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2. Bit manipulation in a register containing a write-only bit
Example 3: BCLR instruction executed designating port 3 control register PCR3
As in the examples above, P3
high-level signal at P3
signals. In this example, the BCLR instruction is used to change pin P3
assumed that a high-level signal will be input to this input pin.
[A: Prior to executing BCLR]
P3
7
Input/output Input
Pin state
Low level High level Low level Low level Low level Low level Low level Low level
PCR3
0
PDR3
1
[B: BCLR instruction executed]
BCLR
#0
,
[C: After executing BCLR]
P3
7
Input/output Output
Pin state
Low level High level Low level Low level Low level Low level Low level High level
PCR3
1
PDR3
1
[D: Explanation of how BCLR operates]
When the BCLR instruction is executed, first the CPU reads PCR3. Since PCR3 is a write-only
register, the CPU reads a value of H'FF, even though the PCR3 value is actually H'3F.
Next, the CPU clears bit 0 in the read data to 0, changing the data to H'FE. Finally, this value
(H'FE) is written to PCR3 and BCLR instruction execution ends.
As a result of this operation, bit 0 in PCR3 becomes 0, making P3
and 6 in PCR3 change to 1, so that P3
To avoid this problem, store a copy of the PCR3 data in a work area in memory. Perform the bit
manipulation on the data in the work area, then write this data to PCR3.
58
and P3
are input pins, with a low-level signal input at P3
7
6
. The remaining pins, P3
6
P3
P3
6
5
Input
Output
0
1
0
0
The BCLR instruction is executed designating PCR3.
@PCR3
P3
P3
6
5
Output
Output
1
1
0
0
and P3
7
to P3
, are output pins that output low-level
5
0
P3
P3
4
3
Output
Output
1
1
0
0
P3
P3
4
3
Output
Output
1
1
0
0
0
change from input pins to output pins.
6
to an input port. It is
0
P3
P3
2
1
Output
Output
1
1
0
0
P3
P3
2
1
Output
Output
1
1
0
0
an input port. However, bits 7
and a
7
P3
0
Output
1
0
P3
0
Input
0
0
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