saacd scan oversample

[Jason Hendrix]

Hi, I'm trying to set up my SAADC to measure VDD and battery voltage on AIN3. According to this post, later versions of the SDK were modified so that one could use scan, burst, and oversample. But I'm using 14.1, and it seems like nrf_drv_saadc_channel_init() is written so that oversample is for 1 channel ONLY. What is the official word on oversampling multiple channels with burst at this time? Thanks.

I'm working on a custom board based on the nRF52832. I'm developing on Win 8.1 using SES and a J-Link Plus. SDK version 14.1.

[Jørgen Holmefjord]

Hi Jason,

As described by Carsten in the thread you are linking, oversample will give correct sampling in scan mode only when burst is enabled for all active channels.

Unfortunately, this is not supported by the SAADC driver in the SDK. It should however be quite simple to add this support if you want to use it. These instructions are for SDK 14.1.0, all changes in nrf_drv_saadc.c:

Add an array called burst to the struct nrf_drv_saadc_cb_t, to keep track of the burst settings for each channel (add between line 93-94):

uint8_t burst[NRF_SAADC_CHANNEL_COUNT];    ///< Burst configurations of SAADC channels.

Add a check to make sure all enabled channels have burst enabled, in nrf_drv_saadc_channel_init() (replace line 314-316):

// Oversampling can be used with only one channel, or if burst is enabled for all active channels.
uint8_t allow_oversample = true;
if(m_cb.active_channels != 0)
{
	for(int i = 0; i < NRF_SAADC_CHANNEL_COUNT; i++)
	{
		if ((m_cb.psel[i].pselp != NRF_SAADC_INPUT_DISABLED) && m_cb.burst[i] != NRF_SAADC_BURST_ENABLED)
		{
			allow_oversample = false;
		}
	}
	if(allow_oversample && p_config->burst == NRF_SAADC_BURST_DISABLED)
	{
		allow_oversample = false;
	}
}
ASSERT((nrf_saadc_oversample_get() == NRF_SAADC_OVERSAMPLE_DISABLED) || allow_oversample);

Store burst setting in struct before initializing the channel in nrf_drv_saadc_channel_init() (add between line 344-345):

m_cb.burst[channel] = p_config->burst;

Clear burst setting in struct when uninitializing channel in nrf_drv_saadc_channel_uninit() (add between line 383-384):

m_cb.burst[channel] = NRF_SAADC_BURST_DISABLED;

I will report this internally, hopefully we can make the driver support this.

Best regards,

Jørgen

[Jason Hendrix]

Thanks Jorgen, that works perfectly for me.

[ftjandra]

Is this now supported in SDK16.0 (scan + burst)? If not, what modifications need to be done and where. I don't think nrf_drv_saadc.c exists anymore.

Thanks.

[Coder_hope]

I am using SDK 15,it this is not supported too,do you figure out this problem?I am using 14bit oversampling, and SAADC_CONFIG_OVERSAMPLE  - Sample period is disabled , the noise is  not reduced..

[Jørgen Holmefjord]

It is not supported in SDK 16. Similar modifications to the ones described above can be added to nrfx_saadc.c.

Scan + burst is supported in SDK 17.0.0 with NRFX_SAADC_API_V2. Unfortunately, there are no examples in the SDK showing the usage of this new driver implementation.

[ankurt]

I was struggling with similar problem and looks like V2 API is good. But my application is based upon V1 api using nrf drv saadc leagcy layer. If no example, then at least a porting guide/document will be very helpful.