HD6433713 datasheet(62/324 Pages) RENESAS

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2.8.2 Notes on Bit Manipulation

The H8/300L CPU executes bit manipulation instructions by a read-modify-write operation on

8-bit data. When bit manipulation instructions are executed in the cases illustrated below, care

must be taken since the operation may affect other bits besides those being manipulated.

Bit manipulation in two registers assigned to the same address (when the source and

destination are different)

Example 1: Timer load register and timer counter

In this example, a bit manipulation instruction is executed in the timer load register and timer

counter of a reloadable timer. Since the timer load register and timer counter share the same

address, the operations take place as follows.

Read: The timer counter value at the time is read.

Modify: The CPU modifies (sets or resets) the bit designated with the instruction. (Other bits

remain the same.)

Write: The modified data is written to the timer load register.

The timer counter is counting based on the system clock (

φ), so the value read is not necessarily

the same as the value in the timer load register. As a result, bits other than the intended bit in the

timer load register may be modified to the timer counter value.

Figure 2-17 shows the reloadable timer configuration.

Figure 2-17 Reloadable Timer Configuration

Example 2: Port data register (pin input and data register)

When a bit manipulation instruction is executed designating a port data register, it may cause

changes in pin I/O states or data register contents other than the intended bit.

Timer counter

Reload

Timer load register

Read

Write

HD6433713 Datasheet(PDF) 62 Page - Renesas Technology Corp