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
7
and P3
6
are input pins, with a low-level signal input at P3
7
and a
high-level signal at P3
6
. The remaining pins, P3
5
to P3
0
, are output pins that output low-level
signals. In this example, the BCLR instruction is used to change pin P3
0
to an input port. It is
assumed that a high-level signal will be input to this input pin.
[A: Prior to executing BCLR]
P3
7
P3
6
P3
5
P3
4
P3
3
P3
2
P3
1
P3
0
Input/output
Input
Input
Output
Output
Output
Output
Output
Output
Pin
state
Low High Low Low Low Low Low Low
level
level level level level level level level
PCR3
0
0
1
1
1
1
1
1
PDR3
1
0
0
0
0
0
0
0
[B: BCLR instruction executed]
The BCLR instruction is executed designating PCR3.
[C: After executing BCLR]
P3
7
P3
6
P3
5
P3
4
P3
3
P3
2
P3
1
P3
0
Input/output
Output Output
Output
Output
Output
Output
Output
Input
Pin
state
Low High Low Low Low Low Low High
level
level level level level level level level
PCR3
1
1
1
1
1
1
1
0
PDR3
1
0
0
0
0
0
0
0
[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
0
an input port. However, bits 7
and 6 in PCR3 change to 1, so that P3
7
and P3
6
change from input pins to output pins.
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.
BCLR #0 , @PCR3
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