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EM1402EVM: External CMOS SPI Control of Gate Driver

Russell Okamura
Prodigy 140 points
Part Number: EM1402EVM
Other Parts Discussed in Thread: BQ76PL455A

Hello, 

I am currently trying to control the on-board switch matrix using SPI communication from an external controller that is not the same host controller that is interfacing with the BQ76PL455A through UART.  Is it possible to communicate through UART using 5V UART and 3.3V SPI?  From reading the schematic, it seems that they are both going to be referenced off the same voltage, VIO, which I believe would be 5V, which could damage my SPI host controller.  My other option would be to level shift the 5V UART logic down to 3.3V , but in that case, I would need to set the DEVCONFIG register to externally supply VDIG and VIO right?   I was wondering what is recommended if someone wants to control the UART separately from the SPI communication.  Can it even be done at different digital voltage levels? 

Thank you in advance, 

Russell Okamura

  • 0
    TI__Mastermind 19795 points
    Hi Russel,

    While I believe it should be possible to supply the SPI device after level shifting the voltage and setting the DEVCONFIG properly, before recommending you try it, I would like to discuss this information with the engineer most familiar with the board this Monday. I will get back to you with the answer!

    Thanks for your patience,
    Vince
  • 0 in reply to Vince
    TI__Mastermind 19795 points
    Hi Russel,

    After discussing with the other engineer, the short answer is that this will cause communications failure. As the logic levels of the entire 455 IC are determined by VIO, if you choose a communications logic level of 3.3V (for SPI) and then attempt to communicate at 5V (for UART), you will violate the abs-max of the communications lines. For reference, absolute max of the part is VIO+0.3. So unfortunately, although supplying the SPI device will work, the communications will fail.

    Let me know if you have any other questions!

    Thanks,
    Vince
  • 0 in reply to Vince
    Prodigy 140 points

    Hi Vince, 

    Thank you for getting back to me.  In response to your answer, I think I will just end up running all communication using a 3.3V digital reference, and level shifting any 5V communication signals down to 3.3V logic.   Also, would it be possible to "link" me in the right direction for trying to control the switch matrix from an external controller.   The datasheet, or datasheets, I should say, can get a bit confusing between the three gate drivers and the single EMP1499Q DC-DC Controller and and the bit shifter for managing the MOSI signal.  Is there any software guide online, similar to the one that was provided in the Passive Balancing BQ76PL455A Evaluation Module architecture.   If not, is there any recommendation as to where to start?   Thank you any help is appreciated.  

    Thanks again,

    Russell