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Original question:

MCT8316Z: Availability of MCT8316Z

MCT8316Z: Evaluation Board for MCT8316Z

Randy Holmberg23
Intellectual 330 points
Part Number: MCT8316Z

Hello,

I have just received the evaluation board for the MCT8316Z. The good news is that I was able to connect a sample of the Maxon motor we are planning to use and it seems to function as expected. One question is regarding the RPM calculated by the Evaluation Web application. The FG_OUT frequency read by the application matches what I see on a scope, but the RPM calculated by the application is about 10X larger than actual. So, for example, when the FG_OUT is 57Hz, the application displays an RPM value of 1663 RPM when the motor is spinning about one tenth that speed. In this case, the motor is slow enough that I can count about 3 rotations per second which would be somewhere around 180 RPM. This Maxon motor has 4 pole pairs and I have selected 4 pole pairs in the application. Please let me know if this is a known bug or if there is something else I need to set in the application to get it to display the correct rotational speed.

Thank you,

Randy Holmberg

  • 0
    TI__Mastermind 26046 points

    Hi Randy, 

    I wrote the application firmware and used the following equation to calculate the commutation speed in RPM. 

    commSpeed = (60.5*FGOUT_avg*3.0)/NumPoles; //FGOUT_DIV = 3 divisions / electrical cycle

    Not sure why I used 60.5 vs 60.0, I thought it was to fine tune the speed but not sure if I got to confirm it on a tachometer. I also multiplied FGOUT_avg (average sum of a buffer of FGOUT values) by 3 if the setting for the FGOUT_SEL = 0b in the SPI version (1/3 of commutation frequency), but for the H/W version it is fixed to commutation frequency, so a multiply of 3 should not happen. The variable NumPoles is misleading in name, but it should be the number of pole pairs. 

    I can see it being off by a factor of 3, not ten, from this equation. This is a bug - can you see if its always off by a factor of 10? I can double check type casting if that improves the application firmware. 

    Thanks,
    Aaron

  • 0
    TI__Mastermind 26046 points

    Another realization I had is that it's actually off by 1663/180 = 9.2 times. 

    I'm actually finding a lot of errors in regards to FGOUT_SEL in the datasheet. I am going to confirm the default setting of the MCT8316Z0T device in the lab today, but the FGOUT_SEL settings should be:

    FGOUT_SEL (0b00) - FGOUT is 3x frequency (I think this is the default for both SPI and H/W)
    FGOUT_SEL (0b01) - FGOUT is 1x frequency 
    FGOUT_SEL (0b00) - FGOUT is 0.5x frequency 
    FGOUT_SEL (0b00) - FGOUT is 0.25x frequency 

    I have some validation data that shows the settings above, but this information did not get carried over properly to the datasheet. Please let me confirm this in the lab today, because this theory makes sense in that the commSpeed is now 9x larger than expected due to the device's 3x setting and the firmware multiplying by 3. 

    Thanks,
    Aaron

  • 0 in reply to Aaron Barrera
    Intellectual 330 points

    Hi Aaron,

    I think you are correct - there is a multiply by 3X which should have been divide by 3 so that explains the the factor of 9X from reported speed to actual. The other little bit is just the difference between 60.5 and 60.0 Rotations / second to Rotations / minute. 

    Thank you for looking through this.

    Randy

  • +1 in reply to Randy Holmberg23
    TI__Mastermind 26046 points

    Hi Randy,

    I checked on the MCT8316ZTEVM in the lab and confirmed that the default setting for FGOUT_SEL = 0b00. FGOUT frequency is 3x the commutation frequency. So the real RPM is equal to (3*3*(60.5/60)) = 9.075 times less than what is reported on the GUI. 

    Thanks,
    Aaron 

  • 0 in reply to Aaron Barrera
    Intellectual 330 points

    Thank you very much for looking into this. Do you know when you expect the GUI to be updated on the website?

  • 0
    TI__Mastermind 26046 points

    The issue is the FW, not the GUI. The FW needs the correction factor and needs to be reflashed onto the MSP430 on the EVM. 

    If you have an MSP430 and follow the instructions in the MCT8316ZTEVM User's Guide, you can fix this yourself in the downloadable firmware and change the 60.5 to 60 and the 3.0 to 0.333333 in that line of code, then re-flash the code onto the MSP430 on the EVM using the instructions in the user's guide. 

    It will take about a week to republish the new revision of FW on the EVM page with that change. 

    Thanks,
    Aaron

  • 0 in reply to Aaron Barrera
    Intellectual 330 points

    Would it be possible to swap out the evaluation board I have with one with updated firmware - I do not have the MSP430 code updater...

  • +1 in reply to Randy Holmberg23
    TI__Mastermind 26046 points

    Hi Randy,

    It will be easier to update the code than send a new EVM since EVMs are very limited in stock now for the next few months. The instructions are laid out in the user's guide on how to download the firmware and re-flash the MCU.

    Could you order a free sample of one of these MSP430 launchpads on samples.ti.com? Any one should be fine

    THanks,
    Aaron

  • 0 in reply to Aaron Barrera
    Intellectual 330 points

    Sounds good - I will see about getting the MSP430 board / eval kit as you mention.