nRF52 SAADC Double Buffering

RE: nRF52 SAADC Double Buffering

Sterling Hughes — Tue, 02 Aug 2016 01:05:53 GMT

I've noticed in most of the examples that nrf_drv_saadc_buffer_convert() is called in the event handler, prior to the resulting buffer being processed.

Does this mean that buffers we pass to nrf_drv_saadc_buffer_convert() are guaranteed not to be filled until the event handler has finished processing? Or, should the results of the buffer in the event handler be processed, prior to calling nrf_drv_saadc_buffer_convert() and re-enabling it for data streaming.

RE: nRF52 SAADC Double Buffering

anasimtiaz — Fri, 27 May 2016 17:59:51 GMT

Very nicely explained. Thanks James!

RE: nRF52 SAADC Double Buffering

James Yu — Fri, 27 May 2016 17:07:56 GMT

Yes it will be in that order. Think of it as a FIFO queue: your first two nrf_drv_saadc_buffer_convert calls sets up the queue as adc_buf[0], adc_buf[1]. When the first saadc_event_handler occurs adc_buf[0] is removed from the queue, and when nrf_drv_saadc_buffer_convert is called again in the event handler adc_buf[0] is added back to the queue making it adc_buf[1], adc_buf[0].

The saadc example is better for understanding how a double buffered scheme could be useful. Instead of adc_buf[2], lets say you have adc_buf[2][1000], so now instead of having 2 buffers with 1 value each you have 2 buffers with 1000 values each. If you need to perform some operations on these 1000 samples it may be hard to maintain a high sampling rate at the same time. But with a double buffered scheme you can perform operations on 1 buffer while at the same time doing conversation on the other.

RE: nRF52 SAADC Double Buffering

anasimtiaz — Fri, 27 May 2016 08:44:14 GMT

Many thanks for the explanation James. So from (3) of your answer, I understand that regardless of sing;e/double buffering scheme nrf_drv_saadc_buffer_convert needs to be called to set up a buffer for conversion before sampling.

However, I have a couple of follow up questions about (2):

Initially, as you said, I have adc_buf[0] and adc_buf[1], and the call to nrf_drv_saadc_buffer_convert() sets up adc_buf[0] for conversion. The next two buffers will then be adc_buf[1] and adc_buf[0] ... do you mean in that order i.e. array element 1 and 0? If so, why?

Secondly, in a double buffered scheme with adc_buf[0] and adc_buf[1], what is the purpose of the second buffer? If adc_buf is an array with 16-bit elements, each sample will be 10-bits so what purpose does it serve to have an extra buffer element?

RE: nRF52 SAADC Double Buffering

James Yu — Thu, 26 May 2016 17:04:31 GMT

Hi,

Yes, you will get a NRF_DRV_SAADC_EVT_DONE when adc_buf[0] result is ready and another when adc_buf[1] is ready.
It sets up the completed buffer for the next conversion in non-blocking mode. So initially you have adc_buf[0] and adc_buf[1] set up as the 2 buffers. Then when you get the first NRF_DRV_SAADC_EVT_DONE event the completed buffer is adc_buf[0]. The nrf_drv_saadc_buffer_convert(p_event->data.done.p_buffer,1) call sets up adc_buf[0] for conversion again, so the next 2 buffers will be adc_buf[1] and adc_buf[0]. If you don't call nrf_drv_saadc_buffer_convert at this point then your next buffer is just adc_buf[1] and is no longer double buffered.
You will still get the NRF_DRV_SAADC_EVT_DONE event when adc_buf[0] is complete. If you want to continue sampling you will have to call nrf_drv_saadc_buffer_convert to set up adc_buf[0] again for conversion, otherwise you won't have any buffers ready for conversion.