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MSP430F5529: ADC12 Pull-up/down resitors

Harvey Ku97
Prodigy 140 points
Part Number: MSP430F5529

Hi,

I wasn't able to confirm in the datasheets whether or not the input pull-up/down resistors can be enabled while utilizing the ADC12 mode.  However, I noticed my ADC channels would read different values depending on whether or not I declared an internal pull-down resistor or not on the ADC ports.  Could you confirm whether or not the MSP430F5529 can utilize the internal pull-down resistors while utilizing the ADC channels? 

Thank You

Harvey

  • 0
    TI__Guru 56685 points
    Hello Harvey,

    The internal pullup/down resistors lie within the digital domain of the GPIO configuration while the ADC is within the Analog domain. this means when the pins are selected to be used as ADC, the pullup/down resistors are not available and not connected to the circuit internally. If you are getting inconsistent values, I would suggest increasing your sample and hold time for the ADC.
  • 0 in reply to Jace H
    Prodigy 140 points

    Thanks for the quick response,  I made your suggested change to the code to increase the sample and hold period from 256 cycles to 1024 cycles but the results are the same.  I still receive different read values; depending if I declare an internal pull down or not after choosing ADC mode.  

    Could it be that declaring an internal pull-down after choosing ADC mode causes the ADC configuration some how to change?  

    Thank you,

    Harvey

  • 0 in reply to Harvey Ku97
    TI__Guru 56685 points
    Hello Harvey,

    As stated before the Resistor Enable bit will be ignored when the GPIO Select bits are selected for ADC Input. That part of the circuit is just not connected. To make sure, I tested this out on the MSP430F5529LP while applying a 2.04V voltage from a digital power supply and changed the resistor enable bit in between measurements. no significant change of results occurred.

    That being said I could see you partially discharging/charging the external circuit to a certain degree of you were to set the pin to GPIO functionality with pulldown/up resistor enabled, and then changing to ADC input. this could affect your measurements a bit. For your issue, have you double checked your sample time calculations to ensure the cycle count increase was enough for your circuit? This equation can be found in section 28.2.5.3 Sample Timing Considerations within the family User Guide.
  • 0 in reply to Jace H
    Prodigy 140 points
    Hi JH,

    I calculated my minimum sampling time and I am well over the requirement, but still have the same issue.
    Is it possible that this could be a grounding/noise issue as mentioned in section 28.2.9 in the user manual?

    Thank You,
    Harvey
  • 0 in reply to Jace H
    TI__Intellectual 1330 points
    JH,
    thanks for checking this behavior on a Launch Pad. I just wondering if you have considered that the IO resistors have an impedance of around 30k ohm. If you drive the input signal with a power supply you won't see any difference at the ADC input signal since the power supply will drive this load easily. But if you have a high impedance input signal like a sensor this could change the input voltage. That's why we are checking, for example, the sampling time always with a series resistance...

    Best regards,
    Tobias

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