Signal Description; External Dma Configuration; Memory-To-Memory Mode - Digi NS7520B-1-C36 Hardware Reference Manual

Signal description

Signal
DREQ_
DACK_
DONE_IN
DONE_OUT

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.
Description
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.
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.
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.
The NS7520 asserts DONE_OUT when the L (Last) bit is set in the DMA buffer
descriptor and the buffer is empty (retired).
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