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BQ76920: Failure to acknowledge address

Tom Eaton
Prodigy 170 points
Part Number: BQ76920

Hello, I just received a BQ76920003. I am trying to cobble together a quick prototype, so I am trying to read some registers from the device to make sure the I2C is working correctly. I am using this circuit.

I am sending the device's address and a low read write bit onto the I2C bus. I would have hoped that the device would ACK this but it does nothing. The trace of the interaction can be seen below.

I think that there be a problem with how I am powering the chip, but I didn't really understand much from the datasheet, so clarification on how to power the chip would be great.

Many thanks,

Tom

  • 0
    TI__Guru 54715 points

    Hi Tom,
    In your simple schematic you do not show capacitors on CAP1 or REGOUT, both of which are recommended for operation. Please see the recommendations at section 6.3 of the bq76920 data sheet.
    After applying power the part will be in the ship mode. An edge of > 1V must be applied to TS1 to boot the part as noted in section 7.4.2 to transition to normal mode where it will ACK the command. TS1 should have a pull down for proper operation. After boot REGOUT should come up to ~ 3.3V as a quick check for normal operation.
    In your simple schematic the pull-ups for I2C are shown from +5V, but the amplitude shows about 2.8V on the waveform. Note in section 6.1 the abs max for SCL and SDA are 3.6V, and in seciton 6.6 that VIH is based on the REGOUT voltage. It seems the signal should work if the part is in normal mode, but as you move on in prototyping be sure you satisfy both the MCU and bq76920 requirements.
    The simple schematic shows most pins disconnected, and other than the comments above the part should respond without full connection, but be sure signals are kept within safe operating conditions.
    It is a big data sheet, but you will need to understand relevant portions for successful operation of the device. Hope this gets you started.