HD64F38602R Datasheet, PDF (68/552 Pages) Renesas Technology Corp – Renesas 16-Bit Single-Chip Microcomputer

Section 2 CPU
2.8.3 Bit-Manipulation Instruction
The BSET, BCLR, BNOT, BST, and BIST instructions read data from the specified address in
byte units, manipulate the data of the target bit, and write data to the same address again in byte
units. Special care is required when using these instructions in cases where two registers are
assigned to the same address, or when a bit is directly manipulated for a port or a register
containing a write-only bit, because this may rewrite data of a bit other than the bit to be
manipulated.
Bit manipulation for two registers assigned to the same address
Example 1: Bit manipulation for the timer load register and timer counter
Figure 2.13 shows an example of a timer in which two timer registers are assigned to the same
address. When a bit-manipulation instruction accesses the timer load register and timer counter of
a reloadable timer, since these two registers share the same address, the following operations takes
place.
1. Data is read in byte units.
2. The CPU sets or resets the bit to be manipulated with the bit-manipulation instruction.
3. The written data is written again in byte units to the timer load register.
The timer is counting, 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 counter may be modified and the
modified value may be written to the timer load register.
Count clock
Timer counter
Reload
Timer load register
Read
Write
Internal data bus
Figure 2.13 Example of Timer Configuration
with Two Registers Allocated to Same Address
Rev. 3.00 May 15, 2007 Page 36 of 516
REJ09B0152-0300