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Rev. 1.0, 07/01, page 40 of 372
•
Prior to executing BCLR instruction
P57 P56 P55 P54 P53 P52 P51 P50
Input/output Input Input Output Output Output Output Output Output
Pin state Low
level
High
level
Low
level
Low
level
Low
level
Low
level
Low
level
Low
level
PCR5 00111111
PDR5 10000000
•
BCLR instruction executed
BCLR #0, @PCR5
The BCLR instruction is executed for PCR5.
•
After executing BCLR instruction
P57 P56 P55 P54 P53 P52 P51 P50
Input/output Output Output Output Output Output Output Output Input
Pin state Low
level
High
level
Low
level
Low
level
Low
level
Low
level
Low
level
High
level
PCR5 1 1111110
PDR5 10000000
•
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 PDR5 data in a work area in memory and
manipulate data of the bit in the work area, then write this data to PDR5.