16F628 datasheet(46/168 Pages) MICROCHIP

46 / 168 page

PIC16F627A/628A/648A

DS40044A-page 44

Preliminary

 2002 Microchip Technology Inc.

5.3

I/O Programming Considerations

5.3.1

BI-DIRECTIONAL I/O PORTS

Any instruction that writes, operates internally as a read

followed by a write operation. The

BCF and BSF instruc-

tions, for example, read the register into the CPU, exe-

cute the bit operation and write the result back to the

are applied to a port with both inputs and outputs

defined. For example, a

BSF operation on bit5 of

PORTB will cause all eight bits of PORTB to be read

into the CPU. Then the

BSF operation takes place on

bit5 and PORTB is written to the output latches. If

another bit of PORTB is used as a bi-directional I/O pin

(e.g., bit0) and 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.

Reading a port register reads the values of the port

pins. Writing to the port register writes the value to the

port latch. When using read-modify-write instructions

(ex.

BCF, BSF, etc.) on a port, the value of the port pins

is read, the desired operation is done to this value, and

this value is then written to the port latch.

Example 5-2 shows the effect of two sequential read-

modify-write instructions (ex.,

BCF, BSF, etc.) on an

I/O port.

A pin actively outputting a Low or High 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 5-2:

READ-MODIFY-WRITE

INSTRUCTIONS ON AN

I/O PORT

5.3.2

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-

16). Therefore, care must be exercised if a write fol-

lowed by a read operation is carried out on the same I/

O port. The sequence of instructions should be such to

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 previ-

ous state of that pin may be read into the CPU rather

than the new state. When in doubt, it is better to sepa-

rate these instructions with a NOP or another instruction

not accessing this I/O port.

FIGURE 5-16:

SUCCESSIVE I/O OPERATION

;Initial PORT settings:PORTB<7:4> Inputs

;

PORTB<3:0> Outputs

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

;not connected to other circuitry

;

PORT latchPORT Pins

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

BCF STATUS, RP0

;

BCF PORTB, 7

;01pp pppp 11pp pppp

BSF STATUS, RP0

;

BCF TRISB, 7

;10pp pppp 11pp pppp

BCF TRISB, 6

;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).

Q2 Q3 Q4

Instruction

fetched

Q2 Q3 Q4

Q2 Q3 Q4 Q1

Q2 Q3 Q4

PC + 1

PC + 2

PC + 3

MOVWF

PORTB

Write to PORTB

MOVF

PORTB, W

Read to PORTB

NOP

Execute

MOVWF

PORTB

Execute

MOVF

PORTB, W

Port pin

sampled here

Execute

NOP

Note

1: This example shows write to PORTB followed by a read from PORTB.

2: Data setup time = (0.25 TCY - TPD) where TCY = instruction cycle and TPD = propagation delay of Q1 cycle to output valid.

Therefore, at higher clock frequencies, a write followed by a read may be problematic.

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