CC1312PSIP: Sensor Controller's ADC Deviation.

Hi Gil,

I will try to find some data on this, let me come back to you today.

Regards,

Arthur

Hi Gil,

From the datasheet:

The variations you are seeing do not seem to match with that plot. Have you enabled input scaling? :

Overall, I would recommend you to look at the reference implementation, in ADCCC26XX.c:ADCCC26XX_convert, which will answer your questions I hope.

Regards,

Arthur

Hello Arthur

Thank you for your reply

Yes. Input scaling is turned on. 

I do apologies...but I did not fully understand your suggestion....

The code we are currently implementing is in the Sensor Controller and it follows the as example given by TI (in the "CC26xx, CC13xx Sensor Controller Studio Getting Started Guide" document). 

Functions relating to the Sensor controller's ADC did not include your recommendation to the best of my knowledge. 

Does ADCCC26XX_convert relate to the "main CPU" ? 

if ADCCC26XX_convert is traced back to the sensor controller, could you email a link to the implementation ?

Thank you 

/Gil 

Hi Gil,

My mistake, I indeed picked the wrong CPU. I will keep looking for more useful information.

Regards,

Arthur

No worries 

Maybe we should consider sampling for a longer time (?) 

Currently sampling before converting takes 42.6usec. 

Best regards

/Gil 

This could be relevant: 

e2e.ti.com/.../ccs-cc1312r-how-to-convert-the-sensor-controller-adc-value-to-voltage

Hello TheGhostOf  (catchy name ;) )

The suggestion in your thread refers to the compensation procedure applied to raw ADC data incoming to the "main cpu".

We use this methodology in our design as well. 

Trouble is that the deviation seen is exhibited despite applying the compensation.

It should also be notes that in a series of experiments done today i tries to increase the voltage pulse width prior to enabling the ADC.

I also tries increasing the ADC sample before convert time in function adcEnableSync() all to no avail. The deviation persists despite the increased voltage pulse, sample before convert times and compensation methodology.

Best regards

/Gil 

The sensor controller return the "raw" value and need the same compensation as done in the ADC driver. Please make sure that you do exactly the same steps. I have seen a fair number of post with similar question (the value deviate from expected) and most of them seem to be caused by FW. To check, use the ADC driver (using the M4) and see if you get the same result.

What do you use as input for these measurements (power supply, voltage divider...) and how do you know that your result is some mV too low? 

Let's do an experiment....

Setup

~~~~~

LP-EM-CC1312PSIP LaunchPad connected to LP-XDS110ET (the smaller portion) which will serve as the platform for the test.

The LP-EM-CC1312PSIP portion shall be powered by a 3V CRA123 battery in order to ensure minimal voltage ripple.  

That same supply voltage shall also be fed as the ADC analog inlet @ DIO25. 

The setup shall run a code through Sensor Controller Studio that will repeatedly sample DIO25 (every second):

macro sampleAuxInput() {

S16 adcValue;

// ADC Assign the AUX Analog Input

adcSelectGpioInput(AUXIO_A_ADC_IN);

// Enable the ADC with VDDS RELATIVE REFERNCE & MANUAL Triggering

adcEnableSync(ADC_REF_VDDS_REL, ADC_SAMPLE_TIME_42P6_US, ADC_TRIGGER_MANUAL); // Changed REF to RELATIVE

// Trigger the ADC Sampling

adcGenManualTrigger();

// Read the ADC Value

adcReadFifo(adcValue);

// Disable the ADC

adcDisable();

// Copy the ADC Value to the output buffer

state.adcValue = adcValue;

}

//---------------------------------------------------------------------------------------------------------------------------

sampleAuxInput();

evhSetupTimer1Trigger(0, 1000, 2);

Reference

~~~~~~~~

Now, let's use task testing to see ADC results through state.adcValue.

If my understanding is correct, since we are supplying the ADC inlet with the same voltage as the 3.3V supply rail - I would expect to get an all F's result from the ADC or very close to that. However, the result differs. Instead of getting a result close to 4096 we get a result around 3980 (setting aside the fluctuations)

Testing with our HW

~~~~~~~~~~~~~~~~

Now, let's return to the original hardware and run the full test code via IAR. 

I shall take 10 raw ADC measurements and make calculation to show the compensated voltage:

 As can be seen, raw ADC results with the original HW seem to be close to that seen with the reference.

Nevertheless, and despite using the compensation - a deviation exists.

This deviation currently creates a problem with our application.

Is this the expected deviation for Sensor Controller ADC? 

Best regards

/Gil  

If I don't remember wrong, the AUXADCValueToMicrovolts function do a third adjustment of the result (I believe the last line in the function, I don't have CCS installed on this PC so I'm not able to easy step through the code). Check the last line in the function. And do you get the correct results if you use this function on your above experiment? 

Here is an excerpt from an older thread (CCS/CC1312R: How to convert the sensor controller adc value to voltage?):

If you look at the adcsinglechannel example, this uses the function ADC_convertRawToMicroVolts to convert the value.

This function looks like this:

/*
 *  ======== ADCCC26XX_convertToMicroVolts ========
 */
uint32_t ADCCC26XX_convertToMicroVolts(ADC_Handle handle, uint16_t adcValue){
    ADCCC26XX_HWAttrs     const *hwAttrs;
    uint32_t                    adjustedValue;

    DebugP_assert(handle);

    /* Get the pointer to the hwAttrs */
    hwAttrs = handle->hwAttrs;

    /* Only apply trim if specified*/
    if (hwAttrs->returnAdjustedVal) {
        adjustedValue = adcValue;
    }
    else {
        uint32_t gain = AUXADCGetAdjustmentGain(hwAttrs->refSource);
        uint32_t offset = AUXADCGetAdjustmentOffset(hwAttrs->refSource);
        adjustedValue = AUXADCAdjustValueForGainAndOffset(adcValue, gain, offset);
    }

    return AUXADCValueToMicrovolts((hwAttrs->inputScalingEnabled ? AUXADC_FIXED_REF_VOLTAGE_NORMAL : AUXADC_FIXED_REF_VOLTAGE_UNSCALED), adjustedValue);
}"

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Assuming it is the same function for CC1312PSI, convertToMicroVolts() calls the gain and offset correction function after retrieving the preprogrammed chip values
However - This is the exact same procedure we undertake in our code and yet a deviation exists.

In should be noted that gain and offset for CC1312PSIP reside in register FCFG1
Since we are using VDDS as reference (relative reference) the values to be taken from FCFG1 are: 
SOC_ADC_REL_GAIN for gain.

But for offset - it would seem that the only value supplied is for absolute reference...
What offset value is used if the reference is relative (the same one)? 
To say that in other words - AUXADCAdjustValueForGainAndOffset() is not restricted to absolute reference. But if that is the case, what is the offset being used when the reference is relative?  

Could that account for the deviation seen?