E X T E R N A L D M A

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DMA transfers. .

External DMA

C H A P T E R

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The external DMA interface provides two external channels for external peripheral support. Each DMA channel moves data from the source address to the destination address. These addresses can specify any peripheral on the AHB bus but, ideally, they specify an external peripheral and external memory.

D M A t r a n s f e r s

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DMA transfers can be specified as burst-oriented to maximize AHB bus efficiency. All transfers are performed in two steps:

1Data is moved from the source address to a 32-byte buffer in the DMA control logic.

2The data is moved from the 32-byte buffer to the destination address. These two steps are repeated until the DMA transfer is complete.

Note: Optimal performance is achieved when both the source address and destination address are aligned.

Initiating DMA transfers

Processor- initiated

External peripheral- initiated

DMA transfers can be initiated in one of two ways: processor-initiated and external peripheral initiated.

The processor must do these steps in the order shown:

1Set up the required buffer descriptors.

2Configure the DMA Control register for each channel.

3Write a 1 to both the CE field and the CG field in the DMA Control register for each channel.

An external peripheral initiates a DMA transfer by asserting the appropriate REQ signal. Software must have set up the required buffer descriptors as well as the DMA Control register for each channel, including setting the CE field to 1, before the REQ signal can be asserted.

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Digi NS9215 manual External DMA, A t r a n s f e r s

NS9215 specifications

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