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Original question:

BQ76PL455A-Q1: Daisy-Chain Communication

BQ76PL455A-Q1: Disposition of COMMx lines

Jeffrey Wright
Expert 2875 points
Part Number: BQ76PL455A-Q1

Hello;

Can you please clarify how the COMMx lines should be connected when they're not used? i.e. do I need to add termination R's as implied in 8.1.2.1 (assuming it's not referring to the internal R's)?

The programing guide says I can 

8.1.2.1 Single-Ended Communication (UART)
Communication with the bq76PL455A-Q1 from a host controller is performed utilizing a UART communication
protocol. The UART interface requires the following configuration:
1. COMML- is connected by 100-kΩ pull-down to DGND.
2. COMML+ is connected by 100-kΩ pull-up to V5VAO.

8.1.2.2.8 Unused Differential Communications Pins
Unused stack communications pins (COMMH+/–, COMML+/–, FAULTH+/–, FAULTL+/–) have internal
terminations; no external pull-up or pull-down resistors are required on these pins, if not used. If not used, leave
the unused pins unconnected. The COMMH drivers cannot be disabled.

...yet the Software design reference says: (SLVA617A)

1.2.5 Disable High-Side Receiver on Differential Interface on Top of Stack
To reduce current consumption and turn off the receipt of undesired messages due to noise from
nonexistent boards above the top-most device, this message turns off the high-side differential receiver
interface on the bq76PL455A-Q1 device at the top of the stack (bq76PL455A-Q1 at Device Address 1 in
this example). Specifically, this message clears the COMM_HIGH_EN bit in the Communication
Configuration register (address 16 to 17) on the top-most device in the chain

  • 0
    TI__Genius 17370 points
    Hi Jeffrey,

    Thanks for the question, the best way to clear this up is to reference Table 2 in the Design Recommendations document: www.ti.com/.../slua791a.pdf which shows the recommended connections for each device in a stack.

    For single ended mode, it is recommended to add external termination for COMML as shown in section 8.1.2.1 as you have above. However, for any other device in the stack you will always have to use COMML+_ to communicate so it will be connected to COMMH of the lower device.

    For disabling of the COMMH drivers, there is a note in section 7.3.10.1 of the datasheet "NOTE The UART receiver (RX), COMMH+/– transmitters, and COMML+/– receivers cannot be disabled." So there is a distinction between COMMH transmitters vs. receivers where you can disable the receiver with the COMM_HIGH_EN bit but you cannot disable the COMMH transmitter.

    Hope this clears it up,

    Taylor
  • 0 in reply to Taylor Vogt
    Expert 2875 points
    Hi Taylor;

    Thanks for the reply. I understand that it's the high-side xmitter and low-side rcvr that cannot be disabled.
    But just one more bit of clarification:

    8.1.2.2.8 Unused Differential Communications Pins
    Unused stack communications pins (COMMH+/–, COMML+/–, FAULTH+/–, FAULTL+/–) have internal
    terminations; no external pull-up or pull-down resistors are required on these pins, if not used. If not used, leave
    the unused pins unconnected.

    This is what is shown in Fig. 36...unconnected.?? Which is correct?...terminate or leave unconnected?

    Thanks!
  • 0 in reply to Jeffrey Wright
    TI__Genius 17370 points
    Hi Jeffrey,

    From a functionality standpoint it is okay to leave the pins unconnected, but from a design recommendations perspective it is better practice and we recommend to include the external termination as shown in the Table 2 link above.

    Best Regards,

    Taylor