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Vbat Issue

Question asked by Alex Schepps on May 26, 2015
Latest reply on Apr 11, 2019 by Lucas Magasweran


I have a customer who is using the tamper/security features of the K21 - 120MHz part.  He is using VBat as a backup voltage.  Vbat is power by either a supercap or bank of capacitors.
These caps are charged by the rest of the system.  There is a case where he wants the user, while the device is running, to be able to press a button to clear out the 32 Vbat register file.   In order for this to happen the Vbat voltage needs to drop below 0.8V per the email chain below.  The issue is that in section 5.1 of the data sheet, it says the following:


“If the VBAT pin is not used, the VBAT pin should
be left
floating. Do not connect VBAT pin to VSS.”


So, based on this he doesn’t want to connect VBAT to VSS via this button.  Is that true that he cannot to this?  Next, as an option, he wants to connect Vbat to ground via a resistor which can them bleed off the voltage quickly.  However, he doesn’t know the limitation on this current is so as to not damage the part if Vdd is still present.  So, can he use a resistor to bleed off the voltage and what are the limitations for that resistor?  Thanks for the help.