Datasheet

11. DMAC
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11.5 Channel Priority and DMA Transfer Timing
If both DMA0 and DMA1 are enabled and DMA transfer request signals from DMA0 and DMA1 are de-
tected active in the same sampling period (one period from a falling edge to the next falling edge of CPU
clock), the DMAS bit on each channel is set to “1” (DMA requested) at the same time. In this case, the DMA
requests are arbitrated according to the channel priority, DMA0 > DMA1. The following describes DMAC
operation when DMA0 and DMA1 requests are detected active in the same sampling period. Figure 11.5.1
shows an example of DMA transfer effected by external factors.
DMA0 request having priority is received first to start a transfer when a DMA0 request and DMA1 request
are generated simultanelously. After one DMA0 transfer is completed, a bus arbitration is returned to the
CPU. When the CPU has completed one bus access, a DMA1 transfer starts. After one DMA1 transfer is
completed, the bus arbitration is again returned to the CPU.
In addition, DMA requsts cannot be counted up since each channel has one DMAS bit. Therefore, when
DMA requests, as DMA1 in Figure 11.5.1, occurs more than one time, the DAMS bit is set to "0" as soon
as getting the bus arbitration. The bus arbitration is returned to the CPU when one transfer is completed.
Figure 11.5.1 DMA Transfer by External Factors
DMA0
DMA1
DMA0
request bit
DMA1
request bit
CPU
INT0
INT1
Obtainment
of the bus
right
An example where DMA requests for external causes are detected active at the same
CPU clock