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[Prior to executing BCLR]
Input/output
Pin state
PCR5
PDR5
[BCLR instruction executed]
BCLR
#0,
[After executing BCLR]
Input/output
Pin state
PCR5
PDR5
[Description on operation]
1. When the BCLR instruction is executed, first the CPU reads PCR5. Since PCR5 is a write-only
register, the CPU reads a value of H'FF, even though the PCR5 value is actually H'3F.
2. Next, the CPU clears bit 0 in the read data to 0, changing the data to H'FE.
3. Finally, H'FE is written to PCR5 and BCLR instruction execution ends.
As a result of this operation, bit 0 in PCR5 becomes 0, making P50 an input port. However, bits 7
and 6 in PCR5 change to 1, so that P57 and P56 change from input pins to output pins. To prevent
this problem, store a copy of the PCR5 data in a work area in memory and manipulate data of the
bit in the work area, then write this data to PCR5.
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P57
P56
P55
Input
Input
Output
Low
High
Low
level
level
level
0
0
1
1
0
0
The BCLR instruction is executed for PCR5.
@PCR5
P57
P56
P55
Output
Output
Output
Low
High
Low
level
level
level
1
1
1
1
0
0
P54
P53
P52
Output
Output
Output
Low
Low
Low
level
level
level
1
1
1
0
0
0
P54
P53
P52
Output
Output
Output
Low
Low
Low
level
level
level
1
1
1
0
0
0
Rev. 1.00, 11/03, page 43 of 376
P51
P50
Output
Output
Low
Low
level
level
1
1
0
0
P51
P50
Output
Input
Low
High
level
level
1
0
0
0
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