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Seeking an Explanation for VMIDON bit

Ehsun
Prodigy 180 points
Other Parts Discussed in Thread: MSP430F47183, MSP430F47187

Hello all,

I recently experienced an issue using the MSP430F47183.  There is an external reference being used for the SD16A.  I had the SD16REFON and SD16VMIDON bits explicitly cleared on my SD16A setup.  This prevented me from getting a temperature reading from the on-board temp sensor.  

By asserting the VMIDON bit, it gave me correct readings.

Can someone explain to me what that bit actually does.  I did read in the user guide that it helps to minimize contention between the internal and externel reference voltages, but I don't understand how that affects the temperature read.

So what does the VMIDON bit really do on the SD16A, and what is the VMID buffer?

Thanks,

Ehsun

  • 0
    Guru 58965 points

    My understanding is as follows.

    (a) If you want to use the internal ref., set REFON=1 and connect a bypass cap. to Vref pin. Optionally, you can also set VMIDON=1 and use Vref pin to supply the buffered internal ref. for external use at Vref pin.

    (b) If you want to use an external ref., connect it to Vref pin and set both REFON=0 and VMIDON=0. But you cannot use the internal temperature sensor under this setting. You have to use the internal ref. when you try to read the temperature sensor.

    (c) If you want to measure the temperature sensor, you have to set REFON=1 and you have to use the internal ref. But if Vref pin is currently connected to an external ref., there will be contention between the internal ref. and the external ref. To avoid this contention, you may set VMIDON=1 to put a buffer between the two. The internal ref. will thus win the war and be used as the ref. 

  • 0 in reply to old_cow_yellow
    Guru 227245 points

    old_cow_yellow said:
    (c)

    Not exactly. If you take a look at the schematics in teh users guide (Figure 29-1) things become more clear.

    For using the temperature sensor, since it is dirven by the internal reference and auto-enables teh internal reference if used, even if AD16REFON isn't set, teh internal reference is active. If VMIDON is clear, the reference output and VREF pin are directly connected, so one is overridden by the other, most likely the eternal reference wins, while current consumption will probably much higher than expected. However, the temeprature sensor data are only correct for the internal reference voltage. Any different applied voltage (whcih woudl be the only reason for an external reference) would significantly change the sensor readings.
    If VMIDON is set, the VREF pin is switched to the buffer output. Now the internal reference is no longer fighting the external one and the temperature sensor works. However, the external reference is now fiughting the reference buffer driver, which has no effect but raising the current consumption even more.

    Other MSPs have separate VREF and VeREF pins, but this one doesn't: external reference can only be used when internal one is off, and using the temperature sensor requires/enables the internal reference. That's the way it is.

    However, the users guide does not state that using the temperature sensor will auto-enable the internal reference. However, your observations imply this as most likely explanation.
    Another possible explanation could be that the itnernal reference is indeed off, but powered by backwards current that are fed through the reference buffer and are pulled down to reference voltage (a scenario where the reference 'wins' whiel being backwards powered by the weakened external reference. In this case, this might even permanenlty affect the performance of the reference output buffer, depending on actual voltages.

  • 0 in reply to Jens-Michael Gross
    Prodigy 110 points

    Just for whatever it is worth.  Our new MSP430F47187 board design has problems with the ADC I can't explain even after many readings and experiments with setting bits.  We have seen readings swing wildly - worse when we do not start up with JTAG - but I have noticed that setting VMIDON seems to reduce our problems

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