PIC17C42 Microchip Technology, PIC17C42 Datasheet - Page 64

• Manufacturer Part Number: PIC17C42
• Description: High-Performance 8-Bit CMOS EPROM/ROM Microcontroller
• Manufacturer: Microchip Technology

PIC17C4X
9.5
9.5.1
Any instruction which writes, operates internally as a
read followed by a write operation. For example, the
BCF and BSF instructions 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 re-written to the data latch of this particu-
lar pin, overwriting the previous content. As long as the
pin stays in the input mode, no problem occurs. How-
ever, if bit0 is switched into output mode later on, the
content of the data latch may now be unknown.
Reading a port 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 (BCF, BSF,
BTG, etc.) on a port, the value of the port pins is read,
the desired operation is performed with this value, and
the value is then written to the port latch.
Example 9-5 shows the effect of two sequential
read-modify-write instructions on an I/O port.
FIGURE 9-9:
DS30412C-page 64
Instruction
Instruction
I/O Programming Considerations
BI-DIRECTIONAL I/O PORTS
RB7:RB0
executed
fetched
SUCCESSIVE I/O OPERATION
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
MOVWF PORTB
write to
PORTB
PC
MOVF PORTB,W
MOVWF PORTB
write to
PORTB
PC + 1
MOVF PORTB,W
Port pin
sampled here
NOP
PC + 2
EXAMPLE 9-5:
; Initial PORT settings: PORTB<7:4> Inputs
;
; PORTB<7:6> have pull-ups and are
; not connected to other circuitry
;
;
;
;
;
;
; 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).
9.5.2
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 9-
9). 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
executing the instruction that reads the values on that
I/O port. 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.
Note:
BCF
BCF
BCF
BCF
PC + 3
NOP
NOP
SUCCESSIVE OPERATIONS ON I/O PORTS
PORTB, 7
PORTB, 6
DDRB, 7
DDRB, 6
A pin actively outputting a Low or High
should not be driven from external devices
in order to change the level on this pin (i.e.
“wired-or”, “wired-and”). The resulting high
output currents may damage the device.
READ MODIFY WRITE
INSTRUCTIONS ON AN
I/O PORT
Note:
This example shows a write to PORTB
followed by a read from PORTB.
Note that:
data setup time = (0.25 T
where T
Therefore, at higher clock
frequencies, a write followed by a
read may be problematic.
T
1996 Microchip Technology Inc.
CY
PD
= instruction cycle.
= propagation delay
PORT latch
----------
01pp pppp
10pp pppp
10pp pppp
10pp pppp
PORTB<3:0> Outputs
CY
- T
PD
PORT pins
---------
11pp pppp
11pp pppp
11pp pppp
10pp pppp
)