PIC16F-ESLASHML datasheet(46/178 Pages) MICROCHIP

46 / 178 page

PIC16F627A/628A/648A

DS40044F-page 44

5.3

I/O Programming Considerations

5.3.1

BIDIRECTIONAL I/O PORTS

Any instruction that writes operates internally as a read

followed by a write operation. The BCF and BSF

instructions, for example, read the register into the

CPU, execute the bit operation and write the result

back to the register. Caution must be used when these

instructions are applied to a port with both inputs and

outputs defined. For example, a BSF operation on bit 5

of PORTB will cause all eight bits of PORTB to be read

into the CPU. Then the BSF operation takes place on

bit 5 and PORTB is written to the output latches. If

another bit of PORTB is used as a bidirectional I/O pin

(e.g., bit 0) 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 bit 0 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 followed 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. Other-

wise, 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 5-16:

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

Instruction

fetched

Q4 Q1

Q2 Q3

PC + 1

PC + 2

PC + 3

MOVWF

PORTB

Write to PORTB

MOVF

PORTB, W

Read to PORTB

NOP

TPD

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.

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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