hd66100 Renesas Electronics Corporation., hd66100 Datasheet - Page 73

• Manufacturer Part Number: hd66100
• Description: H8/3867 Series
• Manufacturer: Renesas Electronics Corporation.

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
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 Input
Pin state
PCR3
PDR3
[B: BCLR instruction executed]
[C: After executing BCLR]
Input/output 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
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.
58
BCLR
#0
P3
Low level High level Low level Low level Low level Low level Low level Low level
0
1
P3
Low level High level Low level Low level Low level Low level Low level High level
1
1
7
7
,
@PCR3
6
. The remaining pins, P3
P3
Input
0
0
P3
Output
1
0
6
6
7
and P3
P3
Output
1
0
P3
Output
1
0
The BCLR instruction is executed designating PCR3.
5
5
6
7
are input pins, with a low-level signal input at P3
and P3
P3
Output
1
0
P3
Output
1
0
6
change from input pins to output pins.
5
4
4
to P3
0
, are output pins that output low-level
P3
Output
1
0
P3
Output
1
0
3
3
0
P3
Output
1
0
P3
Output
1
0
an input port. However, bits 7
2
2
0
to an input port. It is
P3
Output
1
0
P3
Output
1
0
1
1
7
P3
Output
1
0
P3
Input
0
0
and a
0
0