Read multiple ADC samples on single channel

[TS Gowtham]

Hi,

I'm trying to read the battery voltage of a custom board from the VDDHDIV5  channel. I'm using FreeRTOS in my project. I would like to read trigger the ADC to read 5 samples(non-blocking) once every hour. I tried the below pseudo code to trigger the read but I noticed that only one reading is present in the buffer.

nrf_saadc_value_t raw_value[5] = {0};

void saadc_handler(nrf_drv_saadc_evt_t const * p_event)
{
    if(p_event->type == NRF_DRV_SAADC_EVT_DONE)
    {
        raw_val = *(p_event->data.done.p_buffer);
        test = (raw_val*10/1024);
        
        //Notify saadc_thread
    }

}

void saadc_init()
{
    uint32_t err_code;

    //SAADC peripheral configuration
    nrf_drv_saadc_config_t m_saadc_config = 
    {
        .interrupt_priority = APP_IRQ_PRIORITY_LOW,
        .low_power_mode = false,
        .oversample = NRF_SAADC_OVERSAMPLE_DISABLED,
        .resolution = NRF_SAADC_RESOLUTION_10BIT
    };

    // Channel configuration
    //Internal reference is 0.6V
    nrf_saadc_channel_config_t m_channel_config = NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_VDDHDIV5);
    m_channel_config.acq_time = NRF_SAADC_ACQTIME_10US;
    m_channel_config.gain = NRF_SAADC_GAIN1_6;

    err_code = nrf_drv_saadc_init(&m_saadc_config, saadc_handler);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_channel_init(0, &m_channel_config);
    APP_ERROR_CHECK(err_code);
}

void saadc_sample()
{
    nrf_drv_saadc_buffer_convert(raw_value, 5);
    
    for(uint8_t i=0; i<5; ++i)
        nrf_drv_saadc_sample();
}

static void saadc_thread(void * arg)
{
    // Call saadc_sample and wait for a notification. And then delay for 1 hour
}

I tried two more things and noticed different output

• The SAADC example in the SDK. The code was running fine and the ADC values were correct.
• Call saadc_sample in the SAADC handler on NRF_DRV_SAADC_EVT_DONE. The ADC values were correct but this is not a desirable situation as I don't need the SAADC to constantly sample
• Change the size of raw_value to 1. The ADC value was wrong. I tried this with and without oversampling.

MCU - nRF52833-QDAAA0

SDK - nRF5 SDK 17.1.0

With FreeRTOS

[Raoul]

Hi Gowtham,

Due to the Easter holiday season in Norway, we are low in staff. Our FreeRTOS expert is out of office and will be back after Easter to answer your question.

Thanks for your patience!

Best regards,

Raoul

[TS Gowtham]

Hey Raoul,

Hope you had a great Easter! I just wanted to follow up and check if anyone got a chance to take a look at this problem.

Thanks,

Gowtham

[Susheel Nuguru]

Gowtham,

Thanks for waiting. Have you tried your application in the debugger mode? 

It seems like there might be some race conditions in your application causing some deadlock somewhere in your application context. Maybe wrong priorities within other tasks. It is hard to say without knowing all the contexts running in your application. 

If you do not find any race conditions or deadlocks in your application when you run this in the debug mode. Then please share your project so that I try to find the context which probably is starving other contexts.

[TS Gowtham]

Hi Susheel,

There are 2 tasks in the application. One task is to flush the nrf logger module and the other task is to read the values from the ADC.I don't think it's a deadlock or a race condition.

The application was built in the debug configuration and I noticed the problem when I debugging the code. I'll need to remove a few details and then I'll share the project.

Thanks,

Gowtham

[TS Gowtham]

Hi Susheel,

I've attached 2 projects.

1. blink_freertos-adc-test: Based on the blinky_freertos project. I've added a task to sample the SAADC VDDHDIV5 channel. The ADC value read is around 600 when the VDD value is around 4.06V

2. saadc-pcb-test: Based on the saadc peripheral example. I've changed the channel to VDDHDIV5. The ADC value read is around 230 when the VDD value is around 4.06V.

Both the projects are based on the nRF5 SDK v17.1.0 and were tested on the same PCB.

I feel that this seems to be an issue with how the values are read rather than FreeRTOS. 

blinky_freertos-adc-test.zip
saadc-pcb-test.zip