PIC16F84A datasheet(60/337 Pages) MICROCHIP

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PIC16C6X

DS30234D-page 60

5.6

I/O Programming Considerations

5.6.1

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 content

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

(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 5-4 shows the effect of two sequential

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

Applicable Devices

61 62 62A R62 63 R63 64 64A R64 65 65A R65 66 67

EXAMPLE 5-4:

READ-MODIFY-WRITE

INSTRUCTIONS ON AN

I/O PORT

;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, 7

; 01pp pppp

11pp pppp

BCF PORTB, 6

; 10pp 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).

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.

5.6.2

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 5-10). 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 5-10: SUCCESSIVE I/O OPERATION

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

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 prob-

lematic.

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