PIC17C75X
10.8
I/O Programming Considerations
EXAMPLE 10-8: READ MODIFY WRITE
INSTRUCTIONS ON AN
I/O PORT
10.8.1 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 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 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.
; 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
BCF
PORTB, 7
PORTB, 6
; 01pp pppp
; 10pp pppp
11pp pppp
11pp pppp
BCF
BCF
DDRB, 7
DDRB, 6
; 10pp pppp
; 10pp pppp
11pp 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).
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.
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.
Example 10-8 shows the effect of two sequential
read-modify-write instructions on an I/O port.
10.8.2 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 10-17). 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 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.
FIGURE 10-17: SUCCESSIVE I/O OPERATION
Note:
Q4
Q4
Q4
Q1 Q2
Q4
Q3
Q3
Q3
Q3
Q1 Q2
Q1 Q2
Q1 Q2
This example shows a write to PORTB
followed by a read from PORTB.
PC + 3
NOP
PC
PC + 1
PC + 2
NOP
Instruction
fetched
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
Therefore, at higher clock frequencies,
a write followed by a read may be
problematic.
Instruction
executed
MOVWF PORTB MOVF PORTB,W
NOP
write to
PORTB
1997 Microchip Technology Inc.
Preliminary
DS30264A-page 83
MICROCHIP [ MICROCHIP ]
