PIC16C925T-SSLASHCL datasheet(41/182 Pages) MICROCHIP

41 / 182 page

 2001 Microchip Technology Inc.

Preliminary

DS39544A-page 39

PIC16C925/926

4.8

I/O Programming Considerations

4.8.1

BI-DIRECTIONAL I/O PORTS

Any instruction which 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 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 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 con-

tents of the data latch may now be unknown.

Reading the 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

(e.g. BCF, BSF) 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 4-8 shows the effect of two sequential

read-modify-write instructions 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 4-8:

READ-MODIFY-WRITE

INSTRUCTIONS ON AN

I/O PORT

4.8.2

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 4-10). 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 depen-

dent) before the next instruction, which causes that file

to be read into the CPU, is executed. 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 4-10:

SUCCESSIVE I/O OPERATION

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

;

PORTB<3:0> Outputs

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

;not connected to other circuitry

;

PORT latch

PORT pins

;

----------

---------

BCF PORTB,

; 01pp pppp

11pp pppp

BCF PORTB,

; 10pp pppp

11pp pppp

BCF STATUS, RP1 ; Select Bank1

BSF STATUS, RP0 ;

BCF TRISB,

; 10pp pppp

11pp pppp

BCF TRISB,

; 10pp pppp

10pp pppp

;

;Note that the user may have expected the

;pin values to be 00pp ppp. The 2nd BCF

;caused RB7 to be latched as the pin value

;(high).

PC + 1

PC + 2

PC + 3

Q1 Q2 Q3 Q4

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

TPD

Note:

This example shows a write to PORTB

followed by a read from PORTB.

Note that:

data setup time = (0.25TCY - TPD)

where TCY

instruction cycle

TPD

propagation delay

Therefore, at higher clock frequencies,

a write followed by a read may be

problematic.

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