UART Reception + DMA

Question

When I am learning about DMA, I came to know that it can be implemented in UART peripheral operations too. Till now, I have used UART peripheral for simple communication operations. Sometimes I use it for Modbus protocol based projects. Put aside, simple projects, in Modbus protocol projects, I constantly process every byte received via UART receive interrupt to do further process quickly or with very minimal delay.

But DMA explains that it can trigger CPU interrupt after all of its buffer/half of buffers get filled only. Also, there is a situation that I don't know how much data comes via UART reception. If I receive only 4 bytes, then I need to process it. If I am supposed to receive 120 bytes to process, I need to wait for it. So here the length is not constant, at least in my requirements. But using DMA with UART needs constant byte count as far as I know. So, I dropped the idea to use DMA in Modbus based communication projects. Is it correct?

But using DMA free CPU burden more than a regular interrupt method. But in which applications can I use it? In which applications is UART + DMA concept more suitable than normal UART interrupt method? If there is any application link or examples available it will be helpful.

Answer 1

You did not specify the hardware, and you used "CPU" throughout the question. However since you also mentioned half/complete interrupts I am going to assume that you are actually talking about MCU.

using DMA with UART needs constant byte count

Not really. The combination of cyclic DMA mode + half/complete/idle interrupts allows you to process frames of any length. For the hardware that does not support idle interrupts there are alternative implementations, like using idle timer with Rx line wired to timer reset input.

This tutorial has great summary of various DMA methods. You may also find AN3109 by ST useful.

In which applications is UART + DMA concept more suitable than normal UART interrupt method?

Pretty much any packet-based communication (including Modbus) can benefit from DMA. The exceptions are:

• If your communication consists of just a few bytes sent with long idle periods in-between then setting up and processing DMA will unnecessarily complicate the software;
• In time-critical applications if reception of certain bytes (e.g. EOF) requires immediate reaction then using DMA may introduce unacceptable latency;
• Finally, if your application has nothing else to do when there is no data to process AND the MCU is sufficiently fast to process continuous stream of bytes at maximum bandwidth one by one then again, using DMI will introduce unnecessary complexity.

Basically, when making decision whether or not use DMA, consider this - data processing will take exactly the same amount of cycles anyway. Using UART interrupts will add IRQ entry/exit overhead to that per each byte. Using DMA will add same overhead but only to half/complete/idle buffer interrupts. Therefore, any buffer longer than two bytes will theoretically save some cycles for you (in reality you'd need somewhat longer buffer to see the benefits).

Answer 2

If there are termination characters you can search for those since some MCUs have a character match interrupt.

Many protocols also have a byte that indicates the length of the message that is located at a predictable point relative to the beginning of the message. You can use that to know when a full message is completed and how many bytes to parse for that message before you run into the next message in the buffer.

Otherwise, I suppose you could have a timer that ocasionally scans the last n unchecked bytes received for something that signifies a complete message. Or start processing when no bytes are received for a certain length of time which could also imply the end of a transmission.

But if you really do have to inspect every single byte as it comes in then there is no point to a DMA.