mcimx27- Freescale Semiconductor, Inc, mcimx27- Datasheet - Page 56

• Manufacturer Part Number: mcimx27-
• Description: Multimedia Applications Processor
• Manufacturer: Freescale Semiconductor, Inc

Signal Descriptions
3.6.1
After assertion of External DMA Request the DMA burst will start when the corresponding DMA channel
becomes the current highest priority channel. The External DMA Request should be kept asserted until it
is serviced by the DMAC. One External request will initiate at least one DMA burst.
The output External Grant signal from the DMAC is an active-low signal. This signal will be asserted
during the time when a DMA burst is ongoing for an External DMA Request, when the following
conditions are true:
Once the grant is asserted the External DMA Request will not be sampled until completion of the DMA
burst. The priority of the external request will become low, for the next consecutive burst, if another DMA
request signal is asserted.
The waveforms are shown for the worst case—that is, smallest burst (1 byte read/write). Minimum and
maximum timings for the External request and External grant signal are present in the data sheet.
Figure 15
request is de-asserted immediately after sensing grant signal active.
Figure 16
sensing grant signal active such that a new burst is not initiated.
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The DMA channel for which the DMA burst is ongoing has requested source as external DMA
Request (as per RSSR settings).
REN and CEN bit of this channel are set.
External DMA Request is asserted.
External device
Data written to
Ext_DMAReq
Ext_DMAGrant
Data read from
External device
NOTE: Assuming worst case that the data is read/written from/to external device as per the above waveform.
shows the minimum time for which the External Grant signal remains asserted if External DMA
shows the safe maximum time for which External DMA request can be kept asserted, after
Ext_DMAGrant
Direct Memory Access Controller (DMAC)
Ext_DMAReq
Figure 16. Timing Diagram of Safe Maximums for External Request De-Assertion
Figure 15. Assertion of DMA External Grant Signal
Preliminary—Subject to Change Without Notice
i.MX27 and i.MX27L Data Sheet, Rev. 1.2
t
max_req_assert
t
min_assert
t
max_read
Freescale Semiconductor
t
max_write