Bus Arbitration Among Dmac And Other Bus Masters - Renesas H8SX/1500 Series Hardware Manual

Section 7 DMA Controller (DMAC)
7.6.2

Bus Arbitration among DMAC and Other Bus Masters

When DMA transfer cycles are consecutively performed, bus cycles of other bus masters may be
inserted between the transfer cycles. The DMAC can release the bus temporarily to pass the bus to
other bus masters.
The consecutive DMA transfer cycles may not be divided according to the transfer mode settings
to achieve high-speed access.
The read and write cycles of a DMA transfer are not separated. Refreshing, external bus release,
and on-chip bus master (CPU) cycles are not inserted between the read and write cycles of a DMA
transfer.
In block transfer mode and an auto request transfer by burst access, bus cycles of the DMA
transfer are consecutively performed. For this duration, since the DMAC has priority over the
CPU, accesses to the external space is suspended (the IBCCS bit in the bus control register 2
(BCR2) is cleared to 0).
When the bus is passed to another channel or an auto request transfer by cycle stealing, bus cycles
of the DMAC and on-chip bus master are performed alternatively.
When the arbitration function among the DMAC and on-chip bus masters is enabled by setting the
IBCCS bit in BCR2, the bus is used alternatively except the bus cycles which are not separated.
For details, see section 6, Bus Controller (BSC).
A conflict may occur between external space access of the DMAC and a refresh cycle or an
external bus release cycle. Even if a burst or block transfer is performed by the DMAC, the
transfer is stopped temporarily and a cycle of refresh or external bus release is inserted by the BSC
(when the CPU external access does not have priority over a DMAC transfer, the transfer is not
operated until the DMAC releases the bus).
In dual address mode, the DMAC releases the external bus after the external space write cycle.
Since the read and write cycles are not separated, the bus is not released.
An internal space (on-chip memory and internal I/O registers) access of the DMAC and an
external bus release cycle may be performed at the same time.
Rev. 3.00 Mar. 14, 2006 Page 199 of 804
REJ09B0104-0300