Renesas F-ZTAT H8 Series, Hardware Manual

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2. CPU
Rev.3.00 Jul. 19, 2007 page 67 of 532
REJ09B0397-0300
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
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.