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TM4C1233H6PM: Tiva RTC problem

Part Number: TM4C1233H6PM

Hi

We have for several years manufactured two variants of the same product, one with a RTC backup battery installed and one without (the PCBs are identical). 

The product has a specific known error, which we have struggled to identify - as explained in detail in this thread. In short, the MCU (TM4C1233H6PM) becomes non-responsive.

We now have statistical significant data showing a higher failure rate specifically on the variant with the battery installed (we previously suspected the supply circuit to be the problem).

The battery is used to drive the TIVA hibernation RTC clock - such that the time is retained when power is disconnected (we do not use any other features of the hibernation module). The problem occurs even if we do not enable the hibernation module in SW (only install the battery).

The hibernation/RTC/battery circuit is given here (PE is ground). The battery is a 3V coin cell of type CR1225.

The circuit is based on suggestions from this thread. One difference is the value of R_HIB_2, which is selected to keep the unique component count low.

In short, it we install a battery in above circuit, we see a higher failure rate, even if the hibernation module is not enabled in SW.

Any suggestions on why this happen will be highly appreciated.

Thanks

br

Christian

  • My only observation is the value of R_HIB_2 you chose is over 6x the recommended value of 51 Ohms. Is the issue reproducible on a given unit that you could change out that resistor and see if it has an affect?

  • Hi Bob

    We have not tried to replace R_HIB_2 - however we have tried (several times) to remove the battery. When we do so, the problem remains and we need to completely replace the TM4C1233H6PM to get the board working again.

    Do you think there is any point in replacing R_HIB_2 on already broken boards when it does not help to remove the battery? If so, we are happy to try.

    Br

    Christian

  • No, I suspect at that point the TM4C is already damaged. Can you tell me if the time of the damage can be associated with any particular activity? That is such as if it happens during assembly or  once it is used in the field. Does it happen when hooking up, or is it working and then just stops working? Have you taken a failed unit that you removed from the board and done a "diode" check from each pin to ground to identify if any particular pin is damaged?

  • Hi again

    We have collected as much information as possible on the usage up to the failure, but have not been able to detect any pattern. It seems that it just "randomly" stops working, either when it is just turned on or when it has been on some time.

    We have conducted the not conducted diode checks on each pin - but we have done various testing as suggested by you in a previous thread:

    https://e2e.ti.com/support/microcontrollers/other/f/908/t/698514

    I specifically refer to the points 1,2 and 3 in the thread. 

    At that point, it was more unclear to us what the root cause was. Statistics now strongly indicate that it is somehow related to the RTC and in particularly the battery (we have tested with several battery variants with the same result).

    Let me know if you think a test on each pin can provide more insight.

    Thanks

    br

    Christian

  • The reason I suggest testing the pins of a not working unit is that I suspect the device is being damaged with an electrical over-stress. I do not have any other reports of a similar behavior other than the known problem of inserting a battery with no current limiting resistor (R_HIB_2 in the case of your design). Your design is overly protected against that issue. If the damage can be identified as associated with a particular pin, then we have a better idea of where to look in the overall design.

  • Hi Bob

    Thanks for getting back.

    We are working on how to do the diode test. I assume it is best if the MCU is removed from the board - such that any external components do not affect the results. 

    Is it correct that what you are seeking (the diode test) is resistance from PIN to VCC and PIN to GND for each pin?

    If we remove the device from the board, do you have any preferred reference pin for VCC and GND?

    Thanks

    Br

    Christian

  • Yes, you need to remove the failing device from the board. It is best to compare the measurements between the failing device and a good device. No preference as to which Vcc or GND pin used as they are connected together in metal layers internally on the device. 

  • Any update?

  • Hi bob

    We are awaiting delivery of some QFP64 breakout boards. We need these such that we can remove the broken MCU and resolder on the breakout board for easy probing.

    The delivery is set for Monday and we expect to do the measurements shortly after (we may get interrupted by Christmas).

    Br

    Christian