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What Is Direct Memory Access (DMA)?

 

Direct Memory Access (DMA) is a feature of computer systems that allows hardware subsystems to access main system memory (RAM) with minimal intervention from the Central Processing Unit (CPU).

The Direct Memory Access (DMA) hardware peripheral on PIC® microcontrollers (MCUs) allows for direct memory-to-memory and peripheral-to-memory transfers and concurrent operation of the DMA and the CPU (while sharing system bus resources). This enables more flexible usage of eXtreme Low Power (XLP) technologies through core-independent movement of data in low-power modes and improvements in data throughput and latency.

 


 

Why Is DMA Used?

Most CPU operations in MCU-based systems involve the movement of data. In many cases, this can significantly limit the throughput and responsiveness of a system. For example, an interrupt can cause the CPU to stop what it is currently doing to service the interrupt. In low-power applications, this can significantly increase overall power consumption.

The DMA provides a method of offloading data movement operations from the CPU, allowing for concurrent execution of data transfers and CPU processing in a pipelined manner. This reduces CPU load and interrupt overhead, thereby improving system responsiveness and power efficiency. The DMA peripheral can move data through two methods:

  • Moving data while the CPU clock is disabled (e.g. low-power modes and XLP technology, depending on device capabilities)
  • Utilizing available bus cycles when the CPU is idle or waiting for an event

How Is DMA Used?

Configuration Context Switching

Having multiple configurations of peripherals can greatly expand the functionality of a device and lowers unit costs as well as overall BOM cost. However, changing configurations can be CPU intensive as the registers and configuration bits for peripherals need to be altered, which could bring about performance issues in some applications.

The DMA allows for more efficient context-based configuration through a combination of software- and hardware-based triggers that permit automated updates of peripheral registers with reduced CPU involvement.

System Automation

The DMA peripheral can be used to automate system processes involving data transfer. This includes interaction with state-machine-like implementations and reconfiguration of other Core Independent Peripherals (CIPs), providing an efficient method of using a single peripheral in a variety of ways in response to hardware- or software-defined triggers.

Additionally, loop-based processes can be automated through repeated or circular DMA transfers, increasing throughput and reducing CPU intervention.

Arbitrary Waveform Generation

Many applications require arbitrary waveform data, which can be difficult to manage with a software-centric system. Arbitrary waveforms are traditionally generated using Digital Signal Processing (DSP) techniques that require significant computation and code complexity.

The DMA can be used to create these same arbitrary waveforms by reading data sequentially from memory in the form of a lookup table and feeding this to other peripherals such as a Pulse-Width Modulator (PWM) or Digital-to-Analog Converter (DAC).

 

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