Getting analog values from different channels using SAADC example

[arshdeep]

Hello ,

I want to read around 6 analog pins, not necessarily simultaneously. I could afford some delay in between them. i am using nrf52832 dev kit with sdk 15. How do i modify the SAADC example to achieve the same because the example only reads 1 channel. i did found this thread https://devzone.nordicsemi.com/f/nordic-q-a/5561/adc-from-various-pins/19481#19481 which looks similar but i was unable to open the GitHub link provided in the thread. Can you please direct me to the relevant examples. 

Thank you!

[Edvin]

Hello,

It should be fine to measure 6 ADC channels at once.

Attached is a slightly modified project from the SDK15.0.0\examples\peripheral\saadc example. Only the main.c file is changed. It will measure the 6 pins AIN0 - AIN5, and print the values in the UART log.

saadc_x_2.zip

 

Remember that AIN0 is not the same as A0 printed on the PCB. The AINx pins are:

AIN0 = P0.02
AIN1 = P0.03
AIN2 = P0.04
AIN3 = P0.05
AIN4 = P0.28
AIN5 = P0.29

Best regards,

Edvin

[arshdeep]

Hello Edvin, that was a great help. I was wondering what would i have to do to take the readings from the other 2 analog pins? it does not have to be at same time. could be after a delay, but how do i take those readings as well?

Thanks! 

[Edvin]

The two other pins? AIN6 and AIN7?

Just change the channel_X_config in saadc_init() to use NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(pin_nr) with the correct pin_nr. You can find the list of the AIN pins in the list on this page.

AIN6 = P0.30 and AIN7 = P0.31.

 

You can either add them to the list in saadc_init, like the other are added:

nrf_saadc_channel_config_t channel_N_config = NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN#);

and use:

nrf_drv_saadc_channel_init(channel_nr, &channel config);

 

Note that you should match your #define SAMPLES_IN_BUFFER with the number of channels that you use. If you have 6 channels, and SAMPLES_IN_BUFFER is 5, then you will get the values from channel 0, 1, 2, 3, 4 in the first event, and 5, 0, 1, 2, 3 in the second, 4, 5, 0, 1, 2 in the third, and so on.

 

Best regards,

Edvin

[arshdeep]

void saadc_callback(nrf_drv_saadc_evt_t const * p_event)
{
    if (p_event->type == NRF_DRV_SAADC_EVT_DONE)
    {
        ret_code_t err_code;
        uint16_t adc_value;
        uint8_t value[SAADC_SAMPLES_IN_BUFFER*2];
        uint8_t bytes_to_send;
     
        // set buffers
        err_code = nrf_drv_saadc_buffer_convert(p_event->data.done.p_buffer, SAADC_SAMPLES_IN_BUFFER);
        APP_ERROR_CHECK(err_code);
						
        // print samples on hardware UART and parse data for BLE transmission
        printf("ADC event number: %d\r\n",(int)m_adc_evt_counter);
        for (int i = 0; i < SAADC_SAMPLES_IN_BUFFER; i++)
        {
            printf("%d\r\n", p_event->data.done.p_buffer[i]);

            adc_value = p_event->data.done.p_buffer[i];
            value[i*2] = adc_value;
            value[(i*2)+1] = adc_value >> 8;
        }
				
        // Send data over BLE via NUS service. Makes sure not to send more than 20 bytes.
        if((SAADC_SAMPLES_IN_BUFFER*2) <= 20) 
        {
            bytes_to_send = (SAADC_SAMPLES_IN_BUFFER*2);
        }
        else 
        {
            bytes_to_send = 20;
        }
        err_code = ble_nus_string_send(&m_nus, value, bytes_to_send);
        if (err_code != NRF_ERROR_INVALID_STATE) 
        {
            APP_ERROR_CHECK(err_code);
        }
						
        m_adc_evt_counter++;
    }
}

Hello Edvin, thank you for your explanation. Regarding this attached saadc callback function. When is this function called, which interrupt calls this function? and also how do i convert the raw data into voltage. just take the data, then multiply by 5V and divide by 1023 ? or does it function differently? Thanks and Regards! 

[Edvin]

Hello,

In the SAADC example you can see two functions which sets up and enables the saadc callback:

saadc_sampling_event_init() and saadc_sampling_event_enable().

What they do is that they set up a timer and a PPI, and connect the two. This means that the timer interrupt is not called in the application, but when the timeout occurs, the chip will automatically call nrfx_saadc_sample().

When this function is called N times, where N is the size of your SAADC buffer, that is N = SAMPLES_IN_BUFFER, you will get the callback function.

I don't know whether you are familiar with the PPI, but it is a module which, when enabled, can perform tasks without involving the CPU. In this case, it works a bit like an advanced timer.

 

I really recommend that you use this implementation to trigger the samples. It will not work to call nrfx_saadc_sample() N times in a row in the application, because the sample has to finish before calling nrfx_saadc_sample again.

 

Regarding the conversion from the raw SAADC value to voltage, there are a few things to take into consideration:

Your resolution (by default set to 10 bits, your gain, usually set to 1/6. 

Please see this scheeme:

http://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.nrf52832.ps.v1.1%2Fsaadc.html&anchor=saadc_overview

 

From the link: 

RESULT = [V(P) – V(N) ] * GAIN/REFERENCE * 2(RESOLUTION - m)

Which means that V(P) - V(N) = RESULT/ (GAIN/REFERENCE * 2(RESOLUTION - m))


The result is linear, so you can also do some measurements to known voltages (using a voltmeter), and map the result to voltage using this.


Best regards,
Edvin

 

[arshdeep]

Hello Edvin, 

V(p) : voltage at input p 

V(n): voltage at input n, I do not exactly understand this. What are N and P? reference voltage and input voltage? 

Thanks and Regards,

Arshdeep