PIC16C620 Microchip Technology, PIC16C620 Datasheet - Page 30

• Manufacturer Part Number: PIC16C620
• Description: EPROM-Based 8-Bit CMOS Microcontroller
• Manufacturer: Microchip Technology

PIC16C62X
5.3
5.3.1
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 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.
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.
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:
DS30235G-page 30
Instruction
RB7:RB0
RB <7:0>
fetched
PC
I/O Programming Considerations
BI-DIRECTIONAL I/O PORTS
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
MOVWF PORTB
Write to
PORTB
SUCCESSIVE I/O OPERATION
PC
MOVF PORTB, W
Read PORTB
MOVWF
Execute
PORTB
PC + 1
T
PD
Preliminary
Port pin
sampled here
PORTB, W
PC + 2
Execute
NOP
MOVF
EXAMPLE 5-2:
;
;
;
;
;
;
;
;
;
; 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
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 a NOP or another
instruction not accessing this I/O port.
Initial PORT settings:
PORTB<7:6> have external pull-up and are not
connected to other circuitry
BCF PORTB, 7
BCF PORTB, 6
BSF STATUS,RP0
BCF TRISB, 7
BCF TRISB, 6
Execute
PC + 3
NOP
NOP
SUCCESSIVE OPERATIONS ON I/O PORTS
READ-MODIFY-WRITE
INSTRUCTIONS ON AN
I/O PORT
Note:
This example shows write to PORTB
followed by a read from PORTB.
Note that:
Therefore, at higher clock frequencies,
a write followed by a read may be
problematic.
data setup time = (0.25 T
where T
T
to output valid.
PD
1998 Microchip Technology Inc.
= propagation delay of Q1 cycle
; 01pp pppp
; 10pp pppp
;
; 10pp pppp
; 10pp pppp
CY
PORTB<7:4> Inputs
PORTB<3:0> Outputs
PORT latch PORT pins
---------- ----------
= instruction cycle and
CY
11pp pppp
11pp pppp
11pp pppp
10pp pppp
- T
PD
)