PIC16C71X
5.3
I/O Programming Considerations
EXAMPLE 5-3: READ-MODIFY-WRITE
INSTRUCTIONS ON AN I/O
PORT
5.3.1
BI-DIRECTIONAL I/O PORTS
;Initial PORT settings: PORTB<7:4> Inputs
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 BSFoperation 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 to an output, the content of the data
latch may now be unknown.
;
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
BCF PORTB, 6
BSF STATUS, RP0
BCF TRISB, 7
BCF TRISB, 6
; 01pp pppp
; 10pp pppp
;
11pp pppp
11pp pppp
; 10pp pppp
; 10pp pppp
11pp 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).
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.
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.
5.3.2
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 5-6). 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 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.
Example 5-3 shows the effect of two sequential read-
modify-write instructions on an I/O port.
FIGURE 5-6: SUCCESSIVE I/O OPERATION
Q4
Q4
Q4
Q1 Q2
Q4
Q3
Q3
Q3
Q3
Q1 Q2
PC
Q1 Q2
Q1 Q2
Note:
This example shows a write to PORTB
followed by a read from PORTB.
PC + 3
NOP
PC
Instruction
fetched
PC + 1
PC + 2
NOP
MOVWF PORTB MOVF PORTB,W
write to
PORTB
Note that:
data setup time = (0.25TCY - TPD)
RB7:RB0
where TCY = instruction cycle
TPD = propagation delay
Port pin
sampled here
TPD
Therefore, at higher clock frequencies,
a write followed by a read may be
problematic.
Instruction
executed
NOP
MOVWF PORTB
write to
MOVF PORTB,W
PORTB
DS30272A-page 30
1997 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
