217
6. Notes on use of the SLEEP instruction
a. Operation contents
When a DMAC bus cycle is entered immediately after executing a SLEEP instruction,
there are cases when DMA transfer is not carried out correctly.
b. Remedy
•
Stop operation (for example, by clearing the DMA enable bit (DE) in the DMA channel
control register (CHCRn)) before entering sleep mode.
•
To use the DMAC when in sleep mode, first exit sleep mode by means of an interrupt.
In cases when the CPU is not carrying out any other processing but is waiting for the DMAC
to end its transfer during DMAC operation, do not use the SLEEP instruction, but use the
transfer end flag bit (TE) in the channel DMA control register and a polling software loop.
7. Sampling of
DREQ
If
DREQ
is set to level detection in DMA cycle-steal mode, sampling of
DREQ
may take place
before DACK is output. Note that some system configurations involve unnecessary DMA
transfers.
Operation:
As shown in Figure 9.25, sampling of
DREQ
is carried out immediately before the rising edge
of the third-state clock (CK) after completion of the bus cycle preceding the DMA bus cycle
where DACK is output.
If DACK is output after the third state of the DMA bus cycle, sampling of
DREQ
must be
carried out before DACK is output.
Number of states of
DMAC bus cycle
: DMAC bus cycle
Sampling point
1
2
3
4
Figure 9.25
DREQ
Sampling Points
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