Renesas F-ZTAT H8/3642A Series, Hardware Manual, Page: 77 / 551

Section 2 CPU
Rev. 6.00 Sep 12, 2006 page 55 of 526
REJ09B0326-0600
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
7
and P3
6
are input pins, with a low-level signal input at P3
7
and a
high-level signal at P3
6
. The remaining pins, P3
5
to P3
0
, are output pins that output low-level
signals. In this example, the BCLR instruction is used to change pin P3
0
to an input port. It is
assumed that a high-level signal will be input to this input pin.
[A: Prior to executing BCLR]
P3
7
P3
6
P3
5
P3
4
P3
3
P3
2
P3
1
P3
0
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
PCR3 0 0 1 1 1 1 1 1
PDR3 1 0 0 0 0 0 0 0
[B: BCLR instruction executed]
BCLR #0 , @PCR3
The BCLR instruction is executed designating PCR3.
[C: After executing BCLR]
P3
7
P3
6
P3
5
P3
4
P3
3
P3
2
P3
1
P3
0
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
PCR3 1 1 1 1 1 1 1 0
PDR3 1 0 0 0 0 0 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.