PIC16F5X datasheet(33/90 Pages) MICROCHIP

33 / 90 page

 2004 Microchip Technology Inc.

DS41213C-page 31

PIC16F5X

6.8

I/O Programming Considerations

6.8.1

BIDIRECTIONAL 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 bit 5 of PORTB will cause

all eight bits of PORTB to be read into the CPU, bit 5 to

be set and the PORTB value to be written to the output

latches. If another bit of PORTB is used as a

bidirectional I/O pin (say bit ‘0’), 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 bit ‘0’ is switched into

Output mode later on, the content of the data latch may

now be unknown.

Example 6-1 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 6-1:

READ-MODIFY-WRITE

INSTRUCTIONS ON AN I/O

PORT

6.8.2

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 (see

Figure 6-2). 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 6-2:

SUCCESSIVE I/O OPERATION

;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 H'3F'

;

TRIS

PORTB

;10pp pppp

10pp pppp

;

;Note that the user may have expected the

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

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

PC + 1

PC + 2

PC + 3

Q1 Q2 Q3 Q4

Instruction

fetched

RB<7:0>

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

Fetch INST (PC)

Execute INST (PC - 1) Fetch INST (PC + 1)

Execute INST (PC)

Fetch INST (PC + 2)

Execute INST (PC + 1) Fetch INST (PC + 3)

Execute INST (PC + 2)

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