PIC17C756A MICROCHIP [Microchip Technology], PIC17C756A Datasheet - Page 93

• Manufacturer Part Number: PIC17C756A
• Description: High-Performance 8-bit CMOS EPROM Microcontrollers with 10-bit A/D
• Manufacturer: MICROCHIP [Microchip Technology]

10.10 I/O Programming Considerations
10.10.1
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 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 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.
Example 10-10 shows the possible effect of two
sequential read-modify-write instructions on an I/O port.
2000 Microchip Technology Inc.
BI-DIRECTIONAL I/O PORTS
EXAMPLE 10-10: READ-MODIFY-WRITE
; Initial PORT settings: PORTB<7:4> Inputs
;
; PORTB<7:6> have pull-ups and are
; not connected to other circuitry
;
;
;
;
;
; 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).
Note:
BCF
BCF
BCF
BCF
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.
PORTB, 7
PORTB, 6
DDRB, 7
DDRB, 6
INSTRUCTIONS ON AN
I/O PORT
PIC17C7XX
; 01pp pppp
; 10pp pppp
; 10pp pppp
; 10pp pppp
PORT latch
----------
PORTB<3:0> Outputs
DS30289B-page 93
PORT pins
---------
11pp pppp
11pp pppp
11pp pppp
10pp pppp