Section 14 Direct Memory Access Controller (DMAC)
Rev.1.00 Dec. 13, 2005 Page 597 of 1286
REJ09B0158-0100
Bus Modes: There are two bus modes: cycle steal mode and burst mode. Select the mode in the
TB and LCKN bits in CHCR. Moreover, cycle steal mode has normal and intermittent modes that
are specified by the CMS bits in DMAOR.
•
Cycle-Steal Mode
Normal mode1 (DMAOR.CMS = 00, CHCR.LCKN = 0, CHCR.TB = 0)
In cycle-steal normal mode, the SuperHyway bus mastership is given to another bus master
after a one-transfer unit (byte, word, longword, 16-byte, or 32-byte unit) DMA transfer.
When the next transfer request occurs, the DMAC issues the next transfer request, the bus
mastership is obtained from the other bus master and a transfer is performed for one-
transfer unit. When that transfer ends, the bus mastership is passed to the other bus master.
This is repeated until the transfer end conditions are satisfied.
In cycle-steal normal mode, transfer areas are not affected regardless of settings of the
transfer request source, transfer source, and transfer destination.
Figure 14.6 shows an example of DMA transfer timing in cycle-steal normal mode. Transfer
conditions shown in the figure are:
CPU
CPU
CPU
DMAC
DMAC
CPU
DMAC
DMAC
CPU
DREQ
SuperHyway
bus cycle
Bus mastership returned to CPU once
Read/Write
Read/Write
Figure 14.6 DMA Transfer Timing Example in Cycle-Steal Normal Mode 1
(DREQ Low Level Detection)
Normal mode 2 (DMAOR.CMS = 00, CHCR.LCKN = 1, CHCR.TB = 0)
In cycle steal normal mode 2, the DMAC does not keep the SuperHyway bus mastership, is
to obtain the bus mastership in every one transfer unit of read or write cycle.
Figure 14.7 shows an example of DMA transfer timing in cycle steal normal mode 2.
Содержание SH7780 Series
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