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TI 29069 piccolo - ADCREFSEL

Brent Williams
Expert 1065 points

Hello I am using a TI 29069 (100 pin) controlCard with CC5.  I want to use an external 2.5 reference voltage in place of the internal 3.3 V for my ADC sampling.

In the ADC_SOC_Cnf.c file I changed

AdcRegs.ADCCTL1.bit.ADCREFSEL = 0; to 1 =>

AdcRegs.ADCCTL1.bit.ADCREFSEL = 1;
AdcRegs.ADCCTL1.bit.ADCBGPWD = 1; // Power up band gap
AdcRegs.ADCCTL1.bit.ADCREFPWD = 1; // Power up reference
AdcRegs.ADCCTL1.bit.ADCPWDN = 1; // Power up rest of ADC
AdcRegs.ADCCTL1.bit.ADCENABLE = 1; // Enable ADC

When using the internal 3.3 V reference voltage, I get certain digital bit values that convert to correct voltages (i.e. (bits)/4095 * 3300).

When I change to use an internal voltage reference of 2.5 volts, my digital bit count does not change.  It is the same as when I use the internal 3.3 voltage reference.  Thus my voltages are off.

I made sure I had good continuity from my VREFH to pin 66 on the DIMM connector and VREFL to pin 16 on the DIMM connector and that there is 2.5 voltages across the two.  NOTE:  VREFL is pulled to GND.

I also verified ADCREFSEL is set in the (ADCCTL1)

Am I missing something?

Any help would be greatly appreciated.

Thanks,

Brent

  • 0
    TI__Genius 11981 points

    Brent,

      When you calculate you voltages using 2.5V external reference, are you changing your formula to (bits)/4096*2500?  Since the 28069 ADC is ratiometric, the full scale range will change according to the reference voltage.  More information is provided in section 8.1.10.2 of the TRM.

    Regards,

    Joe

  • 0 in reply to Joe Prushing
    Expert 1065 points

    Hi Joe,

    Yes, I am.  

    The issue was actually the SW2 switch on the control card.  Position 1 needed to be in a upward position to connect VREFHI to pin 66 on the DIMM socket.  And likewise VREFLO needed to be in a upward position to connect to pin 16 of the DIMM socket.

    Once this was done I was seeing the correct values.

    Thanks for the response.

    Brent

  • 0 in reply to Brent Williams
    TI__Genius 11981 points

    Brent,

      Great, I'm glad you got it working!  Thanks for posting your solution, I'm sure other will benefit from it.

    Regards,

    Joe