Direct Memory Access (DMA) is a feature in embedded systems that folks often overlook. In a typical microcontroller, the CPU is responsible for transferring data between peripherals and memory. This means the CPU has to read data from a peripheral, process it, and then write it to memory. This process is not only time-consuming but also keeps the CPU busy, preventing it from performing other tasks.
Enter DMA controller. Think of these as a separate co-processor with limited capabilities in your uC. Technically it’s a state machine that can do at least 3 things, Wait for a control input, read from a place, and write to another place. It acts on its own with involvement from CPU freeing it to do other tasks. This is crucial in real-time systems where the CPU needs to perform critical tasks without interruption. By offloading data transfer tasks, DMA can handle large amounts of data quickly and efficiently. Think, continuous input streams from an external accelerometer or an audio stream. CPU kicks in only when the data transfer is complete to process the data, it doesn’t have to concern itself with the transfer. Another key advantage is since the CPU can remain in a low-power state during DMA operations, overall power consumption is reduced. This is vital in battery-powered devices.
Different manufacturers implement DMA in uCs in various ways. Usually you DMA channels, with each channel assigned for a particular task like one for reading from an ADC, one for writing to a USB bus etc. Each can have its own priority of execution. There can even be multiple DMA reading and writing to separate memory regions. It comes in all flavours. One beautiful implementation of DMA is driving RGB WS28 series LEDs in the Neopixel library. You can read more about them in the Adafruit blogs.
So if you haven’t used DMAs before, do give it a shot next time you need a large data transfer. You can effectively utilize DMA to enhance the performance of your embedded solution.
