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PIC16CE625 Datasheet

  • PIC16CE625

  • OTP 8-Bit CMOS MCU with EEPROM Data Memory

  • 1208.41KB

  • 113页

  • Microchip

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PIC16CE62X
5.3
5.3.1
I/O Programming Considerations
BI-DIRECTIONAL I/O PORTS
EXAMPLE 5-2:
READ-MODIFY-WRITE
INSTRUCTIONS ON AN
I/O PORT
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 de铿乶ed. 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 de铿乶ed 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 (鈥渨ired-or鈥? 鈥渨ired-and鈥?.
The resulting high output currents may damage
the chip.
; Initial PORT settings: PORTB<7:4> Inputs
;
;
PORTB<3:0> Outputs
; PORTB<7:6> have external pull-up and are not
; connected to other circuitry
;
;
PORT latch PORT pins
;
---------- ----------
BCF
BCF
BSF
BCF
BCF
PORTB, 7
PORTB, 6
STATUS,RP0
TRISB, 7
TRISB, 6
; 01pp
; 10pp
;
; 10pp
; 10pp
pppp
pppp
pppp
pppp
11pp pppp
11pp pppp
11pp 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
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 铿乴e 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.
FIGURE 5-7:
SUCCESSIVE I/O OPERATION
Q1 Q2 Q3 Q4
PC + 1
MOVF PORTB, W
Read PORTB
Q1 Q2
Q3 Q4
Q1 Q2
Q3 Q4
Note:
This example shows write to PORTB
followed by a read from PORTB.
Note that:
data setup time = (0.25 T
CY
- T
PD
)
where T
CY
= instruction cycle and
T
PD
= propagation delay of Q1 cycle
to output valid.
Q1 Q2 Q3 Q4
PC
Instruction
fetched
RB7:RB0
RB <7:0>
PC
MOVWF PORTB
Write to
PORTB
PC + 2
NOP
PC + 3
NOP
T
PD
Execute
MOVWF
PORTB
Port pin
sampled here
Execute
MOVF
PORTB, W
Execute
NOP
Therefore, at higher clock frequencies,
a write followed by a read may be
problematic.
DS40182A-page 28
Preliminary
漏
1998 Microchip Technology Inc.

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