HD64F2149 Datasheet, PDF (324/1035 Pages) Renesas Technology Corp – Hitachi 16-Bit Single-Chip Microcomputer

10.3 Bus Master Interface
DACNT, DADRA, and DADRB are 16-bit registers. The data bus linking the bus master and the
on-chip supporting modules, however, is only 8 bits wide. When the bus master accesses these
registers, it therefore uses an 8-bit temporary register (TEMP).
These registers are written and read as follows (taking the example of the CPU interface).
• Write
When the upper byte is written, the upper-byte write data is stored in TEMP. Next, when the
lower byte is written, the lower-byte write data and TEMP value are combined, and the
combined 16-bit value is written in the register.
• Read
When the upper byte is read, the upper-byte value is transferred to the CPU and the lower-byte
value is transferred to TEMP. Next, when the lower byte is read, the lower-byte value in
TEMP is transferred to the CPU.
These registers should always be accessed 16 bits at a time using an MOV instruction (by word
access or two consecutive byte accesses), and the upper byte should always be accessed before the
lower byte. Correct data will not be transferred if only the upper byte or only the lower byte is
accessed. Also note that a bit manipulation instruction cannot be used to access these registers.
Figure 10.2 shows the data flow for access to DACNT. The other registers are accessed similarly.
Example 1: Write to DACNT
MOV.W R0, @DACNT
; Write R0 contents to DACNT
Example 2: Read DADRA
MOV.W @DADRA, R0
; Copy contents of DADRA to R0
Table 10.3 Read and Write Access Methods for 16-Bit Registers
Read
Write
Register Name
Word
Byte
Word
Byte
DADRA and DADRB
Yes
Yes
Yes
×
DACNT
Yes
×
Yes
×
Notes: Yes: Permitted type of access. Word access includes successive byte accesses to the
upper byte (first) and lower byte (second).
×: This type of access may give incorrect results.
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