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RTC_B VBAT battery backup & ADC12

Paul Young
Intellectual 310 points

Other Parts Discussed in Thread: MSP430F6638

I'm using an MSP430F6638, ADC12 & RTC_B implemented with its battery backup feature.

I would like to use ADC12 to measure the voltage RTC_B's backup battery. Am I correct in thinking that ADC12 can do this internally? i.e. with no wire connecting an ADC12 channel to the VBAT pin?

Many thanks,

Paul

  • 0
    Guru 227245 points

    The users guide reads that on devices with the Battery Backup System, VBAT can be measured on A12 by setting the BAKADC bit in BAKCTL. However, this info mus tbe collected by reading both, ADC12 and BBS chapter. Also, the datasheet doesn't show this option in the port pin schematics for A12.

  • 0 in reply to Jens-Michael Gross
    Intellectual 310 points

    Thanks for that, made the change & now I'm able to see that I'm measuring a voltage that changes with & without the presence of a coincell (i.e. zero when not there, non-zero when there) but…

    I'm trying to accurately measure the voltage of the battery, according to the multimeter it's 2.97V. I'm using an external Voltage reference of 3V. The raw value I get when I read the ADC is 1334.

    Even taking into account the potential divider used when measuring VBAT that number translates as:

    (1334/4095)*3= 0.98V at the potential divider therefore:

    VBAT = 0.98 * 2 = 1.95V not the 2.97V I'm measuring using the meter.

    Any thoughts?

  • 0 in reply to Paul Young
    Guru 227245 points

    Besides selecting the right reference, there are othe rthings to consider.

    The ADC12 works with charge distribution method. Suring samplign phase, it needs to charge a sampling capacitor to the inptu voltage. Te curred for this is drawn from the source. So depending on capacitor size (see users guide/datasheet), series resistance of the input multiplexer (1k typ) and signal output impedance, a certain charging time is required to charge the samplign capacitor. If your sampling time is too short, you'll read lower values. The users guide has a chapter about this.

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