Renesas F-ZTAT H8 Series Hardware Manual page 93

Bit Manipulation in a Register Containing a Write-Only Bit
Example 3: When a BCLR instruction is executed on PCR3 of port 3
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]
P3
7
Input/output Input
Pin state Low
level
PCR3 0
PDR3 1
[B: BCLR instruction executed]
BCLR #0 ,
[C: After executing BCLR]
P3
7
Input/output Output
Pin state Low
level
PCR3 1
PDR3 1
[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.
and P3 are input pins, with a low-level signal input at P3
7 6
. The remaining pins, P3
6
P3 P3
6 5
Input Output
High Low
level level
0 1
0 0
The BCLR instruction is executed designating PCR3.
@PCR3
P3 P3
6 5
Output Output
High Low
level level
1 1
0 0
to P3 , are output pins that output low-level
5 0
P3 P3
4 3
Output Output
Low Low
level level
1 1
0 0
P3 P3
4 3
Output Output
Low Low
level level
1 1
0 0
Rev.3.00 Jul. 19, 2007 page 67 of 532
and a
7
to an input port. It is
0
P3 P3 P3
2 1
Output Output Output
Low Low Low
level level level
1 1 1
0 0 0
P3 P3 P3
2 1
Output Output Input
Low Low High
level level level
1 1 0
0 0 0
REJ09B0397-0300
2. CPU
0
0