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High Sample Rate with ADC and SoftDevice

I am trying to sample the ADC every 2ms. I notice lots of slowness with BLE advertising and typically can't connect to device over BLE when sampling. I am using PPI, configuring, and starting before the softdevice is enabled.

  1. Should nRF51822 be capable of reading ADC every 1ms to 2ms reliably?

  2. Should I move all the PPI configuration to after the softdevice is enabled and be using the sd_ppi functions?

Thanks!

Edit: Headline, format, added tags.

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  • I note that it is not possible to use the ARM core to 'sample' the ADC value regularly enough, BUT Is it possible to use the EasyDMA in the SPIS devices?

    My proposal:

    The ADC would be set to capture a value using the PPI and its START task and a timer, this gives the regular sample, but not the ability to 'save' the value before it is overwritten.

    To 'save' the data, the SPIS could be set to loop back upon itself, with the TXDPTR set to the ADC result address, and the MAXTX set to 1 for 8-bit samples and 2 for 10 or 9 bit samples. The RXDPTR would then be set to a normal RAM address with a normal buffer size for example 256 to capture 256 8-bit samples.

    Then you connect the MOSI and MISO pins, and the SCK to a suitable clock source (maybe the Master's clock, and then finally connect the CSN to a GPIO that is triggered using the PPI to make a suitably long chip enable signal.

    I know this is convoluted, but the SPIS is the only memory bus master other than those used by the SoftDevice. Is this a feasible, if awkward, solution to regular sampling whilst the SoftDevice is enabled?

    It would be useful to have some feedback to this proposal to help me make my product selection as the ADC capability is one of the nRFs key features (aside from its Bluetooth capabilty).

    Yours,

    Peter Myerscough-Jackopson

  • Stefan,

    I will attempt to implement and keep you posted. To make it work I think I will need to use 2 times, several PPI's, and the ADC. I think that the clock must be generated using one dedicated timer, and the timing for adc conversion and spi must be done using another timer. Of course, this implementation is clunky and will use increased power. I look forward to the rev3. Could you explain a little more about the current implementation and how it will be changed for my understanding? Right now a ble_evt is a background task (of highest priority) and therefore block out all other interrupts from interrupting it? Is the ble_evt a software interrupt or a hardware interrupt? Also, where can i find code for working with the timer peripherals. Does nordic have any documents outlining restricted priority levels for peripherals?

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  • Stefan,

    I will attempt to implement and keep you posted. To make it work I think I will need to use 2 times, several PPI's, and the ADC. I think that the clock must be generated using one dedicated timer, and the timing for adc conversion and spi must be done using another timer. Of course, this implementation is clunky and will use increased power. I look forward to the rev3. Could you explain a little more about the current implementation and how it will be changed for my understanding? Right now a ble_evt is a background task (of highest priority) and therefore block out all other interrupts from interrupting it? Is the ble_evt a software interrupt or a hardware interrupt? Also, where can i find code for working with the timer peripherals. Does nordic have any documents outlining restricted priority levels for peripherals?

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