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BQ76940: Xready some time will rise with TSn got 0 or 125 deg.C

shang Yi Yu
Prodigy 40 points
Part Number: BQ76940

My design has a very low era rate of getting Device Xready.

During that moment reading TSn can catch 0 or 125 deg.C

Multiple cases has the same occurs TS reading.

All cases can get normal TSn reading after I clear Xready bit.

Q1. Is this a Chip issue?

Q2. How Device Xready works. 

Q3. Why it only report 0 or 125 degC during Xready trigger.

Q4. How can I trust Xready result meet FMEA requirement? 

  • 0
    TI__Guru 54715 points

    Hi Shang,

    Q1.  It is by design and nature of the NTC.

    Q2

    The inputs are divided into 3 cell groups.  Cells are converted sequentially every 50 ms, temperature every 2 s.  Each has an ADC and communicates its cell voltage and temperature measurement down to the next lower group every 125ms, 8 times a second.  

    If 5 of 8 communications are corrupt during 1 second, the part assumes the values are not reliable and sets XREADY turning off the FET outputs and setting ALERT to signal the MCU.  The part continues its internal communication which may be OK after a temporary disruption or permanent if it has a partial loss of power.  Register values are not updated if the part has XREADY from the next upper group.

    If XREADY occurs at power up (boot) the registers are cleared.  If XREADY occurs from a problem between the bottom and middle group, none of the registers update, they report 0.  A 0 value is like an open thermistor result and since the circuit has a NTC the gauge or MCU will typically show a very high temperature or a fault.  If XREADY occurs from a problem between the middle and top group the bottom voltages and temperature will continue to convert while the middle and top groups report 0 and a high temperature.

    Q3. If temperature is read immediately at boot it will read 0 and show a high temperature.  Once the value has updated it should have a non-zero voltage value and show a value.  XREADY at start will not allow a register update, XREADY after running some time should leave the value dormant.

    Q4. The XREADY indicates not to trust the values.  If caused by a temporary condition XREADY could occur and be cleared between temperature samples.