Reading ADC with a double buffer

[Alaa]

Hi,

I have a question about ADC reading in ble_app_uart__saadc_timer_driven__scan_mode in SDK 14.

Let's say I'm using a double buffer, and I have 3 ADC channels (so SAADC_SAMPLES_IN_BUFFER=3)

Now I look in p_event->data.done.p_buffer[ i ] . Is the following correct:

p_event->data.done.p_buffer[ 0 ] => Value of channel 0

p_event->data.done.p_buffer[ 1 ] => Value of channel 1

p_event->data.done.p_buffer[ 2 ] => Value of channel 2

I'm not sure because somehow I'm reading almost the same data from [0] and [2], and [1] is about x2 of the other too.

Data example:

201	420	201
199	418	201
201	416	199
201	414	200
199	413	196
194	409	195
197	413	197
198	414	196
195	410	192
197	413	199
198	416	196
198	417	199
197	413	198
200	415	197
200	417	198
197	415	197
196	410	195
196	409	194
195	411	194
200	415	199

Can you assist please?

Thanks

[Edvin]

Hello,

Do you have 3 different gpios measureing these values? Are they connected/shorted to anything? Or do you have one input GPIO, and it stores the values into three different sections in the buffer?

 

Best regards,

Edvin

[Alaa]

3 different GPIOS (AIN0, AIN1, AIN2). And here's the initialization:

    nrf_saadc_channel_config_t channel_0_config =
        NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN0);
    channel_0_config.gain = NRF_SAADC_GAIN1_6;
    channel_0_config.reference = NRF_SAADC_REFERENCE_VDD4;
	
    nrf_saadc_channel_config_t channel_1_config =
        NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN1);
    channel_1_config.gain = NRF_SAADC_GAIN1_6;
    channel_1_config.reference = NRF_SAADC_REFERENCE_VDD4;
	
    nrf_saadc_channel_config_t channel_2_config =
        NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN2);
    channel_2_config.gain = NRF_SAADC_GAIN1_6;
    channel_2_config.reference = NRF_SAADC_REFERENCE_VDD4;
	
    err_code = nrf_drv_saadc_init(&saadc_config, saadc_callback);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_channel_init(0, &channel_0_config);
    APP_ERROR_CHECK(err_code);
    err_code = nrf_drv_saadc_channel_init(1, &channel_1_config);
    APP_ERROR_CHECK(err_code);
    err_code = nrf_drv_saadc_channel_init(2, &channel_2_config);
    APP_ERROR_CHECK(err_code);

I just disabled OVERSAMPLING and now it's working well. Each channel looks independent now. Is there a way to make it work with oversampling enabled? Why would oversampling (of 4x) cause this behavior?

Thanks

[Edvin]

Oversampling should not cause a higher value. When the error occured, did you have oversampling on all the channels, or only one of them? And in addition to oversampling, was the rest of the channel_config settings the same?

 

Best regards,

Edvin

[Alaa]

Hi Edvin,

Yes, channel_config settings for 0,1,2 were the same like appears in the code I posted.

And about oversampling, I just changed the variable SAADC_CONFIG_OVERSAMPLE in sdk_config.h to 2. Do I have to do anything else to turn it on correctly?

Thanks!

[Edvin]

Did you measure the same source with all three pins, or was channel 1 measuring something else?

 

What chip do you use? nRF52832 or nRF52840?

And you say that you used SDK14. Is that SDK14.0.0, SDK14.1.0 or SDK14.2.0?

 

BR,

Edvin