H8SSLASH2218 datasheet(117/750 Pages) RENESAS

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Section 2 CPU

Rev.6.00 Jun. 03, 2008 Page 69 of 698

REJ09B0074-0600

In order to write to a register containing write-only bits, set aside a work area in memory (in on-

chip RAM, for example) and write the data to be manipulated to it. After accessing and

manipulating the data in the work area in memory, write the resulting data to the register

containing write-only bits.

Figure 2.14 Example Flowchart of Method for Accessing Registers Containing Write-Only Bits

Write data to work area

Write data from work area to

Access data in work area

(using either data transfer instructions

or bit manipulation instructions)

Write data from work area to

Write initial value

Change value of register containing

write-only bits

Figure 2.14 Flowchart of Method for Accessing Registers Containing Write-Only Bits

Example: Clearing 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. To start, the initial

value H'F0 to be written to P1DDR is written ahead of time to the work area (RAM0) in memory.

MOV.B

#H'F0,

R0L

MOV.B

R0L,

@RAM0

MOV.B

R0L,

@P1DDR

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