PIC16F010-ESLASHMCSQTP datasheet(39/116 Pages) MICROCHIP

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Preliminary

DS41268C-page 37

PIC12F510/16F506

5.5

I/O Programming Considerations

5.5.1

BIDIRECTIONAL I/O PORTS

Some instructions operate internally as read followed

by write operations. For example, the BCF and BSF

instructions 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/GPIO will

cause all eight bits of PORTB/GPIO to be read into the

CPU, bit 5 to be set and the PORTB/GPIO value to be

written to the output latches. If another bit of PORTB/

GPIO 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, overwrit-

ing 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 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(e.g., PIC16F506)

5.5.2

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 5-14). 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 the 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 5-14:

SUCCESSIVE I/O OPERATION (PIC16F506)

;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:

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

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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