PIC12F508/509/16F505
EXAMPLE 5-1:
READ-MODIFY-WRITE
INSTRUCTIONS ON AN
I/O PORT(e.g. PIC16F505)
5.5
I/O Programming Considerations
5.5.1
BIDIRECTIONAL I/O PORTS
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 BSFoperation on bit 5 of PORTB/GPIO will
cause all eight bits of PORTB/GPIO to be read into the
CPU, bit 5 to be set and the PORTB/GPIO value to be
written to the output latches. If another bit of PORTB/
GPIO is used as a bidirectional I/O pin (say bit 0) 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 bit 0 is
switched into Output mode later on, the content of the
data latch may now be unknown.
;Initial PORTB Settings
;PORTB<5:3> Inputs
;PORTB<2:0> Outputs
;
;
;
PORTB latch PORTB pins
----------
PORTB, 5 ;--01 -ppp
PORTB, 4 ;--10 -ppp
----------
--11 pppp
--11 pppp
BCF
BCF
MOVLW 007h;
TRIS PORTB
;--10 -ppp
--11 pppp
;
Note 1: The user may have expected the pin values to
be ‘--00 pppp’. The 2nd BCFcaused RB5 to
be latched as the pin value (High).
5.5.2
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
causes that file to be read into the CPU. 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 NOPor 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.
FIGURE 5-2:
SUCCESSIVE I/O OPERATION (PIC16F505 Shown)
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
PC + 3
PC
MOVWFPORTB
PC + 1
PC + 2
This example shows a write to PORTB
followed by a read from PORTB.
Instruction
Fetched
MOVFPORTB, W
NOP
NOP
Data setup time = (0.25 TCY – TPD)
where: TCY = instruction cycle.
TPD = propagation delay
RB<5:0>
Port pin
written here
Port pin
sampled here
Therefore, at higher clock frequencies, a
write followed by a read may be problematic.
Instruction
Executed
MOVWF PORTB
MOVF PORTB,W
NOP
(Write to PORTB)
(Read PORTB)
© 2007 Microchip Technology Inc.
Preliminary
DS41236C-page 31
MICROCHIP [ MICROCHIP ]
