC or C bits from the STATUS register. For other
instructions, which do affect STATUS bits, see
Instruction Set Summary.
not writable. Therefore, the result of an instruction with
the STATUS register as destination may be different
than intended.
REGISTER 4-1: STATUS REGISTER (ADDRESS:03h)
R/W-0 R/W-0 R/W-0 R-1 R -1 R/W-x R/W-x R/W-x
RBWUF
bit7 6 5 4 3 2 1 bit0
bit 7: RB WUF: I/O reset bit
bit 6: Unimplemented
bit 5: PA0: Program page preselect bits
bit 4: TO
bit 3: PD
bit 2: Z: Zero bit
bit 1: DC: Digit carry/borrow
bit 0: C: Carry/borrow
—
1 = Reset due to wake-up from SLEEP on pin change
0 = After power up or other reset
1 = Page 1 (200h - 3FFh)
0 = Page 0 (000h - 1FFh)
Each page is 512 bytes.
Using the PA0 bit as a general purpose read/write bit in devices which do not use it for program
page preselect is not recommended, since this may affect upward compatibility with future products.
: Time-out bit
1 = After power-up, CLRWDT instruction, or SLEEP instruction
0 = A WDT time-out occurred
: Power-down bit
1 = After power-up or by the CLRWDT instruction
0 = By execution of the SLEEP instruction
1 = The result of an arithmetic or logic operation is zero
0 = The result of an arithmetic or logic operation is not zero
ADDWF
1 = A carry from the 4th low order bit of the result occurred
0 = A carry from the 4th low order bit of the result did not occur
SUBWF
1 = A borrow from the 4th low order bit of the result did not occur
0 = A borrow from the 4th low order bit of the result occurred
ADDWF SUBWF RRF or RLF
1 = A carry occurred 1 = A borrow did not occur Load bit with LSB or MSB, respectively
0 = A carry did not occur 0 = A borrow occurred
PA0 TO PD Z DC C R = Readable bit
bit (for ADDWF and SUBWF instructions)
bit (for ADDWF, SUBWF and RRF, RLF instr uct i ons)
W = Writable bit
U = Unimplemented bit,
read as ‘0’
- n = Value at POR reset
DS40192C-page 14
1999 Microchip Technology Inc.
PIC16C505
4.4 OPTION Register
The OPTION register is a 8-bit wide, write-only
register, which contains various control bits to
configure the Timer0/WDT prescaler and Timer0.
Note: If TRIS bit is set to ‘0’, the wake-up on
change and pull-up functions are disabled
for that pin (i.e., note that TRIS overrides
OPTION control of RBPU
By executing the OPTION instruction, the contents of
the W register will be transferred to the OPTION
register. A RESET sets the OPTION<7:0> bits.
REGISTER 4-2: OPTION REGISTER
W-1 W-1 W-1 W-1 W-1 W-1 W-1 W-1
RBWU
bit7 6 5 4 3 2 1 bit0
bit 7: RB WU
bit 6: RB PU
bit 5: T0CS : Timer0 clock source select bit
bit 4: T0SE: Timer0 source edge select bit
bit 3: PS A : Prescaler assignment bit
bit 2-0: PS<2:0>: Prescaler rate select bits
RBPU T0CS T 0SE PSA PS2 PS1 PS0 R = Readable bit
: Enable wake-up on pin change (RB0, RB1, RB3, RB4)
1 = Disabled
0 = Enabled
: Enable weak pull-ups (RB0, RB1, RB3, RB4)
1 = Disabled
0 = Enabled
1 = Transition on T0CKI pin (overrides TRIS <RC57>
0 = Transition on internal instruction cycle clock, Fosc/4
1 = Increment on high to low transition on the T0CKI pin
0 = Increment on low to high transition on the T0CKI pin
1 = Prescaler assigned to the WDT
0 = Prescaler assigned to Timer0
Bit Value Timer0 Rate WDT Rate
000
001
010
011
100
101
110
111
1 : 2
1 : 4
1 : 8
1 : 16
1 : 32
1 : 64
1 : 128
1 : 256
1 : 1
1 : 2
1 : 4
1 : 8
1 : 16
1 : 32
1 : 64
1 : 128
and RBWU).
W = Writable bit
U = Unimplemented bit,
read as ‘0’
- n = Value at POR reset
1999 Microchip Technology Inc. DS40192C-page 15
PIC16C505
4.5 OSCCAL Register
The Oscillator Calibration (OSCCAL) register is used to
calibrate the internal 4 MHz oscillator. It contains six
bits for calibration
Note: Please note that erasing the device will
also erase the pre-programmed internal
calibration value for the internal oscillator.
The calibration value must be read prior to
erasing the part, so it can be reprogrammed correctly later.
After you move in the calibration constant, do not
change the value. See Section7.2.5
REGISTER 4-3: OSCCAL REGISTER (ADDRESS 05h) PIC16C505
R/W-1 R/W-0 R/W-0 R/W-0 R/W-0 R/W-0 U-0 U-0
CAL5 CAL4 CAL3 CAL2 CAL1 CAL0
bit7 bit0
bit 7-2: CAL<5:0>: Calibration
bit 1-0: Unimplemented read as ‘0’
— — R = Readable bit
W = Writable bit
U = Unimplemented bit,
read as ‘0’
- n = Value at POR reset
DS40192C-page 16
1999 Microchip Technology Inc.
PIC16C505
4.6 Program Counter
As a program instruction is executed, the Program
Counter (PC) will contain the address of the next
program instruction to be executed. The PC value is
increased by one every instruction cycle, unless an
instruction changes the PC.
For a GOTO instruction, bits 8:0 of the PC are provided
by the GOTO instruction word. The PC Latch (PCL) is
mapped to PC<7:0>. Bit 5 of the STATUS register
provides page information to bit 9 of the PC
(Figure 4-3).
For a CALL instruction, or any instruction where the
PCL is the destination, bits 7:0 of the PC again are
provided by the instruction word. However, PC<8>
does not come from the instruction word, but is always
cleared (Figure 4-3).
Instructions where the PCL is the destination, or
Modify PCL instructions, include MOVWF PC, ADDWF
PC, and BSF PC,5.
Note: Because PC<8> is cleared in the CALL
instructi on or any Modify PCL inst ruction,
all subroutine call s or computed jumps are
limited to the first 256 l ocations of any program memory page (512 words long).
FIGURE 4-3: LOADING OF PC
BRANCH INSTRUCTIONS PIC16C505
GOTO Instruction
11
PC
70
87 0
910
PCL
Instruction Word
PA0
STATUS
4.6.1 EFFECTS OF RESET The Program Counter is set upon a RESET, which
means that the PC addresses the last location in the
last page (i.e., the oscillator calibration instruction.)
After executing MOVLW XX, the PC will roll over to
location 00h and begin executing user code.
The STATUS register page preselect bits are cleared
upon a RESET, which means that page 0 is preselected.
Therefore, upon a RESET, a GOTO instruction will
automatically cause the program to jump to page 0
until the value of the page bits is altered.
4.7 Stack
PIC16C505 devices have a 12-bit wide hardware
push/pop stack.
A CALL instruction will push the current value of stack
1 into stack 2 and then push the current program
counter value , incremen ted b y one, into stac k le v e l 1. If
more than two sequential CALL’s are executed, only
the most recent two return addresses are stored.
A RETLW instruction will pop the contents of stac k le v e l
1 into the program counter and then copy stack level 2
contents into level 1. If more than two sequential
RETLW’s are executed, the stack will be filled with the
address previously stored in level 2. Note that the
W register will be loaded wi th the liter al v alue sp ecif ied
in the instruction. This is particularly useful for the
implementation of data look-up tables within the
program memory.
Note 1: There are no STATUS bits to indicate
stack overflows or stack underflow conditions.
Note 2: There are no instructions mnemonics
called PUSH or POP. These are actions that
occur from the execution of the CALL,
RETLW, and instructions.
CALL or Modify PCL Instruction
11
PC
70
1999 Microchip Technology Inc. DS40192C-page 17
87 0
910
PCL
Instruction Word
Reset to ‘0’
PA0
STATUS
PIC16C505
4.8 Indirect Data Addressing; INDF and FSR Registers
The INDF register is not a physical register.
Addressing INDF actually addresses the register
whose address is containe d in th e FS R reg ister (FSR
pointer
is a
). This is indirect addres sin g.
EXAMPLE 4-1: INDIRECT ADDRESSING
• Register file 07 contains the value 10h
• Register file 08 contains the value 0Ah
• Load the value 07 into the FSR register
• A read of the INDF register will return the value
of 10h
• Increment the value of the FSR register by one
(FSR = 08)
• A read of the INDR register now will return the
value of 0Ah.
Reading INDF itself indirectly (FSR = 0) will produce
00h. Writing to the INDF register indirectly results in a
no-operation (although STATUS bits may be affected).
A simple program to clear RAM locations 10h-1Fh
using indirect addressing is shown in Example 4-2.
EXAMPLE 4-2: HOW TO CLEAR RAM
USING INDIRECT
ADDRESSING
movlw 0x10 ;initialize po i nter
movwf FSR ; to RAM
NEXT clrf INDF ;clear INDF register
incf FSR,F ;inc pointer
btfsc FSR,4 ;all done?
goto NEXT ;NO, clear next
CONTINUE
: ;YES, continue
:
The FSR is a 5-bit wide register. It is used in
conjunction with the INDF reg ister to indirec tly addr ess
the data memory area.
The FSR<4:0> bits are used to select data memory
addresses 00h to 1Fh.
The device uses FSR<6:5> to select between banks
0:3.
FIGURE 4-4: DIRECT/INDIRECT ADDRESSING
Direct Addressing
(FSR)
6 5 4 (opcode) 0
bank select location select
00 01 10 11
1Fh 3Fh 5Fh 7Fh
Bank 0 Bank 1 Bank 2 Bank 3
Data
Memory
00h
0Fh
(1)
10h
Note 1: For register map detail see Section 4.2.
Addresses
map back to
addresses
in Bank 0.
Indirect Addressing
6 5 4 (FSR) 0
bank location select
DS40192C-page 18
1999 Microchip Technology Inc.
PIC16C505
5.0 I/O PORT
As with any other register, the I/O register can be
written and read under program control. However,
read instructions (e.g., MOVF PORTB,W) always read
the I/O pins independent of the pin’s input/output
modes. On RESET, all I/O ports are defined as input
(inputs are at hi-impedance) since the I/O control
registers are all set.
5.1 PORTB
PORTB is an 8-bit I/O register. Only the low order 6
bits are used (RB<5:0>). Bits 7 and 6 are
unimplemented an d rea d as '0 's. Please note that R B3
is an input only pin. The configuration word can set
several I/O’s to alternate functions. When acting as
alternate functions, the pins will read as ‘0’ during port
read. Pins RB0, RB1, RB 3 an d RB 4 c an b e co nfi gure d
with weak pull -ups and a lso with wake-u p on chang e.
The wake-up on change and weak pull-up functions
are not pin selectable. If pin 4 is configured as MCLR
weak pull-up is always off and wake-up on change for
this pin is not enabled.
5.2 PORTC
PORTC is an 8-bit I/O register . O nly the low o rder 6 bits
are used (RC<5:0>). Bits 7 and 6 are unimplemented
and read as ‘0’s.
5.3 TRIS Registers
5.4 I/O Interfacing
The equivalent circuit for an I/O port pin is shown in
Figure 5-1. All port pins except RB3, which is input
only, may be used for b oth inp ut and out put ope ratio ns .
For input operations, these ports are non-latching. Any
input must be present until read by an input instruction
(e.g., MOVF PORTB,W). The outp uts are latc hed and
remain unchanged until th e output latch i s re written. To
use a port pin as output, the corresponding direction
control bit in TRIS mus t be cl eared (= 0). F or us e as a n
input, the corresponding TRIS bit must be set. Any I/O
pin (except RB3) can be programmed individually as
input or output.
FIGURE 5-1: EQUIVALENT CIRCUIT
FOR A SINGLE I/O PIN
Data
Bus
,
WR
Port
W
Reg
TRIS ‘f’
Data
Latch
CK
TRIS
Latch
CK
QD
VDD
Q
QD
Q
P
N
V
SS
I/O
pin
(1)
The output driver control register is loaded with the
contents of the W register by executing the TRIS f
instruction. A '1' from a TRIS register bit puts the
corresponding output driver in a hi-impedance mode.
A '0' puts the contents of the output data latch on the
selected pins, enabling the output buffer. The
exceptions are RB3, which is input only, and RC5,
which may be controlled by the option register. See
Register 4-2.
Note: A read of the ports reads the pins, not the
output data latches. That is, if an output
driver on a pin is enabled and driven high,
but the external system is holding it low, a
read of the p o rt will i n di ca te t h a t th e pi n i s
low.
The TRIS registers are “write-only” and are set (output
drivers disabled) upon RESET.
Reset
Note 1: I/O pins have protection diodes to V
Note 2: See Table 3-1 for buffer type.
(2)
RD Port
DD and VSS.
1999 Microchip Technology Inc. DS40192C-page 19
PIC16C505
TABLE 5-1: SUMMARY OF PORT REGISTERS
Address Name B it 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 B i t 0
Value on
Power-On
Reset
Value on
All Other Resets
N/A TRISB — — I/O control registers
N/A TRISC — — I/O control registers
N/A OPTION RBWU RBPU TOCS TOSE PSA PS2 PS1 PS0
03h STATUS RBWUF — PAO TO PD Z DC C
06h PORTB — — RB5 RB4 RB3 RB2 RB1 RB0
07h PORTC — — RC5 RC4 RC3 RC2 RC1 RC0
Legend: Shaded cells not used by Port Registers, read as ‘0’,
q = depends on condition.
Note 1: If reset was due to wake-up on pin change, then bit 7 = 1. All other rests will cause bit 7 = 0.
5.5 I/O Programming Considerations
5.5.1 BI-DIRECTIONAL I/O PORTS
Some instr uctions opera te intern ally as read followed
by write operations. The BCF and BSF instructions, for
example, read the entire port into the CPU, execute
the bit operation and re-write the result. Caution must
be used when these instructions are applied to a port
where one or more pins are used as input/outputs. For
example, a BSF operation on bit5 of PORTB will cause
all eight bits of PORTB to be read into the CPU, bit5 to
be set and the PORTB v al ue to be written to the output
latches. If another bit of PORTB is used as a bidirectional I/O pin (say bit0) and it is defined as an
input at this time, the input signal present on the pin
itself would be read into the CPU and rewritten to the
data latch of this particular pin, overwriting the
previous content. As long as the pin stays in the input
— = unimplemented, read as ‘0’, x = unknown, u = unchanged,
EXAMPLE 5-1: READ-MODIFY-WRITE
INSTRUCTIONS ON AN
I/O PORT
;Initial PORTB Settings
; PORTB<5:3> Inputs
; PORTB<2:0> Outputs
;
; PORTB latch PORTB pins
; ---------- --------- BCF PORTB, 5 ;--01 -ppp --11 pppp
BCF PORTB, 4 ;--10 -ppp --11 pppp
MOVLW 007h ;
TRIS PORTB ;--10 -ppp --11 pppp
;
;Note that the user may have expected the pin
;values to be --00 pppp. The 2nd BCF caused
;RB5 to be latched as the pin value (High).
5.5.2 SUCCESSIVE OPERATIONS ON I/O PORTS
mode, no problem o ccurs. However, if bit0 is switched
into output mode later on, the content of the data latch
may now be unknown.
Example 5-1 shows the effect of two sequential readmodify-write instructions (e.g., BCF, BSF, etc.) on an
I/O port.
A pin actively outputting a high or a low should not be
driven from exter n al devices at t he sa me tim e in o rder
to change the level on this pin (“wired-or”, “wiredand”). The resulting high output currents may damage
the chip.
The actual write to an I/O port happens at the end of
an instruction cycle, whereas for reading, the data
must be valid at the beginning of the instruction cycle
(Figure 5-2). Therefore, care must be exercised if a
write followed by a read operation is carried out on the
same I/O port. The sequence of instructions should
allow the pin voltage to stabilize (load dependent)
before the next instruction causes that file to be read
into the CPU. Otherwise, the previous state of that pin
may be read into the CPU rather than the new state.
When in doubt, it is better to separate these
instructions with a NOP or another instruction not
accessing this I/O port.
--11 1111 --11 1111
--11 1111 --11 1111
1111 1111 1111 1111
0001 1xxx q00q quuu
--xx xxxx --uu uuuu
--xx xxxx --uu uuuu
(1)
DS40192C-page 20
1999 Microchip Technology Inc.
FIGURE 5-2: SUCCESSIVE I/O OPERATION
PIC16C505
Instruction
fetched
RB<5:0>
Instruction
executed
Q4
Q1 Q2
MOVWF PORTB NOP
Q3
PC PC + 1 PC + 2
Q1 Q2
Port pin
written here
MOVWF PORTB
(Write to PORTB)
Q3
Q4
Q1 Q2
(Read PORTB)
Port pin
sampled here
Q3
Q4
Q1 Q2
Q3
PC + 3
NOPMOVF PORTB,W
NOPMOVF PORTB,W
Q4
This example shows a write to PORTB
followed by a read from PORTB.
Data setup time = (0.25 T
where: T
CY = instruction cycle.
PD = propagation delay
T
CY – TPD)
Therefore, at higher clock frequencies, a
write followed by a read may be problematic.
1999 Microchip Technology Inc. DS40192C-page 21
PIC16C505
NOTES:
DS40192C-page 22
1999 Microchip Technology Inc.
PIC16C505
6.0 TIMER0 MODULE AND TMR0 REGISTER
The Timer0 module has the following features:
• 8-bit timer/counter register, TMR0
- Readable and writable
• 8-bit software programmable prescaler
• Internal or external clock select
- Edge select for external clock
Figure 6-1 is a simplified block diagram of the Timer0
module.
Timer mode is selected by clearing the T0CS bit
(OPTION<5>). In timer mode, the Timer0 module will
increment e very instruction cycle (witho ut prescaler). If
TMR0 register is written, the increment is inhibited for
the following two cycles (Figure 6-2 and Figure 6-3).
The user can work around this by writing an adjusted
value to the TMR0 register.
FIGURE 6-1: TIMER0 BLOCK DIAGRAM
RC5/T0CKI
Pin
T0SE
FOSC/4
0
T0CS
1
(1)
Programmable
Prescaler
PS2, PS1, PS0
Counter mode is selected by setting the T0CS bit
(OPTION<5>). In this mode, Timer0 will increment
either on every rising or falling edge of pin T0CKI. The
T0SE bit (OPTION<4>) determines the source edge.
Clearing the T0SE bit selects the rising edge.
Restrictions on the external clock input are discussed
in detail in Section 6.1.
The prescaler may be used by either the Timer0
module or the Watchdog Timer, but not both. The
prescaler assignment is controlled in software by the
control bit PSA (OPTION<3>). Clearing the PSA bit
will assign the prescaler to Timer0. The prescaler is
not readable or writable. When the prescaler is
assigned to the Timer0 module , prescale v alue s of 1:2,
1:4,..., 1:256 are selectable. Section 6.2 details the
operation of the prescale r.
A summary of registers associated with the Timer0
module is found in Table 6-1.
Data Bus
PSout
1
(2)
3
0
PSA
(1)
(1)
Sync with
Internal
Clocks
CY delay)
(2 T
TMR0 reg
PSout
Sync
8
Note1: Bits T0CS, T0SE, PSA, PS2, PS1 and PS0 are located in the OPTION register.
2: The prescaler is shared with the Watchdog Timer (Figure 6-5).
1999 Microchip Technology Inc. DS40192C-page 23
PIC16C505
FIGURE 6-2: TIMER0 TIMING: INTERNAL CLOCK/NO PRESCALE
PC
(Program
Counter)
Instruction
Fetch
Timer0
Instruction
Executed
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
PC-1
T0
PC PC+1 PC+2 PC+3 PC+4 PC+5 PC+6
MOVWF TMR0
T0+1 T0+2 NT0
MOVF TMR0,W MOVF TMR0,W MOVF TMR0,W MOVF TMR0,W MOVF TMR0,W
NT0+1 NT0+2
Write TMR0
executed
Read TMR0
reads NT0
Read TMR0
reads NT0
Read TMR0
reads NT0
Read TMR0
reads NT0 + 1
Read TMR0
reads NT0 + 2
FIGURE 6-3: TIMER0 TIMING: INTERNAL CLOCK/PRESCALE 1:2
PC
(Program
Counter)
Instruction
Fetch
Timer0
Instruction
Execute
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
PC-1
T0 NT0+1
PC PC+1 PC+2 PC+3 PC+4 PC+5 PC+6
MOVWF TMR0
T0+1
MOVF TMR0,W MOVF TMR0,W MOVF TMR0,W MOVF TMR0,W MOVF TMR0,W
NT0
Write TMR0
executed
Read TMR0
reads NT0
Read TMR0
reads NT0
Read TMR0
reads NT0
Read TMR0
reads NT0
Read TMR0
reads NT0 + 1
TABLE 6-1: REGISTERS ASSOCIATED WITH TIMER0
Value on
Power-On
Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Reset
01h TMR0 Timer0 - 8-bit real-time clock/counter xxxx xxxx uuuu uuuu
N/A OPTION
N/A TRISC
Legend: Shaded cells not used by Timer0,
RBWU RBPU T0CS T0SE PSA PS2 PS1 PS0 1111 1111 1111 1111
— —
RC5
- = unimplemented, x = unknown, u = unchanged.
RC4 RC3 RC2 RC1 RC0
--11 1111 --11 1111
Val ue on
All Other
Resets
T0
DS40192C-page 24
1999 Microchip Technology Inc.
PIC16C505
6.1 Using Timer0 with an External Clock
When an external clock input is used for Timer0, it
must meet certain requirements. The external clock
requirement is due to internal phase clock (TOSC)
synchronization. Also, there is a delay in the actual
incrementing of Timer0 after synchronization.
6.1.1 EXTERNAL CLOCK SYNCHRONIZATION
When no prescaler is used, the external clock input is
the same as the prescaler output. The synchronization
of T0CKI with the internal phase clocks is
accomplished by sampling the prescaler output on the
Q2 and Q4 cycles of the internal phase clocks
(Figure 6-4). Therefore, it is necessary for T0CKI to be
high for at least 2T
and low for at least 2T
20 ns). Refer to the electrical specification of the
desired device.
OSC (and a small RC delay of 20 ns)
OSC (and a small RC delay of
When a presca ler is used, the external clock in put is
divided by the asynchronous ripple counter-type
prescaler, so that the prescaler output is symmetrical.
For the external clock to meet the sampling
requirement, the ripple counter must be taken into
account. Therefore, it is neces sa ry f or T0 C KI to have a
period of at least 4T
OSC (and a small RC delay of
40 ns) divided by the prescaler value. The only
requirement on T0CKI high and low time is that they
do not violate the minimum pulse width requirement of
10 ns. Refer to parameters 40, 41 and 42 in the
electrical specification of the desired device.
6.1.2 TIMER0 INCREMENT DELAY Since the prescaler output is synchronized with the
internal clocks, there is a small delay from the time the
external clock edge occurs to the time the Timer0
module is actually incremented. Figure 6-4 shows the
delay from the external clock edge to the timer
incrementing.
FIGURE 6-4: TIMER0 TIMING WITH EXTERNAL CLOCK
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
External Clock Input or
Prescaler Output
External Clock/Prescaler
Output After Sa mpling
Increment Timer0 (Q4)
Note 1:
Delay from clock input change to Timer0 increment is 3Tosc to 7Tosc. (Duration of Q = Tosc).
Therefore, the error in measuring the interval between two edges on Timer0 input = ± 4Tosc max.
2:
External clock if no prescaler selected; prescaler output otherwise.
3:
The arrows indicate the points in time where sampling occurs.
(2)
Timer0
(3)
(1)
T0 T0 + 1 T0 + 2
Small pulse
misses sampling
1999 Microchip Technology Inc. DS40192C-page 25
PIC16C505
6.2 Prescaler
An 8-bit counter is available as a prescaler for the
Timer0 module or as a postscaler for the Watchdog
Timer (WDT), respectively (Section 7.6). For simplicity,
this counter is being referred to as “prescaler”
throughout this data sheet. Note that the prescaler
may be used by either the Timer0 module or the WDT,
but not both. Thus, a prescaler assignment for the
Timer0 module means that there is no prescaler for
the WDT, and vice-versa.
The PSA and PS<2:0> bits (OPTION<3:0>) determine
prescaler assignment and prescale ratio.
When assigned to the Timer0 module, all instructions
writing to the TMR0 register (e.g., CLRF 1,
MOVWF 1, BSF 1,x, etc.) will clear the prescaler.
When assigned to WDT, a CLRWDT instruction will
clear the prescaler along with the WDT. The prescaler
is neither readable nor writable. On a RESET, the
prescaler contains all '0's.
6.2.1 SWITCHING PRESCALER ASSIGNMENT The prescaler assignment is fully under software
control (i.e., it can be changed “on-the-fly” during
program execution). To avoid an unintended device
RESET, the following instruction sequence
(Example 6-1) must be executed when changing the
prescaler assignment from Timer0 to the WDT.
EXAMPLE 6-1: CHANGING PRESCALER
(TIMER0→WDT)
1.CLRWDT ;Clear WDT
2.CLRF TMR0 ;Clear TMR0 & Prescaler
3.MOVLW '00xx1111’b ;These 3 lines (5, 6, 7)
4.OPTION ; are required only if
; desired
5.CLRWDT ;PS<2:0> are 000 or 001
6.MOVLW '00xx1xxx’b ;Set Postscaler to
7.OPTION ; desired WDT rate
To change prescaler from the WDT to the Timer0
module, use the sequence shown in Example 6-2.
This sequence must be used even if the WDT is
disabled. A CLRWDT instruction should be executed
before switching the prescaler.
EXAMPLE 6-2: CHANGING PRESCALER
(WDT→TIMER0)
CLRWDT ;Clear WDT and
;prescaler
MOVLW 'xxxx0xxx' ;Select TMR0, new
;prescale value and
;clock source
OPTION
FIGURE 6-5: BLOCK DIAGRAM OF THE TIMER0/WDT PRESCALER
TCY ( = Fosc/4)
0
1
Sync
2
Cycles
PS<2:0>
Watchdog
Timer
WDT Enable bit
1
0
0
M
1
PSA
8-bit Prescaler
8
8 - to - 1MUX
0
1
MUX
PSA
TMR0 reg
Note: T0CS, T0SE, PSA, PS<2:0> are bits in the OPTION register.
DS40192C-page 26
WDT
Time-Out
1999 Microchip Technology Inc.
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