PSoC5LP - First 5 ADC reads after component start come back 0

[KyTr_1955226]

Hi everyone,

I'm running into an issue on my Cy8C5468LTI-LP026 when first starting the ADC component.

I typically keep an array of ADC readings for temperature in my code and use it to generate a rolling average.  In order to do this, when I first start up, I need to populate the array with initial values.  I do so with the following functions:

#define ADC_NUM_SAMPLES      16

/* ADC_Read
 * - Reads and returns a single reading from the ADC
*/
static uint16_t ADC_Read(void){
    ADC_SAR_StartConvert();
    while (ADC_SAR_IsEndConversion(ADC_SAR_RETURN_STATUS));
    return ADC_SAR_GetResult16();
}

/* Sample_Temperature
 * - Takes a single ADC read and places it into the next position in the ADC_Vals array
*/
static void Sample_Temperature (void){
    static uint8_t sample_pos = 0;
    uint16_t adc_val = ADC_Read();
    /*ignore obviously faulty readings (T > 180C)*/
    if (adc_val > ADC_TEMP_FAULT_COUNT){
        adc_val = 0;
    } 
    ADC_Vals[sample_pos++] = adc_val;
    if (sample_pos >= ADC_NUM_SAMPLES){
        sample_pos = 0;   
    }
}

/* Init_Temperature
 * - Populates ADC Array and Average Temperature global with starting values
*/
void Init_Temperature (void){
    uint8_t i;
    ADC_SAR_Start();
    for(i = 0; i < ADC_NUM_SAMPLES; i++){
        Sample_Temperature();   
    }
    g_TemperatureAvg = Get_Temp_Avg();
}

What I find here, is when I run Init_Temperature in my initialization, my ADC_Vals array ends up with elements 0-4 being zero, with valid readings starting at element 5.  Does anyone know what's occurring here?  I tried putting a delay (10mS) between starting the component and beginning to take readings, but that didn't seem to have any effect.

I have found a workaround, which is if i simply drop a second for() loop into Init_Temperature after the first one in order to populate the ADC_Vals array again, I have no issues.  Reads after those initial 5 after starting the component don't seem to have a problem.  So I have a workaround, but I'd like to know why it's behaving as it is.

Component is configured as follows:

Any thoughts or suggestions are appreciated.

Thanks!

[Vison_Zhang · Accepted solution]

Firmware issue, change ADC_Read() function as follow can fix the issue.

ADC_SAR_IsEndConversion() API returns 0 when ADC is still calculating the last result, returns 1 only when last conversion is complete.

static uint16_t ADC_Read(void)

{ 

    ADC_SAR_StartConvert(); 

    while (!ADC_SAR_IsEndConversion(ADC_SAR_RETURN_STATUS));   

    return ADC_SAR_GetResult16(); 

} 

[KyTr_1955226]

It's always something really glaringly obvious in hindsight isn't it...

Can't believe I didn't catch that.  Sometimes a 2nd pair of eyes is all it takes

Follow-up question:  If I'm in free-running mode as I am here, presumably  those five '0' values I get are waiting for the first conversion to finish.  Since all my readings past that were fine, does that mean I would only need to worry about waiting for the conversion when I'm populating my values array at the beginning of application execution?

Past that point I'm only calling ADC_Read at a rate of about 1Hz.  After that initial population of the ADC values array there should basically always be a result ready and waiting in the ADC results buffer by the time my next call of ADC_Read(), correct?

Thank You!