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TM4C1290NCPDT: AIN12-AIN13 and PD2-PD3 reading issues

a chida
Prodigy 20 points

Part Number: TM4C1290NCPDT

I'm using PD2-PD3 for analog to digital conversion on one board.
Even if the same voltage is given to PD2-PD3, the converted value may vary from port to port.

For example, when 3.3V is applied to each of PD2 and PD3, the value read by PD2 will be F0 and the value read by PD3 will be FF.

I've made this evaluation board many times before and had no problems with it, but I'm experiencing this problem with several evaluation boards I've made recently.

These have all been resolved by simply replacing the TM4C1290NCPDTI3R with parts.

Do you have any possible causes for this?
If it's a recent known problem or something like that, it's a safe bet.

  • 0
    TI__Guru 72500 points

    I suspect that it is related to the total input resistance and the time to charge the internal sample capacitor. If you convert the same voltage (3.3V) on both pins using a single sequence, and you convert PD3 first (step 0) and then PD2 (step 1), is PD2 still lower? 

  • 0 in reply to Bob Crosby
    Prodigy 20 points

    I tried it, but PD2 still has a low voltage.
    However, when I lowered the total input resistance, PD2 became F4 and PD3 was FF.
    Even if the resistance value was lowered as much as possible, it only changed up to F4, but I was able to confirm the trend of improvement.

    The setting of the external resistance may be one of the causes, but there may be other causes as well.

  • 0 in reply to a chida
    TI__Guru 72500 points

    It may still be an issue of input resistance as the internal resistance of the input multiplexer switches must be included. (up to 2.5K Ohms) What frequency are you running the ADC? Try increasing the sample time by writing to the appropriate "ADC Sample Sequence Sample and Hold Time" register.