HD6433937 Datasheet, PDF (67/521 Pages) Hitachi Semiconductor – Hardware Manual

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, P3₇ and P3₆ are input pins, with a low-level signal input at P3₇ and a
high-level signal at P3₆. The remaining pins, P3₅ to P3₀, are output pins that output low-level
signals. In this example, the BCLR instruction is used to change pin P3₀ to an input port. It is
assumed that a high-level signal will be input to this input pin.
[A: Prior to executing BCLR]
Input/output
Pin state
PCR3
PDR3
P3₇
Input
Low
level
0
1
P3₆
Input
High
level
0
0
P3₅
Output
Low
level
1
0
P3₄
Output
Low
level
1
0
P3₃
Output
Low
level
1
0
P3₂
Output
Low
level
1
0
P3₁
Output
Low
level
1
0
P3₀
Output
Low
level
1
0
[B: BCLR instruction executed]
BSET #0 , @PCR3
The BCLR instruction is executed designating PCR3.
[C: After executing BCLR]
Input/output
Pin state
PCR3
PDR3
P3₇
Output
Low
level
1
1
P3₆
Output
High
level
1
0
P3₅
Output
Low
level
1
0
P3₄
Output
Low
level
1
0
P3₃
Output
Low
level
1
0
P3₂
Output
Low
level
1
0
P3₁
Output
Low
level
1
0
P3₀
Input
High
level
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 P3₀ an input port. However, bits 7
and 6 in PCR3 change to 1, so that P3₇ and P3₆ change from input pins to output pins.
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