HD6417660 RENESAS [Renesas Technology Corp], HD6417660 Datasheet - Page 334

• Manufacturer Part Number: HD6417660
• Description: Renesas 32-Bit RISC Microcomputer
• Manufacturer: RENESAS [Renesas Technology Corp]

actual access addresses. For an instruction fetch to an even word boundary (4n), the CPU
performs longword access. For an instruction fetch to an odd word boundary (4n + 2), the CPU
performs word access.
For a read cycle of a cache-through area or an on-chip peripheral module, the read cycle is first
accepted and then read cycle is initiated. The read data is sent to the CPU via the cache bus.
In a write cycle for the cache area, the write cycle operation differs according to the cache write
methods.
In write-back mode, the cache is first searched. If data is detected at the address corresponding to
the cache, the data is then re-written to the cache. In the actual memory, data will not be re-
written until data in the corresponding address is re-written. If data is not detected at the address
corresponding to the cache, the cache is modified. In this case, data to be modified is first saved to
the internal buffer, 16-byte data including the data corresponding to the address is then read, and
data in the corresponding access of the cache is finally modified. Following these operations, a
write-back cycle for the saved 16-byte data is executed.
In write-through mode, the cache is first searched. If data is detected at the address corresponding
to the cache, the data is re-written to the cache simultaneously with the actual write via the internal
bus. If data is not detected at the address corresponding to the cache, the cache is not modified
but an actual write is performed via the internal bus.
Since the bus state controller (BSC) incorporates a one-stage write buffer, the BSC can execute an
access via the internal bus before the previous external bus cycle is completed in a write cycle. If
the on-chip module is read or written after the external low-speed memory is written, the on-chip
module can be accessed before the completion of the external low-speed memory write cycle. In
read cycles, the CPU is placed in the wait state until read operation has been completed. To
continue the process after the data write to the device has been completed, perform a dummy read
to the same address to check for completion of the write before the next process to be executed.
The write buffer of the BSC functions in the same way for an access by a bus master other than the
CPU such as the DMAC. Accordingly, to perform dual address DMA transfers, the next read
cycle is initiated before the previous write cycle is completed. Note, however, that if both the
DMA source and destination addresses exist in external memory space, the next write cycle will
not be initiated until the previous write cycle is completed.
On-Chip Peripheral Module Access: To access an on-chip module register, two or more
peripheral module clock (Pφ) cycles are required. Care must be taken in system design.
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