CF745 datasheet(26/217 Pages) MICROCHIP

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PIC16C5X

DS30453B-page 26

Preliminary

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

PC + 1

PC + 2

PC + 3

Q1 Q2 Q3 Q4

Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4

Q1 Q2 Q3 Q4

Instruction

fetched

RB7:RB0

MOVWF PORTB

NOP

Port pin

sampled here

NOP

MOVF PORTB,W

Instruction

executed

MOVWF PORTB

(Write to

PORTB)

NOP

MOVF PORTB,W

This example shows a write

to PORTB followed by a read

from PORTB.

(Read

PORTB)

Port pin

written here

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