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I am trying to use DMA for my UART Rx and Tx. Till now I had the freeRTOS version of the serial demo working fine. It still works fine. However, now I have incorporated the UART DMA example, from the example projects. the code is conditionally compiled, so that when a switch _HAS_DMA == 1, only then the DMA engine is configured, ram buffers are configured, and default UART ISRs as required by the FreeRTOS demo are removed.

At this point, whenever I send a serial byte stream, the running project simply gets reset.

I am using MPLAB IDE 8.92, XC16 v1.20, Explorer-16 platform, dspic33fj256gp710 part.

The DMA code included does not use any FreeRTOS API calls. I have setup the project so that StackOverflow is detected using the FreeRTOS configuration option. But the code does not reach the Stackoverflow hook function. I have also included the U2ErrInterrupt ISR to see if incoming bytes are coming in fine, however even that interrupt is not reached.

Has any one faced this before?

interestingly, the UART DMA Loopback example from Microchip website, which uses the MPLAB C30 compiler, works fine on my board.

any pointers on this one? I could not locate any code examples in the FreeRTOS forum on how to use the DMA for UART, but it is suggested to use this method in production code for efficiency.

Need help here.

Thanks and best regards, Vishal

Vishal
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1 Answers1

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OK. I found the culprit. Its me. :)). When setting up the DMA to receive UART interrupts, one should not enable UART interrupt separately in software. Which is what I was doing. In addition, I had conditionally un-compiled the UART ISRs from my code !!!. So in effect, whenever a byte was received by the UART engine, the processor is getting confused as to who will serve this interrupt, DMA or Application code. I thing the PC would point to the designated UART RX ISR vector location, where the processor would not find anything, and this was causing the reset. Or may be there was a race condition setup between the DMA and the processor to serve this interrupt, which was causing the reset.

Now that I have setup UART so that Interrupts are not enabled separately by application, when DMA is going to serve the UART RX, my code is working fine. I am yet to integrate the whole thing with FreeRTOS deferred interrupt processing using binary semaphores, but I hope I will not see any troubles there.

There is not much documented about this though...neither in Microchip manuals nor in the FreeRTOS examples.

Also, I found that when using DMA with UART, as per the manual, the DMA receives WORDS from the UART RX engine, with lower byte having the data, and upper byte having the status. If the DMA is also used for UART Tx, and is set to transfer WORDS to UART TXREG, the two intelligently manage to send only the lower data byte out. So the receiving party still gets expected bytes. This is also not documented well.

I will try to post my code here for future generations though :)).

Vishal
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