Toshiba TMPR4925, Manual

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Chapter 8 DMA Controller
8-20
8.3.14 Arbitration Among DMA Channels
The DMA Controller has an on-chip DMA Channel Arbiter that arbitrates bus ownership among four
DMA channels that use the internal bus (G-Bus). There are two methods for determining priority: the
round robin method and the fixed priority method. (See Figure 8.3.7.) The Round Robin Priority bit
(RRPT) of the DMA Master Control Register (DMMCR) selects the priority method.
•
Fixed priority (DMMCR.RRPT
=
0)
As shown below, Channel 0 has the highest priority and Channel 3 has the lowest priority.
CH0 > CH1 > CH2 > CH3
•
Round Robin method (DMMCR.RRPT
=
1)
The last channel to perform DMA transfer has the lowest priority.
•
After CH0 DMA transfer execution: CH1 > CH2 > CH3 > CH0
•
After CH1 DMA transfer execution: CH2 > CH3 > CH0 > CH1
•
After CH2 DMA transfer execution: CH3 > CH0 > CH1 > CH2
•
After CH3 DMA transfer execution: CH0 > CH1 > CH2 > CH3
a) Fixed Priority is selected
b) Round Robin Priority is selected
Figure 8.3.7 DMA Channel Arbitration
8.3.15 Restrictions in Access to PCI Bus
The PCI Controller detects a bus error if the DMA Controller performs one of the following accesses
to the PCI Bus.
•
Burst transfer exceeding 8 words (PCICSTATUS.TLB)
•
Address Increment value –4 Burst transfer (PCICSTATUS.NIB)
•
Address Increment Value 0 Burst transfer (PCICSTATUS.ZIB)
•
Dual Address Burst transfer when the setting for DMSARn, DMDARn, or DMCNTRn is not a
word boundary (PCICSTATUS.IAA)
In addition, Single Address transfers between an external I/O device and the PCI Bus are not
supported. Data transfer is not performed, but no error is detected.
Channel 0 Channel 1 Channel 2 Channel 3
Channel 0
Channel 2
Channel 1 Channel 3