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How to setup external ADC Reference with VREF pin?

Cedric Bodemer
Prodigy 90 points
Other Parts Discussed in Thread: TM4C1294NCPDT

I'm trying to use an external voltage reference  with the Tiva C LaunchPad ADC - but for some reason it just won't use it...

I followed the instructions on http://www.ti.com/lit/ds/symlink/tm4c1294ncpdt.pdf page 1164 and 1861. The latter page states:

"VREFA+ = Positive external voltage reference for ADC, when VREF field in the ADCCTL register is 0x1"

I assume "VREFA+" is the "VREF" pin 34 (see http://www.ti.com/lit/ug/spmu365a/spmu365a.pdf page 15) on the Tiva C LaunchPad and I got a REF5040 divided to 3.079V connected to that pin.

This is the code I'm setting things up with:

#define ADC_REF_EXT_3V          0x00000001  // External 3V reference

void ADCInit(void) {
	MAP_SysCtlPeripheralEnable(SYSCTL_PERIPH_ADC1);

	MAP_ADCReferenceSet(ADC1_BASE, ADC_REF_EXT_3V);  // this define is 0x1 like demanded by page 1861

	MAP_ADCHardwareOversampleConfigure(ADC1_BASE, 1);

	MAP_ADCSequenceConfigure(ADC1_BASE, 3, ADC_TRIGGER_TIMER, 0);
	MAP_ADCSequenceStepConfigure(ADC1_BASE, 3, 0, ADC_CTL_CH3 | ADC_CTL_IE | ADC_CTL_END);
	MAP_ADCSequenceEnable(ADC1_BASE, 3);

	MAP_ADCIntEnable(ADC1_BASE, 3);
	MAP_IntEnable(INT_ADC1SS3);
	MAP_ADCIntClear(ADC1_BASE, 3);
}

void ADCPinInit(void) {
	MAP_SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOE);

	MAP_GPIOPinTypeADC(GPIO_PORTE_BASE, GPIO_PIN_0);
}

However, all the samples I get are based on the internal voltage reference - exactly like setting

MAP_ADCReferenceSet(ADC1_BASE, ADC_REF_INT);

Am I missing something here or is this feature somehow broken on the Tiva C? Unfortunately I couldn't find anything on this issue yet...

Do I also get page 1861 right, that only 2.4V - 3.3V references are supported and there is no way around this?

  • 0
    TI__Guru**** 244440 points

    Hello Cedric,

    How have you configured the ADC Analog Inputs. That is is not mentioned in your code.

    Regards

    Amit

  • 0
    Guru 26105 points

    Hi,

    On my C board, the micro is indeed NCPDT - but both the user manual and schematics show up REFA+ pin single, fixed, on pin 9, not 34. GNDA is  pin 10.

    Also, there is REF5030, really with 3.000V - you can use it without additional divider.

    If using the external reference, take care to remove R41 from the board.

    Petrei

  • 0 in reply to Petrei
    Prodigy 90 points

    Thank you both for the quick answers.

    I updated the first post with the pin configuration.

    As for the VREF pin: I mean the pin on the side of the LaunchPad, not the pin number of the chip. According to http://www.ti.com/lit/ug/spmu365a/spmu365a.pdf page 15 it is pin 34...

    Is removing R41 mandatory for using an external voltage reference?

  • 0 in reply to Cedric Bodemer
    Guru 26105 points

    Hi,

    Yes, absolutely - see the schematic in board's user manual.

    Also, to avoid confusing pin names in the future, suggest to name the pin prefixed by connector name, like X1-34.

    Petrei