HD64F38024HV Datasheet, PDF (93/684 Pages) Renesas Technology Corp – Hardware Manual Renesas 8-Bit Single-Chip Microcomputer H8 Family/H8/300L Super Low Power Series

Section 2 CPU
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, P3₇ and P3₆ are input pins, with a low-level signal input at P3₇ and a
high-level signal at P3₆. The remaining pins, P3₅ to P3₀, are output pins that output low-level
signals. In this example, the BCLR instruction is used to change pin P3₀ to an input port. It is
assumed that a high-level signal will be input to this input pin.
[A: Prior to executing BCLR]
P37
P36
P35
P34
P33
P32
P31
P30
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]
P37
P36
P35
P34
P33
P32
P31
P30
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.
As a result of this operation, bit 0 in PCR3 becomes 0, making P3₀ an input port. However, bits 7
and 6 in PCR3 change to 1, so that P3₇ and P3₆ change from input pins to output pins.
Rev. 8.00 Mar. 09, 2010 Page 71 of 658
REJ09B0042-0800