PIC12F519-ESLASHMS datasheet(32/96 Pages) MICROCHIP

32 / 96 page

PIC12F519

DS41319B-page 30

6.4

I/O Programming Considerations

6.4.1

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 GPIO will cause

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

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

latches. If another bit of 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, 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

6.4.2

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 6-6).

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 6-6:

SUCCESSIVE I/O OPERATION

;Initial GPIO Settings

;GPIO<5:3> Inputs

;GPIO<2:0> Outputs

;

GPIO latch

GPIO pins

;

----------

BCF

GPIO, 5

;--01 -ppp

--11 pppp

BCF

GPIO, 4

;--10 -ppp

--11 pppp

MOVLW

007h;

TRIS

GPIO

;--10 -ppp

--11 pppp

;

Note

The user may have expected the pin values to

be ‘--00 pppp’. The 2nd BCF caused GP5 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

GP<5:0>

MOVWF GPIO

NOP

Port pin

sampled here

NOP

MOVF GPIO

, W

Instruction

Executed

MOVWF GPIO

(Write to GPIO)

NOP

MOVF PORTB,W

This example shows a write to GPIO followed

by a read from GPIO.

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

PIC12F519-E/MS Datasheet(PDF) 32 Page - Microchip Technology

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