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BQ79656-Q1: UART Multidrop Communication

Micah Black
Prodigy 60 points
Part Number: BQ79656-Q1
Other Parts Discussed in Thread: BQ79616

There is not much detail given in the datasheet about the UART multidrop bus.

Could you confirm if multiple 'base' devices can be connected to the same UART bus (assuming their addresses are different)?

Are there any special considerations I should be aware of when connecting in this manner?

  • 0
    TI__Prodigy 540 points

    Hi Micah, 

    UART does not support more than two devices on the same bus. If you are looking to have a setup with multiple bases, I would suggest using the BQ79600 as the base devices and utilizing SPI communication. You can also support multiple devices by having a singular BQ79656 base and connecting the rest of the monitors by VIF (COMH <->COML). More information is on page 49 and onward of the BQ79656 datasheet on the UART and VIF communication. If none of those options work for you, I would suggest exploring the industrial battery monitors that are capable of I2C communication. 

    Sincerely,

    Michelle W. 

  • 0 in reply to Michelle Williams
    Prodigy 60 points

    Section 9.3.6.1.6 of the device datasheet (pg 71) seems to indicate that multiple devices can be used on the same UART.
    In this case, the TX driver of the devices should be disabled unless they are sending a response to avoid any collisions on the UART lines. The datasheet seems to suggest that is implemented by the order that the responses are sent. Is this correct?


  • 0 in reply to Micah Black
    TI__Prodigy 540 points

    Good point; this feature is not as frequently used compared with the daisy chain configuration. Let me get back to you on how to implement the multidrop functionality. 

    Sincerely,

    Michelle W. 

  • 0 in reply to Micah Black
    TI__Prodigy 540 points

    Hi Micah,

    From what I can find in the internal BQ79616 documentation, the multidrop functionality is meant to have each base device paired with a local microcontroller and then all of the microcontrollers are controlled by an MCU through CAN. UART still should not have more than two devices on the same bus. If you have an NDA on file with TI, I can further explain the multidrop functionality with diagrams as well over email. 

    Sincerely,

    Michelle W. 

  • 0 in reply to Michelle Williams
    Prodigy 60 points

    I understand that configuration with a microcontroller at each device. But I think it is different than the Multidrop Configuration that the datasheet seems to describe. It mentions stack devices and a specific addressing sequence which would not be required if each device has its own microcontroller.

    When you say that "UART still should not have more than two devices on the same bus", I understand that requirement for a general UART bus, but it seems this chip has a way of putting the TX pin in high-impedance while another device is transmitting based on that datasheet paragraph.

  • 0 in reply to Micah Black
    TI__Prodigy 540 points

    I understand the confusion of the multidrop configuration on the datasheet. However, the addressing of devices refers to having one MCU and local microcontrollers that will address each BQ monitor since each device needs a separate address for broadcast writes/reads to function correctly. Furthermore, the documentation that I have on the multidrop functionality only points to using local microcontrollers controlled by a single MCU rather than splicing multiple devices on the same UART bus. 

    Sincerely,

    Michelle W. 

  • 0 in reply to Michelle Williams
    Prodigy 60 points

    Thanks for looking into this. I'll continue my prototype design with a local microcontroller for each chip, but will add some jumpers to bypass the micro to test it out.