PIC16C505T-20/SL データシートの表示（PDF） - Microchip Technology

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PIC16C505T-20/SL

14-Pin, 8-Bit CMOS Microcontroller

Microchip Technology 

PIC16C505

TABLE 5-1: SUMMARY OF PORT REGISTERS

Address

Name

Bit 7

Bit 6 Bit 5

Bit 4 Bit 3 Bit 2 Bit 1 Bit 0

Value on

Power-On

Reset

Value on

All Other Resets

N/A

TRISB

—

— I/O control registers

--11 1111 --11 1111

N/A

TRISC

—

— I/O control registers

--11 1111 --11 1111

N/A

OPTION

RBWU RBPU TOCS TOSE PSA PS2 PS1 PS0 1111 1111

1111 1111

03h

STATUS

RBWUF —

PAO

TO

PD

Z

DC

C

0001 1xxx q00q quuu(1)

06h

PORTB

—

—

RB5

RB4 RB3 RB2 RB1 RB0 --xx xxxx

--uu uuuu

07h

PORTC

—

—

RC5

RC4 RC3 RC2 RC1 RC0 --xx xxxx

--uu uuuu

Legend: Shaded cells not used by Port Registers, read as ‘0’, — = unimplemented, read as ‘0’, x = unknown, u = unchanged,

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 instructions operate internally 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 value to be written to the output

latches. If another bit of PORTB is used as a bi-

directional 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

mode, no problem occurs. 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 read-

modify-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 external devices at the same time in order

to change the level on this pin (“wired-or”, “wired-

and”). The resulting high output currents may damage

the chip.

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

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.

DS40192C-page 20

© 1999 Microchip Technology Inc.

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