PIC12C5XX
TABLE 5-1:
SUMMARY OF PORT REGISTERS
Value on
Power-On
Reset
Value on
All Other Resets
Address
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3 Bit 2 Bit 1 Bit 0
N/A
N/A
03H
TRIS
—
—
--11 1111
1111 1111
0001 1xxx
--11 1111
1111 1111
OPTION
GPWU
GPPU
T0CS
PAO
T0SE
TO
PSA
PD
PS2
Z
PS1
DC
PS0
C
(1)
STATUS
GPWUF
—
q00q quuu
GPIO
(PIC12C508/
PIC12C509/
PIC12C508A/
PIC12C509A/
PIC12CR509A)
--xx xxxx
11xx xxxx
--uu uuuu
11uu uuuu
06h
06h
—
—
GP5
GP5
GP4
GP4
GP3
GP3
GP2
GP2
GP1
GP1
GP0
GP0
GPIO
(PIC12CE518/
PIC12CE519)
SCL
SDA
Legend: Shaded cells not used by Port Registers, read as ‘0’, — = unimplemented, read as '0', x= unknown, u= unchanged,
q = see tables in Section 8.7 for possible values.
Note 1: If reset was due to wake-up on change, then bit 7 = 1. All other resets will cause bit 7 = 0.
5.4
I/O Programming Considerations
EXAMPLE 5-1: READ-MODIFY-WRITE
INSTRUCTIONS ON AN
I/O PORT
5.4.1
BI-DIRECTIONAL I/O PORTS
;Initial GPIO Settings
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 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.
;
;
;
;
;
GPIO<5:3> Inputs
GPIO<2:0> Outputs
GPIO latch GPIO pins
---------- ----------
BCF
BCF
MOVLW 007h
TRIS GPIO
GPIO, 5
GPIO, 4
;--01 -ppp
;--10 -ppp
;
--11 pppp
--11 pppp
;--10 -ppp
--11 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).
5.4.2
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.
DS40139E-page 22
1999 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
