PIC16CE62X
5.3
I/O Programming Considerations
BI-DIRECTIONAL I/O PORTS
EXAMPLE 5-2: READ-MODIFY-WRITE
INSTRUCTIONS ON AN
5.3.1
I/O PORT
;Initial PORT settings: PORTB<7:4> Inputs
;
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 BSFoperation on bit5
of PORTB will cause all eight bits of PORTB to be read
into the CPU. Then the BSF operation takes place on
bit5 and PORTB is written to the output latches. If
another bit of PORTB is used as a bidirectional I/O pin
(e.g., 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 re-written 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.
;
PORTB<3:0> Outputs
;PORTB<7:6> have external pull-up and are not
;connected to other circuitry
;
;
;
PORT latch PORT pins
---------- ----------
BCF PORTB, 7
BCF PORTB, 6
BSF STATUS,RP0
BCF TRISB, 7
BCF TRISB, 6
;01pp pppp 11pp pppp
;10pp pppp 11pp pppp
;
;10pp pppp 11pp pppp
;10pp pppp 10pp pppp
;
;Note that the user may have expected the pin
;values to be 00pp pppp. The 2nd BCF caused
;RB7 to be latched as the pin value (High).
5.3.2
SUCCESSIVE OPERATIONS ON I/O PORTS
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
(ex. 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.
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 5-7). 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 be
such to 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 an NOP or another
instruction not accessing this I/O port.
Example 5-2 shows the effect of two sequential
read-modify-write instructions (ex., BCF, BSF, etc.) on
an I/O port.
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.
FIGURE 5-7: SUCCESSIVE I/O OPERATION
Note:
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
This example shows write to PORTB
followed by a read from PORTB.
PC + 1
MOVWF PORTB MOVF PORTB, W
PC + 2
NOP
PC + 3
NOP
PC
Instruction
fetched
PC
Note that:
Write to
PORTB
Read PORTB
data setup time = (0.25 TCY - TPD)
where TCY = instruction cycle and
TPD = propagation delay of Q1 cycle
to output valid.
RB7:RB0
RB <7:0>
Port pin
sampled here
Therefore, at higher clock frequencies,
a
write followed by a read may be
problematic.
TPD
Execute
MOVWF
PORTB
Execute
MOVF
PORTB, W
Execute
NOP
DS40182A-page 28
Preliminary
1998 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
