STM8L and STM8S resets abruptly and takes a lot of current during sleep

[allantommathew]

Hi friends,

We are using STM8L(STM8L051F3) and STM8S (STM8S003F3) for a project. I has been designed, developed and deployed. It works as expected - STM8L as a transmitter, wakes up from stand-by mode, collect some data, transmits it and goes back to stand-by mode and STM8S as a receiver, receives it and represents it visually. This set up has been deployed and working for over a year now.

Now we redesigned the board with some extra functionality. We made the board and soldered the components but the new board continuously resets after 10-20 mins of operation. This is evident in both STM8L and STM8S. After a "cool off time" of about 10-20 mins the board again works but back to the erratic behaviour after 10-20 mins.

On further evaluation we saw:

• inconsistencies with the NRST pin. The voltage of NRST pin is not constant at 3.3V but changes abruptly. This brought us to a similar problem faced by another person (link) but the findings were inconclusive.
  It would vary between 1-2V when the debugger is not connected and between 2.2-2.6V when the debugger is connected.
• during the standby mode, the sleep current that the board takes is very high (it was around 0.8µA to 1.5µA in the previous design but it is around 35µA to 2mA, which is very high for a battery operated device).
  

Thus, we understand that the new design is not working properly.


We tried doing some more things for analysis:

• The new board was very similar to the old one. So we tried running the previous hex file (the deployed one) on the new board, but it showed the same problem (so there may not be an error in the code).
  
• The new micro-controller was soldered on the old board with the old code (the deployed hex file). But the old board started having the same issues as above (so we felt it has something to do with the micro-controller).

Please let me know if more information is needed, regarding schematic or waveforms or anything that can aid in decoding the problem. I would be obliged if anyone could guide me in the right direction.

Thank you.

With regards,
Allan.