H8SSLASH2218 datasheet(116/750 Pages) RENESAS

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Section 2 CPU

Rev.6.00 Jun. 03, 2008 Page 68 of 698

REJ09B0074-0600

P17

P16

P15

P14

P13

P12

P11

P10

I/O

Output

Input

P1DDR

Read value

The BCLR instruction performs bit manipulation on the read value, which is H'F8 in this example.

It clears bit 4 to 0.

P17

P16

P15

P14

P13

P12

P11

P10

I/O

Output

Input

P1DDR

After bit

manipulation

Following bit manipulation the data is written to P1DDR and the BCLR instruction terminates.

P17

P16

P15

P14

P13

P12

P11

P10

I/O

Output

Input

Output

Input

P1DDR

Write value

The contents of P1DDR should have been overwritten with a value of H'E0, but in fact a value of

H'E8 was written to the register. This changed pin 13, which should have been an input pin, to an

output pin. In this example we assumed that the value of bit 1 in P1DDR was 1. However, since

the values of bits 7 to 0 in P1DDR are all undefined when read, there is the possibility that

individual bit values could be changed from 0 to 1 or from 1 to 0. To prevent this from happening,

the recommendations in section 2.9.4, Accessing Registers Containing Write-Only Bits, should be

followed when changing the values of registers containing write-only bits.

In addition, the BCLR instruction can be used to clear flags in internal I/O registers to 0. In such

cases it is not necessary to read the relevant flag beforehand so long as it is clear that it has been

set to 1 by an interrupt processing routine or the like.

2.9.4

Accessing Registers Containing Write-Only Bits

Using data transfer instructions or bit manipulation instructions on registers containing write-only

bits can result in undefined values being read. To prevent the reading of undefined values, the

procedure described below should be used to access registers containing write-only bits.

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