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BQ76PL455A TX Pin Fault

MJ15
Prodigy 160 points
Other Parts Discussed in Thread: BQ76PL455A

I have a BQ76PL455A device monitoring a 16S cell pack.  It is interfaced via serial port to the UART on an Atmel AT90CAN128 microcontroller with 100R series resistors on both the TXD and RXD lines.  VIO for the BQ76PL455A is 5V.  I have had several of these circuits working successfully for many weeks but I have now had 2 of the BQ76PL455A devices enter a state where the TXD (Pin 38) signal from the device becomes stuck at 2V and all communications from the devices cease.

Can you suggest what might be causing this state with the BQ76PL455A and how I might resolve this?  Thanks.

  • TI__Intellectual 2795 points

    2V seems very strange. Can you please explain what your VIO supply is being sourced from? Is this with an EVM board, or your own design? If your own design, please share your schematic (privately if necessary) to review.

    If communications is disrupted because of a noisy environment (RX pin sees noise), the device can think a start bit has been received and fail to respond to legitimate communications. In such a case, the first response is to issue a COMM_CLEAR to the device by having the MCU hold its TX pin (PL455A RX) low for ~100us. This will clear the start of frame detection and allow the receiver to accept a start bit again.

    If communication is still not possible, the next response would be to issue a COMM_RESET, which is done by having the MCU hold its TX pin (PL455A RX) low for greater than ~200us. This will clear the start of frame detection and allow the receiver to accept a start bit again, and also reset all communications related registers to default values. It will also set the UART baud rate to 250KHz, regardless of the EEPROM values. The MCU will need to change its UART baud rate to 250KHz, then retry communication with the device, re-initialize communication register values (if other than defaults) then continue.

  • in reply to Stephen Holland
    Prodigy 160 points

    Thanks for your reply Stephen.

    The 5V supply is from an Atmel DVK90CAN1 eval board.  I have measured 5V directly at the bq76xx device.  I have sent you the PSU schematic from the EVM and our schematic page with the bq76xx device.

    I have tried the COMM_CLEAR and COMM_RESET commands just now with no success.  I tried 100us and 200us pulses as well as slightly longer in each case and verified with a logic analyser.