BQ76952: Unable to establish communication on I2C with STM32L452

Hello Varij,

Do you have logic analyzer captures of the communication? Does this seem okay? Is there a NACK?

The device should be ON if REG18 is to 1.8-V. What is your battery voltage?

Are you writing the correct address to the device? The following FAQ explains what is the address of the device:

• What is the I2C device address of the BQ769x2 family?

Best Regards,

Luis Hernandez Salomon

Hi Luis 

The battery voltage is around 51V.

We have made 24 nos. of board for initial prototyping and validation purpose, but in only 5 boards , we are having this issue not in all boards. But if it occurs how we should identify and try to resolve.

The device address we have written is correct.

Below are the scope snippets for the board which doesnt have this once and one which has the issue.

Non working board below

Thanks and regards 

Varij prajapati

Hello Varij,

Do you have any schematic for this design? The rise time seems pretty slow, is the RC constant high? is it possible to decrease the capacitance/resistance here?

Maybe a smaller pull-up resistor for SCL/SDA.

Have you attempted swapping IC's between a good board and bad board to confirm if it follows the IC?

Best Regards,

Luis Hernandez Salomon

Hi Luis,

Please find the below image of the schematic of AFE section as well as MCU.

Yes, it is possible to reduce the pull up resistance right now its 10k. But for capacitance I don't think so, any suggestion from your side.

No, I have not attempted to swap the IC from working board to non-working board.

Let me know if you need any more info from my side.

Thanks and Regards,

Varij prajapati

Hello Varij,

For now could you attempt doing both of these things and see if it improves things? The RC constant seems very long, so I am concerned this may be affecting the signal integrity between the devices. 

From the schematic I cannot tell any problems, but do try decreasing the pull-ups to maybe 4-kOhm and see if it improves this. And if not, try performing the ABA swap (Good IC to bad board, Bad IC to known good board) and see if the problem is reproduceable on either.

Best Regards,

Luis Hernandez Salomon

Hello Luis,

Just a normal concern, in 19 boards from the lot is not having any issue like this so I was just curious why this is into some boards. We have faced this issue same problem in previous project also same architecture TI+ STM micro and unable to resolve that.

I will perform the ABA swap first and then I will go to towards decreasing the pullup resistor to half of the existing i.e. 5k.

I will let you know the updates on the above things soon tomorrow before 12pm. IST.

Thanks and Regards,

Varij prajapati

Hello Varij,

I am really sorry to hear that! I am not sure why you had this problem in the past. We have used an STM MCU in the past with the BQ76952 plenty of times with no problems, so I am hoping we can figure this case out and make all of your boards working!

Do let me know your findings tomorrow. I will also bring this up internally to see if there are any additional ideas.

Best Regards,

Luis Hernandez Salomon

Hi Luis,

As suggested by you performing the ABA swap (Good IC to bad board, Bad IC to known good board). We have noticed that both the boards started working.

But its little bit weird.

In previous scope snippets, I believe it was probe issue we didn't receive better waveforms. But hereby attaching the new waveform with 10k pullup.

I have replaced the pull up with 4.7k and then cross verified the rise time. It was approximate same.

Looking forward to doing more trials and suggestion in order to find the exact issue.

Thanks and Regards,

Varij prajapati

Hello,

I am out of office until Wednesday. Responses will be delayed until then.

Best Regards,

Luis Hernandez Salomon

Hello Varij,

Sorry for the wait here! Just got back .

If the swap fixed it, could it have been that there was a poor solder connection in the I2C lines? The first scope shots shared definitely looked like the RC constant was very large. If the solder was not great, maybe it increases the RC due to parasitic resistance/capacitance. I'd advice to check continuity on the board.

It is likely that the long RC was almost surely affecting communications, this new scope shoot looks a lot better. When the parts were swapped, they were essentially being re-soldered, so it is possible that there was a better solder done on the IC connections this time, causing the problem to go away. 

I would recommend to place a non-good board into the reflow oven to see if the device starts working as intended.

Best Regards,

Luis Hernandez Salomons

Hi Luis,

Thanks for your input and prompt support!!

Sure, we will put the non-good board into the reflow oven aur try to remove the AFE and resolder is again.

Thanks and Regards,

Varij Prajapati

Hello Varij,

Sounds good! Do let us know the results .

Best Regards,

Luis Hernandez Salomon