3 Software Oversampling

The example used for this application note utilizes SysConfig with driverlib for the configuration of the ADC, ePWM, and other peripherals. Within the program code, set up the ADC to minimize overhead such that more time can be used between conversions to do a control loop. For the example used in this application note, the SOCs are configured in burst mode with round-robin priority, so that the SOCs are triggered together and accumulated without missing a value when oversampling. An interrupt is set up to trigger once the last SOC, SOC15 for F28003x, reaches the end of conversion. The interrupt runs the corresponding ISR, which stores the ADC result and accumulate multiple SOC results if oversampling is enabled.

The ePWM triggers the SOCs here, however software triggers and CPU timer triggers are also available. Take care when choosing the period of the trigger to maintain uniform sampling of the SOCs, and appropriate conversion time with respect to the rest of the control loop. Once the last SOC in the burst sequence issues an end-of-conversion signal, the ISR executes the control loop. In this example, the control loop consists of a simple accumulation function for oversampling and storing the results. Avoid averaging the values because this effectively reduces measurement precision by discarding information contained in the lower bits of the result. The final result is stored in memory before the next burst is triggered.

The following code is an example of baseline sampling with the burst ISR setup:

__interrupt void adcA1ISR(void)
{
    //
    // Clear the interrupt flag
    //
    ADC_clearInterruptStatus(ADCA_BASE, ADC_INT_NUMBER1);

    //
    // 1X Oversampling
    //
    lv_results[nloops++] = ADC_readResult(myADC0_RESULT_BASE, ADC_SOC_NUMBER0);

    //
    // Check if overflow has occurred
    //
    if(true == ADC_getInterruptOverflowStatus(ADCA_BASE, ADC_INT_NUMBER1))
    {
        ADC_clearInterruptOverflowStatus(ADCA_BASE, ADC_INT_NUMBER1);
        ADC_clearInterruptStatus(ADCA_BASE, ADC_INT_NUMBER1);
    }

    //
    // Check if all results are stored
    //
    if(nloops >= numBins)
    {
        //
        // Disable ADC interrupt
        //
        ADC_disableInterrupt(myADC0_BASE, ADC_INT_NUMBER1);
        ESTOP0;
    }

    //
    // Acknowledge the interrupt
    //
    Interrupt_clearACKGroup(INTERRUPT_ACK_GROUP1);
}

An example of oversampling a signal at 8 × with ISRs is as follows:

__interrupt void adcA1ISR(void)
{
    //
    // Clear the interrupt flag
    //
    ADC_clearInterruptStatus(ADCA_BASE, ADC_INT_NUMBER1);

    //
    // Accumulate 8 ADC results to oversample 8X
    //
    lv_results[nloops++] = (ADC_readResult(myADC0_RESULT_BASE, ADC_SOC_NUMBER0) + ADC_readResult(myADC0_RESULT_BASE, ADC_SOC_NUMBER1) +
            ADC_readResult(myADC0_RESULT_BASE, ADC_SOC_NUMBER2) + ADC_readResult(myADC0_RESULT_BASE, ADC_SO