We're using a BQ78350 to manage a BQ76940 AFE in an e-bike application. We lose SMBus connectivity with the BQ78350 when the system is connected to the bike's electric motor under load. Communications errors increase as the motor load increases and eventually the SMBus fails completely. The motor draws a maximum of 12A @ 36V. There are no problems when we operate the BMS using an equivalent [noiseless] resistive load.
I've pasted an oscilloscope trace of the SMBus showing successful and failed communications. The trace on the left shows a successful SMBus transfer, the trace on the right shows an unsuccessful transfer. In both cases, you can see the BQ78350 ACKs the host's 8-bit data (highlighted by red arrows). In both cases the clock is temporarily suspended following the ACK. On the left, the data bus is released and floats high. On the right, the data bus begins to float high then is almost immediately driven low (you see a brief spike after the ACK when the clock goes low). We're not sure whether it's the host or slave BQ78350 driving the data signal low, but suspect it's the BQ78350. Is this potentially the BQ78350 reacting to multiple transmission errors trying to reset the SMBus? Note that this condition is reproducible using the BQ76940EVM with EV2300 as well as our design, which closely follows the BQ76940EVM reference design layout and components.
Any advice and troubleshooting ideas are welcome.
Thanks,
Mark
