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External peripheral DMA support
146
NS7520 Hardware Reference, Rev. D 03/2006
Signal description
External DMA configuration
The REQ bit in the DMA Control register must be set to 1 (external source) when
configuring for external DMA, and the DMA output signals must be selected at GPIO
PORTA. All other settings are the same as for any other DMA transfer.
The DMA channel 3/5 signals — DREQ1_, DACK1_, and DONE1_ — are multifunctional
and shared with GPIO PORTA. The DMA channel 4/6 signals — DREQ2_, DACK2_, and
DONE2_ — are multifunctional and shared with GPIO PORTC.
Memory-to-memory mode
In memory-to-memory mode, one of the source buffer pointers or destination buffer
pointers points to the external device (accomplished by tying the device to a chip
select). Each movement of data involves two autonomous operations: reading from
the source and writing to the destination.
Figure 20 shows the signals needed for external memory-to-memory transfers.
Signal Description
DREQ_ An input to the NS7520, sourced by the external device. All transfers are
initiated when the external device asserts DREQ_ low. When the external
device wants a DMA transfer (either read or write), it asserts the DREQ_
signal. DREQ_ can stay low until all data transfers are complete, or it can be
removed as needed to control execution of the DMA.
DACK_ An input to the external device, sourced by the NS7520. When a bus
transaction occurs, the NS7520 asserts DACK_; DACK_ may or may not
return high between each data transfer.
The external device can drive write data to the bus as soon as DACK_ is
active.
DONE_IN The external device asserts DONE_IN to retire the current DMA descriptor.
When the NS7520 receives the DONE_IN signal, it closes the current buffer
and sets the NCIP bit in the DMA Status register. The number of bytes that
were received is in the buffer length field in the DMA buffer descriptor.
DONE_IN can be asserted only when DACK_ is asserted.
DONE_OUT
The NS7520 asserts DONE_OUT when the L (Last) bit is set in the DMA
buffer descriptor and the buffer is empty (retired).