PIC16C926T-IL View Datasheet(PDF) - Microchip Technology
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PIC16C926T-IL Datasheet PDF : 182 Pages
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PIC16C925/926
4.8 I/O Programming Considerations
4.8.1 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 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 bi-directional 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 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 con-
tents 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
(e.g. BCF, BSF) 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 4-8 shows the effect of two sequential
read-modify-write instructions 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.
EXAMPLE 4-8:
READ-MODIFY-WRITE
INSTRUCTIONS ON AN
I/O PORT
;Initial PORT settings: PORTB<7:4> Inputs
;
PORTB<3:0> Outputs
;PORTB<7:6> have external pull-ups and are
;not connected to other circuitry
;
;
PORT latch PORT pins
;
---------- ---------
BCF PORTB, 7 ; 01pp pppp 11pp pppp
BCF PORTB, 6 ; 10pp pppp 11pp pppp
BCF STATUS, RP1 ; Select Bank1
BSF STATUS, RP0 ;
BCF TRISB, 7 ; 10pp pppp 11pp pppp
BCF TRISB, 6 ; 10pp pppp 10pp pppp
;
;Note that the user may have expected the
;pin values to be 00pp ppp. The 2nd BCF
;caused RB7 to be latched as the pin value
;(high).
4.8.2
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 4-10). 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 depen-
dent) 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.
FIGURE 4-10:
SUCCESSIVE I/O OPERATION
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
PC
PC
Instruction
Fetched
MOVWF PORTB
write to
PORTB
PC + 1
MOVF PORTB,W
PC + 2
NOP
PC + 3
NOP
RB7:RB0
Instruction
Executed
Port pin
sampled here
TPD
MOVWF PORTB MOVF PORTB,W
write to
PORTB
NOP
Note:
This example shows a write to PORTB
followed by a read from PORTB.
Note that:
data setup time = (0.25TCY - TPD)
where TCY = instruction cycle
TPD = propagation delay
Therefore, at higher clock frequencies,
a write followed by a read may be
problematic.
2001 Microchip Technology Inc.
Preliminary
DS39544A-page 39
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