PIC16F570 datasheet(31/134 Pages) MICROCHIP

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DS40001652C-page 31

PIC16F527

6.7

I/O Programming Considerations

6.7.1

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 rewrite 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 bidirec-

tional 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(e.g. PIC16F527)

6.7.2

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 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

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.

FIGURE 6-2:

SUCCESSIVE I/O OPERATION

;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 1:

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).

PC + 1

PC + 2

PC + 3

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

Instruction

Fetched

RB<5:0>

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.

Data setup time = (0.25 TCY – TPD)

where: TCY = instruction cycle.

TPD = propagation delay

Therefore, at higher clock frequencies, a

write followed by a read may be problematic.

(Read PORTB)

Port pin

written here

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