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Measuring the Power that is being supplied

Steve Smith1
Prodigy 10 points

Hi,

I am currently using an M430 F2274 on an ez430 rf2500T board.

I am working on trying to use this setup, along with the temperature measuring demo program that comes with my kit, to measure the wattage that is being supplied by whatever is supplying power to the board. 

My approach is to place a shunt resistor on the development pins of the ez340 rf2005t to measure the current from a known voltage, and since the demo program inherently measures the voltage being suppled, I can use these values to find the power.

Unfortunately, I haven't been able to figure out what all of the register values must be set to in order to measure an external voltage using the demo program (the external voltage would be placed across the shunt resistor), including the values of the SREF, INCH, and ADC10AE0 values.

Any help with what the register values need to be changed to in order to accomplish this will be greatly appreciated!

Thanks

  • 0
    Guru 227245 points

    Teh registers are described in the users guide.
    However, you have a little problem with your concept.

    YOu cannot measure the voltage across the shunt resistor.
    The MSP measures voltage relative to GND, and the maximum voltage it can measure is the reference used. VCC, as it is unknown, cannot be used as reference, It would always give you '100% of reference' reading. And the supply side would be even above VCC.

    What you can do is using two 1:1 voltage dividers between both sides of the shunt and GND. Then use one of the MSPs internal references and see what voltage readings you get from both voltage dividers. Then twide the difference is the voltage across the shunt.
    However, the voltage on one side is knwos, do you say? THen there is an easier way: the MSPs ADC has an internal 1:1 voltage divider for VCC. So you can measure VCC (or rather VCC/2) against one of the internal references (1.5 or 2.5V).

    I think, the demo code is using VCC as reference. SO you need to enable the internal references (1.5 vor VCC <3V or 2.5 for VCC>=3V) by settign the proper control bits, then wait a little so the reference can settle, and then use the 'VREF/VSS' options instead of the (default and perhaps not explicitely set in the demo code) 'VCC/VSS' option for the reference when programming the ADC

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