H8SSLASH2224 datasheet(186/1071 Pages) RENESAS

186 / 1071 page

Section 2 CPU

Rev. 5.00 Aug 08, 2006 page 100 of 982

REJ09B0054-0500

The bit manipulation operation is performed on this value that was read. In this example, bit 4 will

be cleared for H'F8.

P17

P16

P15

P14

P13

P12

P11

P10

I/O

Output

Input

P1DDR

After bit

manipulation

000

After the bit manipulation operation, this data will be written to P1DDR, and the BCLR

instruction completes.

P17

P16

P15

P14

P13

P12

P11

P10

I/O

Output

Input

Output

Input

P1DDR

000

Write value

000

Although the instruction was expected to write H'E0 back to P1DDR, it actually wrote H'E8, and

P13, which was expected to be an input pin, is changed to function as an output pin. While this

section described the case where P13 was read out as a 1, since the values read are undefined

when P17 to P10 are read, when this bit manipulation instruction completes, bits that were 0 may

be changed to 1, and bits that were 1 may be changed to 0. To avoid this sort of problem, see

section 2.9.4, Access Methods for Registers with Write-Only Bits for methods for modifying

registers that include write-only bits.

Also note that it is possible to use the BCLR instruction to clear to 0 flags in internal I/O registers.

In this case, if it is clear from the interrupt handler or other information that the corresponding flag

is set to 1, then there is no need to read the value of the corresponding flag in advance.

2.9.4

Access Methods for Registers with Write-Only Bits

Undefined values will be read out if a data transfer instruction is executed for a register that

includes write-only bits, or if a bit manipulation instruction is executed for a register that includes

write-only bits. To avoid reading undefined values, use methods such as those shown below to

access registers that include write-only bits.

The basic method for writing to a register that includes write-only bits is to create a work area in

internal RAM or other memory area and first write the data to that area. Then, perform the desired

access operation for that memory and finally write that data to the register that includes write-only

bits.

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