BQ76920: 920 ceases communicating

Hi Jeffrey,

Since you are using the BQ78350, can you provide a log of the event where the BQ76920 stops running and then starts running again when the current is reduced? If you are able to connect using BQStudio, that might be the easiest way to capture a log file (click the 'Start Log' button on the Register screen).

When you say it stops running, do you mean that the device shuts down and needs to be rebooted, or do you mean it is not responding on the I2C bus, or some other symptom?

Best regards,

Matt

Hi Matt; 

I've asked Dan to capture that log file...

It's not crashing, but it stops toggling the Alert, holding it high for several hundred milliseconds until it's reset by the 78350 (I assume). What's odd is that I've not seen this behavior on the Eval board...I can dial up an AOLD threshold voltage at SRN, see the SafetyAlert and SafetyStatus fire....but my host u/C would still see the Alert toggling every 250 ms. ???? 

Now however, my driver is randomly catching SMBus timeouts when this occurs. The big difference is that now we have the actual BMS board which I can't connect to bqStudio, and so I'm left with only the IDE to debug this.

You saw Dan's scope capture: but I thought that the 78350 would not clear the fault and tell the 920 to reenable the discharge fet until and unless the OCD event recovered?  

HI Matt;

Yes, the same issue...and as I asked previously, I didn't think that the 78350 would clear the fault until/unless the OCD event had recovered? But the scope capture shows that it's being cleared, even thought there's still >100mV at SRN (where I set the AOLD threshold)?

The AOLD threshold is set by the BQ78350 (the BQ78350 will use this value to set the BQ76920). It is under the Protections:AOLD:Threshold and Delay register.

If he is able to observe the I2C waveform during the Alert, it might help to see if there any additional faults present.

Also, when the over-current fault triggers, it should open the DSG FET. Do you think you might be losing the ground connection for your host when this happens?

Matt

I've asked for that capture the log file but they don't think it's necessary, since it seems you've confirmed that that stretched Alert signal (~800ms) is correct behavior and so the problem is with my driver...I'm still not sure that's correct since I never see this on the eval board.  The BMS is using high-side disconnect fets, driven by the 76200, so there's no loss of gnd.

And like I mentioned, I'm programming the 920's OCD to 100mV, and its SCD to 155mV, which translates into ~267A and ~414 A....this all works perfect on the eval board, but not on the new BMS. 

So bottom line: is this stretched Alert correct behavior? As I said, I thought that the 78350 would not reenable the fet(s) until/unless the fault cleared????

THanks

Hi Jeff,

The BQ76920 will not reenable the FETs until the fault is cleared. That's good that you have high-side FETs - that means no loss of gnd. 

I'm hoping the I2C traffic will shed some light. In Dan's waveform, you can see a quick I2C transaction each time the Alert pulse goes high and the Alert pin goes back low (this is for measurements where Alert is signalling new data is ready). When it goes high due to the fault, there is a ton of I2C traffic. It might just be checking the 920 status register repeatedly, I'm not sure. The Alert pin is also sensitive to noise and can be driven high to indicated a fault - hopefully that is not occurring, but the I2C traffic will show this.

Matt

Ok thanks Matt; I've asked again for that log file. tbd

Hello;

I've uploaded 2 .gifs and the bqStudio log during an SCD event. Once the 920 detects an OLD event (~267A), its Alert signal slows to 1 sec intervals, but no other problems occur. At his point, I can still talk to the 78350. However,  when the 920 encounters an SCD event (~414A), the 920's Alert signal starts toggling at ~2ms intervals. 

Is this known and expected behavior of the 920 during H/W protection events?

Tue Feb 18 10:11:31 EST 2020

Device Name = bq78350_R1
Firmware Version = V1_04_BLD0026

Design Capacity = 9000
Design Voltage = 3600
Specification Info = 0x0031
Manufacturer Date = 1980-1-1
Serial Number = 0x0001
Manufacturer Name = Texas Instruments
Device Name = bq78350
Device Chemistry = LION

Sample,DateTime,ElapsedTime,ManufAccess,RemCapAlarm,RemTimeAlarm,BattMode,@Rate(@),@TimeFull,@TimeEmpty,@RateOK,Temperature,Voltage,Current,AvgCurr,MaxErr,RSOC,ASOC,RemCap,FullChgCap,RunTimeEmty,RunTimeEmty,AvgTimeFull,ChgCurr,ChgVolt,BattStat,CycleCnt,PendingEdv,SoH,OpStatA,OpStatB,TempRange,ChgStat,GaugeStat,MfgStat,SafetyAlertAB,SafetyStatAB,SafetyAlertCD,SafetyStatCD,PFAlertAB,PFStatAB,PFStatAB,CellVolt1,CellVolt2,CellVolt3,CellVolt4,CellVolt5,CellVolt6,CellVolt7,CellVolt8,CellVolt9,CellVolt10,CellVolt11,CellVolt12,CellVolt13,CellVolt14,CellVolt15,ExtAvgCellVolt,TS1 Temp,TS2 Temp,TS3 Temp,CellTemp,FetTemp,GaugeInternalTemp,LogRowTime(ms),LogStatus
1,2020-02-18 10:11:35,4.001,0x0129,300,10,0x6001,0,65535,65535,1,18.5,14440,-32767,-32767,100,0,0,0,9000,0,0,65535,0,0,0x0BD0,30,324,100,0x2901,0x0000,0x01,0x12,0x4245,0x0010,0x4000,0x00F8,0x0000,0x0000,0x0000,0x0000,0x0000,3543,3631,3638,3628,0,0,0,0,0,0,0,0,0,0,0,3475,18.5,-273.2,-273.2,18.5,-273.2,29.0,1073,SUCCESS
2,2020-02-18 10:11:39,8.002,0x0329,300,10,0x6001,0,65535,65535,1,18.5,14440,-32767,-32767,100,0,0,0,9000,0,0,65535,0,0,0x0BD0,30,324,100,0x2903,0x0000,0x01,0x12,0x4245,0x0010,0x4000,0x00E8,0x0000,0x0000,0x0000,0x0000,0x0000,3543,3631,3638,3628,0,0,0,0,0,0,0,0,0,0,0,3476,18.5,-273.2,-273.2,18.5,-273.2,29.0,1201,SUCCESS