20.1 Overview

The Direct Memory Access Controller (DMAC) contains both a Direct Memory Access (DMA) engine and a Cyclic Redundancy Check (CRC) engine. The DMAC can transfer data between memories and peripherals and therefore, off-load these tasks from the CPU. It enables high data transfer rates with minimum CPU intervention, and frees up CPU time. With access to all peripherals, the DMAC can handle automatic transfer of data between communication modules.

The DMA part of the DMAC has several DMA channels, which can receive different types of transfer triggers and generate transfer requests from the DMA channels to the arbiter (Refer to the Block Diagram). The arbiter will select one DMA channel at a time to act as the active channel. When an active channel has been selected, the fetch engine of the DMAC will fetch a transfer descriptor from the SRAM and store it in the internal memory of the active channel, which will then execute the data transmission.

An ongoing data transfer of an active channel can be interrupted by a higher prioritized DMA channel. The DMAC will write back the updated transfer descriptor from the internal memory of the active channel to SRAM, and grant the higher prioritized channel a start transfer as the new active channel. Once a DMA channel is done with its transfer, interrupts and events can be generated optionally.

The DMAC has four bus interfaces:

  • The data transfer bus is used for performing the actual DMA transfer.
  • The AHB/APB Bridge bus is used when writing and reading the I/O registers of the DMAC.
  • The descriptor fetch bus is used by the fetch engine to fetch transfer descriptors before data transfer can be started or continued.
  • The write-back bus is used to write the transfer descriptor back to SRAM.

All buses are AHB host interfaces except the AHB/APB Bridge bus, which is an APB client interface.

The CRC engine can be used by software to detect an accidental error in the transferred data and to take corrective action, such as requesting the data to be sent again or simply not using the incorrect data.