H8SSLASH2218 datasheet(115/750 Pages) RENESAS

115 / 750 page

Section 2 CPU

Rev.6.00 Jun. 03, 2008 Page 67 of 698

REJ09B0074-0600

The instructions BSET, BCLR, BNOT, BST, and BIST perform the following operations in the

order shown:

1. Read data in byte units

2. Perform bit manipulation on the read data according to the instruction

3. Write data in byte units

Example: Using the BCLR instruction to clear pin 14 only of P1DDR for port 1

P1DDR is an 8-bit register that contains write-only bits. It is used to specify the I/O setting of the

individual pins in port 1. Reading produces invalid data. Attempting to read from P1DDR returns

undefined values.

In this example, the BCLR instruction is used to set pin 14 as an input port. Let us assume that

pins 17 to 14 are presently set as output pins and pins 13 to 10 are set as input pins. Thus, the value

of P1DDR is initially H'F0.

P17

P16

P15

P14

P13

P12

P11

P10

I/O

Output

Input

P1DDR

To change pin 14 from an output pin to an input pin, the value of bit 4 in P1DDR must be changed

from 1 to 0 (H'F0 to H'E0). Now assume that the BCLR instruction is used to clear bit 4 in P1DDR

to 0.

BCLR

#4, @P1DDR

However, using the above bit manipulation instruction on the write-only register P1DDR can cause

problems, as described below.

The BCLR instruction first reads data from P1DDR in byte units, but in this case the read values

are undefined. These undefined values can be 0 or 1 for each bit in the register, but there is no way

of telling which. Since all of the bits in P1DDR are write-only, undefined values are returned for

all of the bits when the register is read. In this example the value of P1DDR is H'F0, but we will

assume that the value returned when the register was read is H'F8, which would give bit 3 a value

of 1.

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