Section 14 Direct Memory Access Controller (DMAC)
Rev.1.00 Dec. 13, 2005 Page 598 of 1286
REJ09B0158-0100
CPU
CPU
CPU
DMAC
CPU
DMAC
CPU
CPU
DMAC
DMAC
DREQ
Read
Bus mastership retured to CPU once
Read
Write
Write
CPU
SuperHyway
bus cycle
Figure 14.7 DMA Transfer Timing Example in Cycle-Steal Normal Mode 2
(DREQ Low Level Detection)
Intermittent mode 16 (DMAOR.CMS = 10, CHCR.LCKN = 0 or 1, CHCR.TB = 0),
intermittent mode 64 (DMAOR.CMS = 11, CHCR.LCKN = 0 or 1, CHCR.TB = 0)
In intermittent mode of cycle steal, the DMAC returns the SuperHyway bus mastership to
other bus master whenever a one-transfer unit (byte, word, longword, or 16-byte or 32-byte
unit) is complete. If the next transfer request occurs after that, the DMAC issues the next
transfer request after waiting for 16 or 64 clocks in Bck count, and obtains the bus
mastership from other bus master. The DMAC then transfers data of one-transfer unit and
returns the bus mastership to other bus master. These operations are repeated until the
transfer end condition is satisfied. It is thus possible to make lower the ratio of bus
occupation by DMA transfer than cycle-steal normal mode.
When the DMAC issues again the transfer request, DMA transfer can be postponed in case
of entry updating due to cache miss.
The intermittent modes, however, must be cycle steal mode in all channels 0 to 5 or 6 to11
for the corresponding transfer channel.
Figure 14.8 shows an example of DMA transfer timing in cycle steal intermittent mode.
Transfer conditions shown in the figure are:
DREQ
CPU
CPU
CPU
More than 16 or 64 Bck
DMAC DMAC
CPU
CPU
DMAC DMAC
CPU
Read/Write
Read/Write
SuperHyway
bus cycle
Figure 14.8 Example of DMA Transfer Timing in Cycle Steal Intermittent Mode
(DREQ Low Level Detection)
Содержание SH7780 Series
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