Datasheet

 1997-2013 Microchip Technology Inc. DS30561C-page 31
PIC12C67X
TABLE 5-1: SUMMARY OF PORT REGISTERS
5.4 I/O Programming Considerations
5.4.1 BI-DIRECTIONAL I/O PORTS
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 BSF operation on bit5
of GPIO will cause all eight bits of GPIO to be read into
the CPU. Then the BSF operation takes place on bit5
and GPIO is written to the output latches. If another bit
of GPIO is used as a bi-directional I/O pin (i.e., 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, overwrit-
ing 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.
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
(i.e., 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.
Example 5-1 shows the effect of two sequential read-
modify-write instructions on an I/O port.
EXAMPLE 5-1: READ-MODIFY-WRITE
INSTRUCTIONS ON AN
I/O PORT
;Initial GPIO Settings
; GPIO<5:3> Inputs
; GPIO<2:0> Outputs
;
; GPIO latch GPIO pins
; ---------- ----------
BCF GPIO, 5 ;--01 -ppp --11 pppp
BCF GPIO, 4 ;--10 -ppp --11 pppp
MOVLW 007h ;
TRIS GPIO ;--10 -ppp --10 pppp
;
;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).
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.
Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Value on
Power-on
Reset
Value on
all other
Resets
85h TRIS
— — GPIO Data Direction Register --11 1111 --11 1111
81h OPTION GPPU
INTEDG T0CS T0SE PSA PS2 PS1 PS0 1111 1111 1111 1111
03h STATUS
IRP
(1)
RP1
(1)
RP0 TO PD ZDCC0001 1xxx 000q quuu
05h GPIO
SCL
(2)
SDA
(2)
GP5 GP4 GP3 GP2 GP1 GP0 11xx xxxx 11uu uuuu
Legend: Shaded cells not used by Port Registers, read as ‘0’, — = unimplemented, read as '0', x = unknown, u = unchanged,
q = see tables in Section 9.4 for possible values.
Note 1: The IRP and RP1 bits are reserved on the PIC12C67X; always maintain these bits clear.
2: The SCL and SDA bits are unimplemented on the PIC12C671 and PIC12C672.