Selective sampling of ADC channels at run time

[SChary]

I am using a 52810 custom board and the SAADC module.

It is used for sampling one analog input through AIN2 and the battery voltage directly by setting the pin to VDD

As per the documentation, since more than one channel is configured/enabled, it goes into scan mode.

i need to measure the analog input at AIN2 for 15 seconds.

However, i do not need to measure the battery voltage in every scan. I only want to measure it once every 5 seconds to check for voltage drop.

Is this possible ?

Regards

Chary BS

[Einar Thorsrud]

Hi Chary,

You will always sample all configured channels. So if you only want to sample battery voltage every now and then you need to reconfigure the SAADC. An alternative could be to also sample the battery voltage every time, but ignore the samples most of the time unless when you want to process it.

Einar

[SChary]

Hello Einar,

Thanks for the clarification ....

Is the following code correct in order to add the second channel to the SAADC initial configuration? I just added channel to use VDD as input.. andinitialized it.

<code>

void saadc_init(void)
{
    ret_code_t err_code;
    nrf_saadc_channel_config_t channel0_config =
        NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN2);

    nrf_saadc_channel_config_t channel1_config =
        NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_VDD);   //To measure the battery voltage.

    err_code = nrf_drv_saadc_init(NULL, saadc_callback);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_channel_init(0, &channel0_config);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_channel_init(1, &channel1_config);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_buffer_convert(m_buffer_pool[0], SAMPLES_IN_BUFFER);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_buffer_convert(m_buffer_pool[1], SAMPLES_IN_BUFFER);
    APP_ERROR_CHECK(err_code);
}

</code>

Regards

Chary BS

[SChary]

And also, is there a way to know which channel's readings are being returned in the callback ? OR do we have to maintain a static variable as a flag to toggle or sequence through the number of channels every time the callback is called?

[SChary]

Hi Einar,

This example from Nordic confirmed the answer to my question - it seems a trivial question in hind sight :-)

https://github.com/NordicPlayground/nRF52-ADC-examples/blob/master/ble_app_uart__saadc_timer_driven__scan_mode/main.c

But it would be good to have a channel index returned along with the callback, so that we can identify the data source and apply any processing logic based on that.... just a suggestion from my side !

Cheers

Chary BS

[Einar Thorsrud]

Hi Chary,

Good to hear you found it. Essentially it is just the offset in the buffer. So the first channel is at position 0, second enabled channel in position 1 and so on. I will forward your suggestion, though I do not expect any updates in the API any time soon.

Einar