Normal operation
- The data sheet reference schematic shows Vcc to the EEPROM supplied from the Vout pin of the bq2060/bq2060A. The gauge only powers the EEPROM when the EEPROM is needed, saving the power consumption of the EEPROM except for the very short intervals when reading or writing to the EEPROM is required. Normally, there is a 10msec delay after writing this data, before Vout is powered down, giving the EEPROM time to complete the write operation.
Issue
- All versions of the bq2060/bq2060A have a low probability timing issue when the gauge updates the CycleCount and FullChargeCapacity values stored in the external EEPROM. There is about a 1% probability that the gauge will initiate the update to the EEPROM, but will incorrectly detect a busy condition and shut down Vcc to the EEPROM without the normal 10msec delay. The gauge will retry the operation in about 1 second and will again write the desired value to the EEPROM with the normal 10msec delay before power is switched off. Therefore, CycleCount and/or FullChargeCapacity values will be written correctly in the EEPROM and the short delay after the first aborted write operation has historically caused no known issues.
Problem
- One of the predominant EEPROM manufacturers has recently changed the architecture of their EEPROM design and the combination of this change with a short delay in Vcc application by the gauge has resulted in unacceptable results. The issue is that the entire 8-byte block will be erased when the write operation is started and the short Vcc application would prevent restoring the data. The end result is that the other 6 bytes in the 8-byte block will be left at 0xFF. This issue can result in corrupting the EEPROM values for Misc Options (bq2060A only), Safety Overtemperature (bq2060A only), Charging Voltage, Pack Capacity, Cycle Count Threshold, and Pack Configuration. If the gauge subsequently does a reset for any reason, the RAM will be refreshed from EEPROM and these potentially corrupted values would upset normal gauging operation.
Resolution
- The easiest solution is to power the EEPROM continually instead of using the Vout switched power from the gauge. Existing pcb assemblies can be modified by bridging the gauge pin 6 (Vout) to pin 7 (Vcc). The Vout pin is driven by a FET switch connected to Vcc, so shorting from pin 6 to pin 7 will not cause any stress or reliability issues with the gauge. The additional current drain may be less than 1uA, depending on the particular EEPROM used.
