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BQ27421 memory clarification

Rudy Garcia
Intellectual 380 points
Other Parts Discussed in Thread: BQ27421-G1, BQ27411-G1

Hi,

I am a bit confused regarding the type of internal memory in the BQ27421.

The datasheet refers to non-volatile memory (OTP) and volatile memory (RAM).
The technical reference manual refers to non-volatile memory ROM and volatile memory (RAM).

The technical reference manual also states that most of the data memory is in RAM but some are located in ROM.

1. Is the OTP (one time programmable) memory the same as the ROM?
2. Refer to the paragraph below from the datasheet.
     a) If I send a reset sub-command (0x0041) to the device do all the data memory revert back to default values as defined in the datasheet?
     b) If the battery voltage is disconnected from the BAT pin do all the data memory revert back to default values as defined in the datasheet?

From the datasheet section 10.2.1 "Design Requirements":

As shipped from the Texas Instruments factory, many bq27421-G1 parameters in OTP NVM are left in the
unprogrammed state (zero) while some parameters directly associated with the CHEMID are preprogrammed.
This partially programmed configuration facilitates customization for each end application. Upon device reset, the
contents of OTP are copied to associated volatile RAM-based Data Memory blocks. For proper operation, all
parameters in RAM-based Data Memory require initialization — either by updating Data Memory parameters in a
lab/evaluation situation or by programming the OTP for customer production. Chapter 6 in the bq27421-G1
Technical Reference Manual (SLUUAC5) shows the default value that is present.

  • 0
    Genius 3970 points

    Hi Rudy,
    I'm not a TI employee but have had experience with many of their fuel gauges. In response to your questions:

    1. As far as I know, the OTP is the same as ROM, in the sense that the fuel gauge copies some of the contents of the ROM into RAM, then uses the RAM for normal operation. I believe the TRM's mentions of OTP memory being unprogrammed refers to the bq27411-G1, which is similar to the bq27421 but omits the sense resistor, has a different package and leaves the OTP memory blank for the user to program themselves. (TI reuses many elements of their datasheets and TRMs, so this may have slipped through the cracks, but that is purely speculation on my part.)
    2. a) Yes, using the RESET subcommand 0x0041 will cause the data memory to revert to the defaults.
      b) Yes, disconnecting the battery from the fuel gauge (as well as any power supplied through the SRX pin) will also cause the data memory to revert to the defaults once power is re-applied to the gauge.