H8S2245 datasheet(109/818 Pages) RENESAS

109 / 818 page

Section 2 CPU

Rev.3.00 Mar. 26, 2007 Page 67 of 772

REJ09B0355-0300

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

order.

1. Read the data in byte units

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

Write the data back in byte units

Example: Using the BCLR instruction to clear only bit 4 in the port 1 P1DDR register.

The P1DDR register consists of 8 write-only bits and sets the I/O direction of the port 1 pins.

Reading this register is invalid. When read, the values returned are undefined.

Here we present an example in which P14 is specified to be an input port using the BCLR

instruction. Currently, P17 to P14 are set to be output pins and P13 to P10 are set to be input pins.

At this point, the value of P1DDR is H'F0.

P17

P16

P15

P14

P13

P12

P11

P10

I/O

Output

Input

P1DDR

To switch P14 from the Output pin to the input pin function, the value of P1DDR bit 4 must be

changed from 1 to 0 (H'F0

→ H'E0). Here we assume that the BCLR instruction is used to clear

P1DDR bit 4.

BCLR

#4,@P1DDR

However if a bit manipulation instruction of the type shown above is used on P1DDR, which is a

write-only register, the following problem may occur.

Although the first thing that happens is that data is read from P1DDR in byte units, the value read

at this time is undefined. An undefined value is a value that is either 0 or 1 in the register but reads

out as an arbitrary value whose relationship to the actual value is unknown. Since the P1DDR bits

are all write-only bits, every bit reads out as an undefined value. Although the actual value of

P1DDR at this point is H'F0, assume that bit 3 becomes a 1 here, and the value read out is H'F8.

P17

P16

P15

P14

P13

P12

P11

P10

I/O

Output

Input

P1DDR

Read value

000

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