nrf52805: ADC polling mode issue

[Gokulnath]

Hardware setup: Custom board using BC805M-P (with App protect)
Software: nrf5 SDK S112 v17.0.2 ->Segger V7.30

My Objective: 

measure the Li-ion battery voltage(4.2V -100% till 3.6V-0%) through an external resistive divider & internal 0.6v ref voltage. (I'm not worried about the current consumption as of this moment. would like to make this concept work & all the other optimisations shall see later. Regarding optimisations yes I'm aware of this page: https://devzone.nordicsemi.com/nordic/nordic-blog/b/blog/posts/measuring-lithium-battery-voltage-with-nrf52)

What works so far?
for the moment my example measured voltage value is being transmitted over BLE through the battery service. it's all working well.
But the ADC is just measuring some garbage value (random) which is clearly not correct.

My concept of ADC polling mode is taken from this post & modified for my work. https://devzone.nordicsemi.com/f/nordic-q-a/14486/measuring-the-battery-voltage-with-nrf52832/129422

// Input range of External Vdd measurement = (0.6 V)/(1/5) = 3 V
// 3.0 volts ->  16383 ADC counts with 14-bit sampling:  5461 counts per volt
// 3.0 volts ->  4095 ADC counts with 12-bit sampling:  1365 counts per volt

#define ADC12_COUNTS_PER_VOLT 5461

void Adc12bitPolledInitialise(void)
{
    uint32_t timeout = 10;
    nrf_saadc_channel_config_t myConfig =
    {
        .resistor_p = NRF_SAADC_RESISTOR_DISABLED,
        .resistor_n = NRF_SAADC_RESISTOR_DISABLED,
        .gain       = NRF_SAADC_GAIN1_5,            // (1/5) Gain
        .reference  = NRF_SAADC_REFERENCE_INTERNAL, // 0.6V internal Ref Voltage
        .acq_time   = NRF_SAADC_ACQTIME_40US,       // See max source resistancetable
        .mode       = NRF_SAADC_MODE_SINGLE_ENDED,
        .burst      = NRF_SAADC_BURST_DISABLED,
        .pin_p      = NRF_SAADC_INPUT_AIN2,         // AIN2 for input Pin
        .pin_n      = NRF_SAADC_INPUT_DISABLED
    };

    nrf_saadc_resolution_set((nrf_saadc_resolution_t) 3);   // 2 is 12-bit , 3 for 14-bit 
    nrf_saadc_oversample_set((nrf_saadc_oversample_t) 2);   // 2 is 4x, about 150uSecs total
    nrf_saadc_int_disable(NRF_SAADC_INT_ALL);
    nrf_saadc_event_clear(NRF_SAADC_EVENT_END);
    nrf_saadc_event_clear(NRF_SAADC_EVENT_STARTED);
    nrf_saadc_enable();

    NRF_SAADC->CH[1].CONFIG =
              ((myConfig.resistor_p << SAADC_CH_CONFIG_RESP_Pos)   & SAADC_CH_CONFIG_RESP_Msk)
            | ((myConfig.resistor_n << SAADC_CH_CONFIG_RESN_Pos)   & SAADC_CH_CONFIG_RESN_Msk)
            | ((myConfig.gain       << SAADC_CH_CONFIG_GAIN_Pos)   & SAADC_CH_CONFIG_GAIN_Msk)
            | ((myConfig.reference  << SAADC_CH_CONFIG_REFSEL_Pos) & SAADC_CH_CONFIG_REFSEL_Msk)
            | ((myConfig.acq_time   << SAADC_CH_CONFIG_TACQ_Pos)   & SAADC_CH_CONFIG_TACQ_Msk)
            | ((myConfig.mode       << SAADC_CH_CONFIG_MODE_Pos)   & SAADC_CH_CONFIG_MODE_Msk)
            | ((myConfig.burst      << SAADC_CH_CONFIG_BURST_Pos)  & SAADC_CH_CONFIG_BURST_Msk);

    NRF_SAADC->CH[1].PSELN = myConfig.pin_n;
    NRF_SAADC->CH[1].PSELP = myConfig.pin_p;
}

void ble_Update_BatteryVoltage(void)
{
    // Enable command & Turn on the Power
    nrf_gpio_pin_write(ADC_SWITCH, 1); //turning on the voltage bridge on with a transistor
    nrf_saadc_enable();

    uint16_t result = 9999;         // Some recognisable dummy value
    uint32_t timeout = 100000;       // Trial and error
    volatile int16_t buffer[8];

    NRF_SAADC->RESULT.PTR = (uint32_t)buffer;
    NRF_SAADC->RESULT.MAXCNT = 1;

    nrf_saadc_event_clear(NRF_SAADC_EVENT_END);
    nrf_saadc_task_trigger(NRF_SAADC_TASK_START);
    nrf_saadc_task_trigger(NRF_SAADC_TASK_SAMPLE);

    if (timeout != 0)
    {
        result = ((buffer[0] * 1000L)+(ADC12_COUNTS_PER_VOLT/
        5)) / ADC12_COUNTS_PER_VOLT;
    }

    while (0 == nrf_saadc_event_check(NRF_SAADC_EVENT_END) && timeout > 0)
    {
        timeout--;
    }
    nrf_saadc_task_trigger(NRF_SAADC_TASK_STOP);
    nrf_saadc_event_clear(NRF_SAADC_EVENT_STARTED);
    nrf_saadc_event_clear(NRF_SAADC_EVENT_END);

    // Disable command & turn off the Power to ADC Bridge to reduce power consumption
    nrf_saadc_disable();
    
    nrf_gpio_pin_write(ADC_SWITCH, 0); //turning off the voltage bridge on with a transistor
    ble_bas_battery_level_update(&m_bas, result, m_conn_handle); 
}

The ADC result value is clearly not accurately working when I'm just calling the function manually upon a button press. ble_Update_BatteryVoltage();

What am I doing wrong?

Thanks, for any valuable input I'm new to NRF. 
Gokunath 

[hmolesworth]

Using the code posted with the floating-point percentage calculation: This data comes from your code using a 2.996 volt input (slightly different resistors, but that is not the point here):

-Raw SAADC-   --------Scaled-------   --------Percent1-----    -------Percent2-----
 Dec   Hex     Decimal     Hex         Decimal      Hex         Decimal      Hex
==== ======   ========== ==========   ========== ==========    ========== ==========
2979 0x0BA3    2979.00   0x453A3000      39.86  0x421F6DB7       39.86 0x421F6DB7
2972 0x0B9C    2972.00   0x4539C000      38.86  0x421B6DB7       38.86 0x421B6DB7
2965 0x0B95    2965.00   0x45395000      37.86  0x42176DB7       37.86 0x42176DB7
2964 0x0B94    2964.00   0x45394000      37.71  0x4216DB6D       37.71 0x4216DB6D
2966 0x0B96    2966.00   0x45396000      38.00  0x42180000       38.00 0x42180000
2966 0x0B96    2966.00   0x45396000      38.00  0x42180000       38.00 0x42180000
with Burst 4 enabled
2981 0x0BA5    2981.00   0x453A5000      40.14  0x42209249       40.14 0x42209249
2970 0x0B9A    2970.00   0x4539A000      38.57  0x421A4925       38.57 0x421A4925
2972 0x0B9C    2972.00   0x4539C000      38.86  0x421B6DB7       38.86 0x421B6DB7
2965 0x0B95    2965.00   0x45395000      37.86  0x42176DB7       37.86 0x42176DB7
2966 0x0B96    2966.00   0x45396000      38.00  0x42180000       38.00 0x42180000
2966 0x0B96    2966.00   0x45396000      38.00  0x42180000       38.00 0x42180000

ints truncate floats; also as an aside floats (on the nRF52 hardware) are only 32-bit inclusive of sign and exponent, so have less precision than 32-bit ints. Compiler implicit conversion is compiler-dependent.

The data above is generated by adding these lines to the code :

// Input range = (0.6 V)/(1)   = 0.6V -> 3.393 V with 931k/200K dividers
// 3.0 volts -> 14486 ADC counts with 14-bit sampling: 4828.8 counts per volt
#define ADC12_COUNTS_PER_VOLT 4829

    // Disable command & turn off the Power to ADC Bridge to reduce power consumption
    nrf_gpio_pin_write(ADC_SWITCH, 0); //Turning off the voltage divider transistor
    nrf_saadc_disable();
    nrf_saadc_task_trigger(NRF_SAADC_TASK_STOP);
    nrf_saadc_event_clear(NRF_SAADC_EVENT_STARTED);
    nrf_saadc_event_clear(NRF_SAADC_EVENT_END);
    float f_result = (float)((buffer[0] * 1000L) * 1.667 * 16384);
    f_result = result; ///0.140625;
    int32_t i_result = result; ///0.140625;
    float percent_result1 = volttopercent(i_result);
    float percent_result2 = volttopercent(f_result);
    char InfoPacket[180] = "";
    // -Raw SAADC-   -------Scaled-----   ------Percent1----   ------Percent2----
    //  Dec   Hex    Decimal   Hex        Decimal     Hex      Decimal    Hex
    // ==== ======   ======= ==========   ======= ==========   ======= ==========
    // 2968 0x0B98   2968.00 0x45398000     38.29 0x42192492     38.29 0x42192492
    snprintf(InfoPacket, sizeof(InfoPacket), "-Raw SAADC-   -------Scaled-----   ------Percent1----   ------Percent2----\r\n");  uartSend(InfoPacket, strlen(InfoPacket));
    snprintf(InfoPacket, sizeof(InfoPacket), " Dec   Hex    Decimal   Hex        Decimal     Hex      Decimal    Hex\r\n");     uartSend(InfoPacket, strlen(InfoPacket));
    snprintf(InfoPacket, sizeof(InfoPacket), "==== ======   ======= ==========   ======= ==========   ======= ==========\r\n"); uartSend(InfoPacket, strlen(InfoPacket));
    snprintf(InfoPacket, sizeof(InfoPacket), "%4d 0x%04X  %8.2f 0x%08X    %6.2f 0x%08X    %6.2f 0x%08X\r\n", result, result, f_result, *(uint32_t *)&f_result, percent_result1, *(uint32_t *)&percent_result1, percent_result2, *(uint32_t *)&percent_result2);
    uartSend(InfoPacket, strlen(InfoPacket));

My post here is to show how to check the algorithm is doing the required actions ..

Edit: I commented out the peculiar 0.140625 divider, correctd the hex float print and changed the % calc to use mV and correct min & max for my test setup

[hmolesworth]

I corrected the hex float print and changed the % calculation to use mV and correct min & max for my test setup with 2032 coin cell

float volttopercent(float vadc)
{
  float Vmax = 3400; // mV was 4.2;
  float Vmin = 2700; // mV was3.63;
  float percentage;
  percentage = ((vadc -Vmin)/(Vmax - Vmin))*100;
  return percentage;
}

Results look more sensible:

-Raw SAADC-   ------Scaled------   ------Percent1----   -----Percent2------
 Dec   Hex    Decimal   Hex        Decimal   Hex        Decimal      Hex
==== ======   ======= ==========   ======= ==========   =======  ==========
2979 0x0BA3   2979.00 0x453A3000     39.86 0x421F6DB7     39.86  0x421F6DB7