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Stellaris LaunchPad, ADC behaviour

teoprimo
Prodigy 40 points


Hi guys,

I've choosen for my last university project the amazing Stellaris Launchpad.

Everything goes fine until I've used the integrated ADC:

A simple two steps sequence is used to measure I-V of a device:

Configuration:

GPIOPinTypeADC(GPIO_PORTE_BASE, GPIO_PIN_4);
GPIOPinTypeADC(GPIO_PORTE_BASE, GPIO_PIN_5);
ADCSequenceDisable(ADC0_BASE, 2);
ADCSequenceConfigure(ADC0_BASE, 2, ADC_TRIGGER_PROCESSOR, 0);
ADCSequenceStepConfigure(ADC0_BASE, 2, 0, ADC_CTL_CH8);
ADCSequenceStepConfigure(ADC0_BASE, 2, 1, ADC_CTL_CH9 | ADC_CTL_IE | ADC_CTL_END );
ADCSequenceEnable(ADC0_BASE, 2);
Call:

while(i < 300)
{
ADCIntClear(ADC0_BASE, 2);
ADCProcessorTrigger(ADC0_BASE, 2);
GPIOPinWrite(GPIO_PORTA_BASE, GPIO_PIN_7, 0);
while(!ADCIntStatus(ADC0_BASE, 2, false))
{
}
ADCSequenceDataGet(ADC0_BASE, 2, ADCvalue);
UARTCharPut(UART0_BASE, (0x3F & (ADCvalue[0] >> 6)));
UARTCharPut(UART0_BASE, (0x40 | (0x3F & ADCvalue[0])));
SysCtlDelay(1000000);
UARTCharPut(UART0_BASE, (0x80 | (0x3F & (ADCvalue[1] >> 6))));
UARTCharPut(UART0_BASE, (0xC0 | (0x3F & ADCvalue[1])));

if (ADCvalue[1] < 20) break;
i ;
}

As you can see, I put ADC datas into UART buffer with a simple two packets communication protocol that marks with a mask the packet (0x3F, 0x40 an so on).

Sometimes, two three times every call, samples of the first channel are changed with the samples of the second channel. It seems that ADC samples first the second channel and then the first one.

Can you help me?

Thank you so much!

  • 0
    Guru 117855 points

    Code extract does not show this required set-up/config...  (and there may be more - this jumps out!)

         // Enable the ADC peripherals and the associated GPIO port
        //
        ROM_SysCtlPeripheralEnable(SYSCTL_PERIPH_ADC0);
        ROM_SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOE );

    This is missed time & again - if you have done this it should be shown...

    Another thought - suspect that it is better to "unpack" your ADC FIFO - it may be that you have an over or underflow - or your FIFO is NOT empty after reading the data.

    Here's one method:  Note: we used this for a different sequence than yours - you must adapt.

            //
            // Disable the sequence.
            //
            HWREG(ADC0_BASE + ADC_O_ACTSS) &= ~ADC_ACTSS_ASEN0;
            //
            // Drain the Sequence FIFO.
            //
            while(!(HWREG(ADC0_BASE + ADC_O_SSFSTAT0) & ADC_SSFSTAT0_EMPTY))
            {
                //
                // Read the next sample.
                //
                ulTemp = HWREG(ADC0_BASE + ADC_O_SSFIFO0);
            }

            //
            // Clear any overflow/underflow conditions that might exist.
            //
            HWREG(ADC0_BASE + ADC_O_OSTAT) = ADC_OSTAT_OV0;
            HWREG(ADC0_BASE + ADC_O_USTAT) = ADC_USTAT_UV0;

        // Read the samples from the ADC FIFO.
        //
        g_pusADC0DataRaw[0] = HWREG(ADC0_BASE + ADC_O_SSFIFO0);   //   Note: your data is much safer copied to var - as shown here!
        g_pusADC0DataRaw[1] = HWREG(ADC0_BASE + ADC_O_SSFIFO0);


       

     

  • 0 in reply to cb1_mobile
    Genius 5695 points

    This might be of interest as well:

    http://e2e.ti.com/support/microcontrollers/stellaris_arm_cortex-m3_microcontroller/f/473/t/212263.aspx

    have fun...

  • 0 in reply to cb1_mobile
    Prodigy 40 points

    sorry for my late answer!

    Why should I have an underflow or an overflow? I wait until ADC generates an interrupt!

    Thank you so much!

  • 0
    Guru 47900 points

    Suggest that you try this:

    while(!ADCIntStatus(ADC0_BASE, 2, false))
    {
    }

    ADCIntClear(ADC0_BASE, 2);   // add this - also leave it - where it exists now - top instruct w/in while...

    ADCSequenceDataGet(ADC0_BASE, 2, ADCvalue);

    var0 = ADCvalue[0];

    var1 = ADCvalue[1];

    and then use your:

    UARTCharPut(UART0_BASE, (0x3F & (var0 >> 6)));   // this to insure that you don't over-flow the ADC FIFO 

    and similarly for var1...

    The fact that you confirm that your ADC data is "intermixed" - strongly points to over or under-flow.  You may succeed just with the 2nd ADCIntClear() - but try with the code as suggested first - then you may experiment by "backing out" the copy of the FIFO data.