Vbat Issue

[PHXFAE]

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.

[Jorge_Gonzalez]

Hello Alex Schepps:

The note in the datasheet should be more a recommendation for noise immunity when not using VBAT pin, but not a restriction.

However a better solution if the customer wants to reset VBAT Register File with a switch, might be placing a normally closed switch between the supercap and the VBAT pin, this way he can press the switch and isolate the capacitor voltage from the pin.

Regards!,

Jorge Gonzalez

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[PHXFAE]

HI Jorge,

I am a little confused about your thought about noise immunity.  I could see that there would be a recommendation to tie a pin to ground or VCC for noise immunity but not requiring that it be left floating.  So, before I tell the customer it's OK to tie this pin to ground, I'd like to really make sure it won't damage the part.  The data sheet says explicitly not to tie this to VSS.  There must be a reason for that.  I don't fell comfortable telling the customer to tie this to VSS without a good explanation that this won't hurt the part.  Remember that he is wanting to tie this to ground while VCC is powering the part.  We discussed the other suggestion that you had but there was a reason he couldn't do that which I can't remember right now.

[Jorge_Gonzalez]

Hello Alex:

Sorry if my previous response was confusing. I checked and as the datasheet indicates the VBAT pin should not be connected to VSS, although we cannot share the internal configuration to better explain.

The only solution would be as per my suggestion, letting the VBAT pin floating to clear VBAT register file. I tried this approach and it worked in a TWR-K21F120M board using the next simple test code:

#define    VBAT_REG    (*((unsigned int*)0x4003E000))
int main(void)
{
    int counter = 0;
    VBAT_REG = 0x55555555;
    for(;;)
    {       
           counter++;
    } 
    return 0;
}

Using a coin battery the register can be written, but when removing jumper J7 (VBAT pin connection) the register gets cleared as expected.

Regards!

Jorge Gonzalez

[lucasrangit]

Using a coin battery the register can be written, but when removing jumper J7 (VBAT pin connection) the register gets cleared as expected.

@Jorge_Gonzalez, that is strange that you are able to able to access the VBAT register file without VBAT connected. I just learned that if it is not connected or below a threshold, the CPU will hard fault. https://community.nxp.com/message/1137628#comment-1137817 

[PHXFAE]

HI Jorge,

Thanks for your reply.  I will check with the customer to see if he can implement a switch between the battery and the VBAT pin.  This customer has an NDA with Freescale for the security/tamper functions.  If he wants to use a resistor to bleed off the voltage rather than your method can someone tell him what resistor he can use?

[PHXFAE]

Hi Jorge,

The customer specifically asked if they can connect VBAT to VSS using a 33Kohm resistor.  Can you please answer that question?

Thx, Alex

[Jorge_Gonzalez]

Hello Alex:

I double checked my previous response with the designers, this was their feedback:

The "Do not connect VBAT to VSS" warning is for latchup concerns in a system if VBAT was tied directly to a hard ground. Since this is a momentary event that has some impedance, the precaution does not apply. Discharging the VBAT cap with a resistor will not harm the internal circuits. There  are no power domain crossing concerns since VBAT and VDD are isolated from each other.

So there should be no problem with the resistor. Please let your customer know this.

Regards!

Jorge Gonzalez