CF745 データシートの表示（PDF） - Microchip Technology

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CF745

EPROM/ROM-Based 8-Bit CMOS Microcontroller Series

Microchip Technology 

PIC16C5X

5.6 I/O Programming Considerations

5.6.1 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 PORT Settings

; PORTB<7:4> Inputs

; PORTB<3:0> Outputs

;PORTB<7:6> have external pull-ups and are

;not connected to other circuitry

;

;

PORT latch PORT pins

;

---------- ----------

BCF PORTB, 7 ;01pp pppp 11pp pppp

BCF PORTB, 6 ;10pp pppp 11pp pppp

MOVLW 03Fh

;

TRIS PORTB

;10pp pppp 10pp pppp

;

;Note that the user may have expected the pin

;values to be 00pp pppp. The 2nd BCF caused

;RB7 to be latched as the pin value (High).

5.6.2 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, which causes that file to be

read into the CPU, is executed. 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.

FIGURE 5-2: SUCCESSIVE I/O OPERATION

Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4

Instruction

fetched

PC

MOVWF PORTB

PC + 1

MOVF PORTB,W

PC + 2

NOP

PC + 3

NOP

RB7:RB0

Instruction

executed

Port pin

written here

Port pin

sampled here

MOVWF PORTB MOVF PORTB,W

(Write to

(Read

PORTB)

PORTB)

NOP

This example shows a write

to PORTB followed by a read

from PORTB.

DS30453B-page 26

Preliminary

© 1998 Microchip Technology Inc.

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