H838024R datasheet(113/697 Pages) RENESAS

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Section 2 CPU

Rev. 7.00 Mar 10, 2005 page 71 of 652

REJ09B0042-0700

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

Pin state

Low level High level Low level Low level Low level Low level Low level Low level

PCR3

00111111

PDR3

10000000

[B: BCLR instruction executed]

BCLR

@PCR3

The BCLR instruction is executed designating PCR3.

[C: After executing BCLR]

P37

P36

P35

P34

P33

P32

P31

P30

Input/output Output

Output

Input

Pin state

Low level High level Low level Low level Low level Low level Low level High level

PCR3

11111

PDR3

10000000

[D: Explanation of how BCLR operates]

When the BCLR instruction is executed, first the CPU reads PCR3. Since PCR3 is a write-only

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.

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