PIC17C44-08SLASHL datasheet(64/240 Pages) MICROCHIP

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PIC17C4X

DS30412C-page 64

9.5

I/O Programming Considerations

9.5.1

BI-DIRECTIONAL I/O PORTS

Any instruction which writes, operates internally as a

read followed by a write operation. For example, the

BCF

and BSF instructions 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 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 it is deﬁned as an input at this time, the

input signal present on the pin itself would be read into

the CPU and re-written to the data latch of this particu-

lar pin, overwriting the previous content. As long as the

pin stays in the input mode, no problem occurs. How-

ever, if bit0 is switched into output mode later on, the

content of the data latch may now be unknown.

Reading a port 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 (BCF, BSF,

BTG

, etc.) on a port, the value of the port pins is read,

the desired operation is performed with this value, and

the value is then written to the port latch.

Example 9-5 shows the effect of two sequential

read-modify-write instructions on an I/O port.

EXAMPLE 9-5:

READ MODIFY WRITE

INSTRUCTIONS ON AN

I/O PORT

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

;

PORTB<3:0> Outputs

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

; not connected to other circuitry

;

PORT latch

PORT pins

;

----------

---------

;

BCF

PORTB, 7

01pp pppp

11pp pppp

BCF

PORTB, 6

10pp pppp

11pp pppp

;

BCF

DDRB, 7

10pp pppp

11pp pppp

BCF

DDRB, 6

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).

9.5.2

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 9-

9). 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

executing the instruction that reads the values on that

I/O port. 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.

Note:

A pin actively outputting a Low or High

should not be driven from external devices

in order to change the level on this pin (i.e.

“wired-or”, “wired-and”). The resulting high

output currents may damage the device.

FIGURE 9-9:

SUCCESSIVE I/O OPERATION

Note:

This example shows a write to PORTB

followed by a read from PORTB.

Note that:

data setup time = (0.25 TCY - TPD)

where TCY = instruction cycle.

Therefore, at higher clock

frequencies, a write followed by a

read may be problematic.

TPD = propagation delay

PC + 1

PC + 2

PC + 3

Q1 Q2 Q3 Q4

Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4

Q1 Q2 Q3 Q4

Instruction

fetched

RB7:RB0

MOVWF PORTB

write to

PORTB

NOP

Port pin

sampled here

NOP

MOVF PORTB,W

Instruction

executed

MOVWF PORTB

write to

PORTB

NOP

MOVF PORTB,W

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