PIC16C5X
5.6
I/O Programming Considerations
BI-DIRECTIONAL I/O PORTS
EXAMPLE 5-1: READ-MODIFY-WRITE
INSTRUCTIONS ON AN
I/O PORT
;Initial PORT Settings
; PORTB<7:4> Inputs
5.6.1
Some instructions operate internally as read followed
by write operations. The BCFand BSFinstructions, for
example, read the entire port into the CPU, execute
the bit operation and re-write the result. Caution must
be used when these instructions are applied to a port
where one or more pins are used as input/outputs. For
example, a BSFoperation on bit5 of PORTB will cause
all eight bits of PORTB to be read into the CPU, bit5 to
be set and the PORTB value to be written to the output
; PORTB<3:0> Outputs
;PORTB<7:6> have external pull-ups and are
;not connected to other circuitry
;
;
;
PORT latch PORT pins
---------- ----------
BCF
BCF
MOVLW 03Fh
TRIS PORTB
PORTB, 7
PORTB, 6
;01pp pppp
;10pp pppp
;
11pp pppp
11pp pppp
latches. If another bit of PORTB is used as
a
;10pp pppp
10pp pppp
bi-directional I/O pin (say 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.
;
;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).
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 5-2). 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
allow the pin voltage to stabilize (load dependent)
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 5-1 shows the effect of two sequential
read-modify-write instructions (e.g., BCF, BSF, etc.) on
an I/O port.
A pin actively outputting a high or a low 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.
FIGURE 5-2: SUCCESSIVE I/O OPERATION
Q4
Q4
Q4
Q1 Q2
Q4
Q3
Q3
Q3
Q3
Q1 Q2
PC
Q1 Q2
Q1 Q2
PC + 3
NOP
PC + 1
PC + 2
NOP
Instruction
fetched
MOVWF PORTB MOVF PORTB,W
This example shows a write
to PORTB followed by a read
from PORTB.
RB7:RB0
Port pin
written here
Port pin
sampled here
Instruction
executed
MOVWF PORTB MOVF PORTB,W
NOP
(Write to
PORTB)
(Read
PORTB)
DS30453B-page 26
Preliminary
1998 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
