RFPIC12C509AG Datasheet, PDF (26/104 Pages) Microchip Technology – 18/20-Pin 8-Bit CMOS Microcontroller with UHF ASK/FSK Transmitter

rfPIC12C509AG/509AF
TABLE 5-1: SUMMARY OF PORT REGISTERS
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
N/A
TRIS
—
—
--11 1111
--11 1111
N/A
OPTION
GPWU GPPU T0CS T0SE PSA PS2 PS1 PS0 _{1111 1111}
1111 1111
03H
STATUS
GPWUF
—
PAO
TO
PD
Z
DC
C
0001 1xxx
q00q quuu⁽¹⁾
06h
Legend:
GPIO
—
—
GP5
GP4 GP3 GP2 GP1 GP0 --xx xxxx
--uu uuuu
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
5.4.1 BI-DIRECTIONAL I/O PORTS
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-direc-
tional 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.
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 --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 volt-
age 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 instruc-
tions with a NOP or another instruction not accessing
this I/O port.
DS70031A-page 24
Preliminary
© 2001 Microchip Technology Inc.