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BQ76PL455A: Brief Questions on Cap. comm.

Cameron Perl
Expert 5385 points
Part Number: BQ76PL455A
Other Parts Discussed in Thread: BQ79616,

Hey team,

I had a customer who was originally looking at the BQ79616, but due to lead time is looking to some other options to begin prototyping with. This resulted in some questions related to the BQ76PL455A and BQ89606. The questions are as follows:

  • If I used the BQ79606 as a bridge IC, would it be able to communicate with the 76PL455A using the capacitive communication architecture?  They appear to be the same interface (almost the same protocol format).  I'm not sure if there's been hardware improvements between the families that would prohibit this kind of operation.
  • I recall the BQ76PL455a having distancing limitations with the capacitive communication interface (based on wiring, filters, etc).  What are the limits of the BQ79606 and BQ79616?  As I understand it, the interface should implement at a minimum capacitive isolation and dual chokes or isolation transformers to get the best performance at cable distances greater than 2m.  How far beyond 2m is possible? 10m, 25m more?  For instance, in the BQ79606 datasheet, it specifies how to calculate the daisy chain cable capacitance.  Maybe I'm missing something, but I didn't find a call out for the capacitance limit.  The Bq79616 datasheet doesn't mention anything about cable capacitance as well.
  • For the BQ79606, 79616, and 76PL455A, as messages travel up and down the capacitive communication bus, if devices are connected to the UART (ie MCU), would those devices also see the message?  If so, could they theoretically respond as well by responding back over the UART

I appreciate your feedback!

Cameron

  • 0
    TI__Genius 17370 points

    Hi Cameron,

    1) No you cannot mix 606, 455, and 616 devices in the same stack.

    2) For 606 and 616, depending on the isolation and pcb and if the customer follows our waveform recommendations for signal quality, we could support 25m cabling but as you say it may depend on the cable characteristics. We don't have a strict cap spec but rather we request customers to validate on their setup to ensure reliability. However, we recommend to minimize the distance to ensure best performance.

    3) The devices are stacked in daisy chain so they would respond via our daisy chain protocol on a twisted pair rather than UART. Only the base device communicates via UART in regular stacking applications. There is a less popular option to do multi-drop, best explained in the 606 datasheet where each device talks via UART to separate MCU.

    Regards,

    Taylor

  • 0 in reply to Taylor Vogt
    TI__Expert 5385 points

    Hey Taylor quick clarification / followup on the first question -

    "I’m not looking to mix the devices.  I was wondering if I could use the 606 as a communication translator to communicate with the 455’s.  The communication interfaces look similar.  Basically, it would look like this: MCU (UART) -> 606 (as communication IC, described in the user guide, implements daisy chain comm) -> 455 … 455 … 455."

  • 0 in reply to Cameron Perl
    TI__Genius 17370 points

    Hi Cameron,

    By mixing devices I am referring to not being able to use the devices in the same stack even if they look like similar protocols. So your example would also be mixing the devices.

    Regards,

    Taylor