HD6433640 HITACHI [Hitachi Semiconductor], HD6433640 Datasheet - Page 65

• Manufacturer Part Number: HD6433640
• Description: H8/3644 Series Hardware Manual
• Manufacturer: HITACHI [Hitachi Semiconductor]

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]
Input/output
Pin state
PCR3
PDR3
[B: BCLR instruction executed]
[C: After executing BCLR]
Input/output
Pin state
PCR3
PDR3
[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
BCLR
#0
P3
Input
Low
level
0
1
P3
Output
Low
level
1
1
,
7
7
6
. The remaining pins, P3
@PCR3
P3
Input
High
level
0
0
P3
Output
High
level
1
0
7
6
6
and P3
6
P3
Output
Low
level
1
0
P3
Output
Low
level
1
0
7
The BCLR instruction is executed designating PCR3.
are input pins, with a low-level signal input at P3
and P3
5
5
6
change from input pins to output pins.
5
P3
Output
Low
level
1
0
P3
Output
Low
level
1
0
to P3
4
4
0
, are output pins that output low-level
P3
Output
Low
level
1
0
P3
Output
Low
level
1
0
3
3
0
an input port. However, bits 7
P3
P3
1
Output
Low
level
1
0
Output
Low
level
0
0
2
2
to an input port. It is
P3
Output
Low
level
1
0
P3
Output
Low
level
1
0
1
1
7
P3
Output
Low
level
1
0
P3
Input
High
level
0
0
and a
0
0
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