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BQ25895: I2C line stuck low (SCL)

Part Number: BQ25895

We have discovered some sort of glitch state or failure mode where the SCL pin on the BQ25895 becomes stuck pulling low, thus stalling the I2C bus indefinitely.  Once the device enters this state it persists across power cycles, even after power is completely removed and all caps are discharged.  In this state (while powered on, or power removed) the pin shows a low resistance to GND (measures around 9 - 28 ohms). 

It seems almost like the BQ25895 is doing clock-stretching (holding SCL low); except that it remains in this state indefinitely.  

Isolating the Issue:

We have isolated the issue specifically down to this pin on the BQ25895 device, it is not caused by other devices on the bus.  We are puzzled how this is even possible, however we have now seen this on 3 different devices.  We have so far been unable to reproduce the problem on demand, so are still unclear what conditions could be causing the BQ25895 to enter this state.  Particularly puzzling is why it remains in this glitch state across power cycles.

During rework, in one case hot-air reflowing the module fixed it, only for the problem to return several hours later.  On another board, the problem went away after cutting traces, probing, power cycling several times, etc.  It worked for a while, then problem returned for several minutes, then problem cleared again.  It seems that momentarily shorting the SCL line's pull-up resistor (forcing the line high) will also temporarily reset/fix this error condition; though this does not seem like a reliable long-term solution.

Some background on the circuit / application: 

The application microcontroller is powered from the SYS supply rail.  We are disabling the BQ25895 watchdog.  We are also not using PMID.  After the system is brought out of ship mode with a button press, the application processor gets powered on, does some things, then eventually sends the command to place the BQ25895 back in ship mode to power itself off.  Normally this has been working great; until the BQ25905 pin becomes locked up.  We are using 4.7k ohm pull-up resistors (from the microcontroller's 3.3V supply).  

[Circuit snippet schematic]

[Edit - We are wondering if it might have something to do with our button QON circuit.  We are sharing this button with the microcontroller input pin, which pulls this pin up to 3.3V through internal pullups.  Also QON pin should have internal 200k pullup as well (pulling up to ~5v or Vbatt depending on supply present), according to BQ25895 datasheet.  Including an image of this circuit below.]

Any help or suggestions would be greatly appreciated (1) what might be causing this condition, or (2) how to best recover from this (since power cycling does not reset the problem).

Thank you!

  • Hello Kevin,

    The charger should recover from a missed communication within a few seconds, when you say power cycle do you remove all power connections including the battery?

    If all power is removed (including the battery) everything should be reset if it's related to the registers or any memory.

    Sincerely,

    Wyatt Keller

  • Hi Wyatt, I agree, we understand that fully removing power should reset the chip back to default.  However, in this case it is not.  We are seeing the pull to GND persist across power cycles when this is occurring. 

    To confirm, we isolated the SCL trace all the way back to the pin, so we know it is just this chip as the problem.  And to verify power cycling, we fully disconnected the board.  No battery supply, no debugger, no USB, then shorted across power supply caps in different areas (including bypass caps near the BQ25895) to ensure power rails were fully discharged, then verified with multimeter, so there is no residual power that might be keeping any state.  Yet we still measured a low resistance to GND on this pin; measured resistance using ohmmeter while powered off, and saw the same resistance pulling to GND once power was then re-applied.  

    Truly puzzling how this is even possible.  Additionally we found that when we momentarily apply a short across the pull-up resistor to drive the SCL line high directly, we see it can fix the issue for a while (for a couple mins or couple hrs) before the issue returns.  

    So we believe there could be some sort of transient or state condition that might be putting the BQ25895 into this glitched state (pulling down on SCL), but we have so far been unable to track down the problem to reproduce the issue. 

  • Hello Kevin,

    I would not recommend sharing the QON button function with other parts of your design, if you disable the other functions you have linked with QON is the issue resolved? How many devices do you see this happening to?

    If when the device PORs the issue is still occurring that leads me to the hardware, which could be the QON pin.

    Sincerely,

    Wyatt Keller