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Current measurement on MSP430

Other Parts Discussed in Thread: CC1120, MSP430F5438A

I'm using the TrxEB (rev.1.7) which is equipped with the MSP430F5438A, and I'm using the CC1120 (rev.1.1) as radio. I would very much like to use an oscilloscope to measure how much current is used and to get a current profile so as to achieve greater insight into what steps/parts of my program is consuming the most current and why. I'm not very experience however with these practical matters so I'm wondering if anyone has an idea as to how I can achieve this? I know the TrxEB is equipped with 3 current measurement pin pairs, MCU_PWR, IO_PWR and RF_PWR. Can i simply connect one of these pin pair to an oscilloscope and that's that?

  • It is not that simple. For current measurement you need Ammeter. You can use oscilloscope but some "tricks" are required: http://www.hereticalbuilders.com/showthread.php?t=126

    Regards,
    Maciej 

  • sigurd stoll said:
    MCU_PWR, IO_PWR and RF_PWR. Can i simply connect one of these pin pair to an oscilloscope and that's that?

    No. You need current shunt resistor to monitor voltage drop on it, so using ohms law you will know current going through shunt resistor. Note that your scope ground does not float so if you want to connect ground of scope to other than ground connector of your circuit, you need to isolate your circuit..

    I do not measure high side shunts (on positive terminals) but low side instead, for whole circuit including CPU and other loads like RF. I am using ~6v battery, 3.3V LDO, 100ohms resistor betweem battery minus terminal and GND of device. Scope's ground is connected to GND of device. Yes, you will get negative voltage drop but use "invert polarity" function on scope if needed. This approach gives you freedom to use 2nd channel for other measurements or trigger which supposedly you will need to see consumption patterns.

    Oh, and remember that msp430 low power versus active mode consumption can differ more than 10000 times but most scopes have 8 bits giving 256 counts range meaning not enough resolution. Here's easy solution is multimeter and test code "generating" consumption for power mode you want to measure.

  • Thank you very much for a quick and informative reply, not sure if I can get my hands on a pair of current probes though. I've appended a figure which describes the setup I'll be

    using ( the figure is a modified version of figure 6 from SLAA378D found on www.ti.com). So my next question is this: Do you have any experience with the TrxEB? Where,

    physically, should i connect the wires to my TrxEB?

  • Well, you mean 2-channel oscilloscope here? Nice idea! Use math -> difference between two channels. Then you can have common ground for supply, scope and device. Otherwise if you measure using single channel, dont forget to isolate power supply (use 2xAA batteries for instance) and device.

  • I have worked on similar project.
    2 channel mean Oscilloscope can measure 2 signals at a time. By connecting 2 probes to 2 different source we can see teir values on the scree. Best way is to digital multimeter.
  • >Best way is to digital multimeter.
    Digital multimeter is _very_ slow instrument to measure current consumption of microcontroller. To get meaningful measurement using multimeter - you really shall understand what and how you measure. Otherwise you will get random readings you are wondering about in the other thread

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