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BQ76952: TS2 setting and Full Access mode question

Eohan George22
Intellectual 620 points
Part Number: BQ76952

1. While debugging our application, we found that we couldn't write to data memory settings anymore after a test in which we wrote some other settings to OTP. Our understanding is that you can write some registers
to OTP, and then continue to simply write to others only in RAM (for the sake of testing). It seems like writing to OTP (or some other event) put it into the sealed mode, and that we can't unseal it as we didn't get the keys and take note of them when the chip was still in FULLACCESS.

2. We need to use TS2 as a weak pull up in the normal mode for our circuitry. If TS2 is set as an ADC, can a pull be set ? We have TS2 config set to the default value of 0x00(which should have an 18k pull up to 1.8v), but TS2 shows 0v in normal mode.

2a. Would this work in thermistor mode ? 

2b. If it does, can we prevent the chip from ignoring(such as a high temp or low temp fault condition etc) the TS2 measurement as a thermister value. 


Is our understanding correct on all of this? The sealed issue has halted our experiments.

  • 0
    TI__Guru 54715 points

    Hi Eohan,

    1. OTP has limited writes.  You can make a fixed number of changes, but only 1 change to a bit.  If you set the keys and seal the part it is stuck.  If the default keys don't work replace the part.

    2. If TS2 is a thermistor input or ADC input.  Thermistor inputs have a pull up while selected for measurement, the rest of the time they are off to minimize current draw.  See table 13-10 in the Technical Reference Manual.

    2a.  Unfortunately the pull up for thermistor mode is only energized when the thermistor is measuring.

    2b. See the table 13-10.  Notice the Measurement type can be set to "thermistor temperature measurement, reported but not used for protections"  But again the pull up is only energized during the thermistor measurement, not all the time as you would like. 

    You will likely need to provide your own pull up.  Note that the maximum value for ADC inputs is REG18.  REG18 is available externally, if using it for a pull up do not load significantly, avoid coupling noise into the part on the REG18.

  • 0 in reply to WM5295
    Intellectual 620 points

    Regarding

    1. I didn't follow what you meant by "You can make a fixed number of changes, but only 1 change to a bit." I'm reading section 3.2 in https://www.ti.com/lit/ug/sluuby2/sluuby2.pdf?ts=1608258087399 . But is not quite clear about the two page and the 8 signature concept. Its definitely complicated enough to warrant a flow chart diagram for this !! 

    2. If TS2 is permanently connected to REG18 pin, does it create any issues for using TS2 to wake up the chip in shutdown mode ? 

  • 0 in reply to Eohan George22
    TI__Guru 54715 points

    Hi Eohan,

    1. Yes, see section 3.2.  My comment of only 1 change to a bit perhaps is worded poorly.  In section 3.2 it describes "At power-up, the device will XOR each setting in the first OTP image with the corresponding setting in the second OTP image and with the default value for the corresponding setting..."

    So you can set the bit once and change it once.  You can't toggle it back an forth more than once in OTP.  You can write it to memory as many times as desired.

    The limited number of changes relates to the signature count limitation: "The device supports up to eight different signature values, so up to eight partial changes in OTP can be made..."

  • 0 in reply to WM5295
    Intellectual 620 points

    1a. So you can write to a page only once, correct ? So, after you write to page 1, any other changes would have to be in page 2. is that right ? 

    1b. How does the eight different signature(crc like ? ) be useful if a page can be written to only once ? 

    2. Any suggestion for this question ? connecting TS2 to REGIN permanently ?

  • 0 in reply to Eohan George22
    TI__Guru* 81034 points

    Hi Eohan,

    Ideally, you should plan to program OTP one time. If you make a mistake and need to change back a register it is possible since there are two OTP images that are XOR'd together. So you can set a register to a value and then change it back, but after that you cannot change that register again in OTP. Normally you wouldn't plan to write different values to the OTP of the same location once you have determined the setting you want.

    The eight different signatures are not needed if you program all of your settings at the same step. If production is done in multiple steps like calibration as one step (program calibration values to OTP calibration registers) then settings configured at another production step and programmed to OTP, it might be useful. Or if you decide to use a feature you were not using which requires setting some new registers in OTP, it might be useful.

    For #2, I think you mean REG18? I would not recommend tying anything to the REG18 pin since any noise or loading on this pin may affect the performance of the device. If you try to drive TS2 low to wake the device from Shutdown, you might load REG18 which could reset the device. Connecting a pullup from TS2 to one of the REG1 or REG2 outputs (you can set these to 1.8V) might be better, it seems like this would still be a problem - it may prevent the device from fully entering Shutdown. The REG1/REG2 outputs will turn off during Shutdown, so TS2 would be pulled to 0V.

    Best regards,

    Matt