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Cortex M4 (LPC4370): how do I detect ADC threshold crossing while moving data in a DMA driven double buffer?

Andrea Bettati

Hi to you all,

I'm currently working on a project involving the LPC-Link2 as a eval. board for it's LPC4370 (for a complete explanation see this question).
What I'm trying to do is:

  • Continuously sample external analog signal (using on-board ADCHS)
  • Transfer data to RAM (I'm using a ping-pong buffer: let's call the sub-buffers s[0] and s[1])
  • When a threshold crossing is detected by the ADCHS -> begin data processing

At the moment I managed to be fine with the first 2 bullets, my question is: what is the fastest (and most elegant) way to process data without loosing (too much) samples and whitout interferring with DMA activity?

I tried the following procedure:

  • Use ADCHS interrupt (highest priority) to capture the address at which the DMA is currently pointing (eg. in sub-buffer s[0])
  • Process the data in the DMA ISR (eg. when s[1] is being filled)

BUT, it seems that I'm haviong troubles because of the cortex M4 ISR overhead. A drawing will better explain this:

The * in the image means:

  • ADCHS sampled the threshold-crossing responsible semple in s[1], thus flagging to process s[1]
  • By the time the DMA ISR gets the ADCHS flag s[1] is being filled and the DMA ISR won't process it.

How do I solve this? If anything isn't that clear I will be happy to explain it better.

Any help would be highly appreciated!

Regards,

Andrea

Parents
  • 0 in reply to Thibaut ZEISSLOFF

    Hi Thibaut, I saw in the past few months that this microcontroller is really rarely used and (I think) a bit underrated. I feel somewhat lost in trying to understand how it works. Anyway, maybe I'll just switch back to my previous solution as you said, but I'm also considering to purchase from NXP a Pro licence to have 1 year of email support.

    I'd really like to thank you for the help and I won't forget to update this post if I'll get this thing work.

    All the best,
    Andrea

Reply
  • 0 in reply to Thibaut ZEISSLOFF

    Hi Thibaut, I saw in the past few months that this microcontroller is really rarely used and (I think) a bit underrated. I feel somewhat lost in trying to understand how it works. Anyway, maybe I'll just switch back to my previous solution as you said, but I'm also considering to purchase from NXP a Pro licence to have 1 year of email support.

    I'd really like to thank you for the help and I won't forget to update this post if I'll get this thing work.

    All the best,
    Andrea

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