PIC12C672 MICROCHIP [Microchip Technology], PIC12C672 Datasheet - Page 20

• Manufacturer Part Number: PIC12C672
• Description: 8-Pin, 8-Bit CMOS Microcontroller with EEPROM Data Memory
• Manufacturer: MICROCHIP [Microchip Technology]

PIC12CE5XX
5.4
5.4.1
Some instructions operate internally as read followed
by write operations. The BCF and BSF instructions, 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 BSF operation on bit5 of GPIO will cause all
eight bits of GPIO to be read into the CPU, bit5 to be
set and the GPIO value to be written to the output
latches. If another bit of GPIO is used as a 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.
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:
DS40172A-page 20
Instruction
Instruction
executed
GP5:GP0
fetched
I/O Programming Considerations
BI-DIRECTIONAL I/O PORTS
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
MOVWF GPIO
PC
SUCCESSIVE I/O OPERATION
MOVF GPIO,W
MOVWF GPIO
(Write to
GPIO)
Port pin
written here
PC + 1
MOVF GPIO,W
(Read
Port pin
sampled here
GPIO)
NOP
PC + 2
Preliminary
PC + 3
NOP
NOP
EXAMPLE 5-1:
;Initial GPIO Settings
; GPIO<5:3> Inputs
; GPIO<2:0> Outputs
;
;
;
;
;Note that the user may have expected the pin
;values to be --00 pppp. The 2nd BCF caused
;GP5 to be latched as the pin value (High).
5.4.2
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.
BCF
BCF
MOVLW 007h
TRIS
SUCCESSIVE OPERATIONS ON I/O
PORTS
GPIO, 5
GPIO, 4
GPIO
This example shows a write to GPIO followed
by a read from GPIO.
Data setup time = (0.25 T
where: T
Therefore, at higher clock frequencies, a
write followed by a read may be problematic.
T
READ-MODIFY-WRITE
INSTRUCTIONS ON AN
I/O PORT
CY
PD
;--01 -ppp
;--10 -ppp
;--10 -ppp
;
= instruction cycle.
= propagation delay
GPIO latch
----------
1997 Microchip Technology Inc.
CY
--11 pppp
--11 pppp
--11 pppp
– T
----------
GPIO pins
PD
)