Dead coincell and tamper detection

[adriengravouill]

Hi community,  

I'm developing a custom board with the IMX6SL processor, based on EDK.

I will use integrated RTC and tamper detection, which both required an external coincell. I need some help to choose the coincell correctly. My system will run only when it is supplied from a wall USB adapter, so no other battery available.

1. It seems that's when system is off, it drains up to 250µA from the coincell, which is huge.  A standard rechargeable CR2032 capacity is 40mAh. So, the RTC can work 6 day before being out of power. After that, I will need to plug the board to my wall adapter to make the board working again : is the low voltage of the coincell a problem?
2. PF0100 integrates a Lithium charger, which current charging is 60µA, so it will take something like 27 days to charge the RTC's coincell?
3. Finally, is the first case, when battery will be out of power, what will happen with tamper detection when I will bring back the power?

Best regards,

[igorpadykov]

1. this is max. value, typical 41uA as gives Table 12 i.MX6SL Datasheet

http://cache.freescale.com/files/32bit/doc/data_sheet/IMX6SLCEC.pdf

Low voltage of the coincell (below values in Table 9. Operating Ranges)

may not guarantee proper SNVS block operaton (time, tamper functions)

2. right

3. >when battery will be out of power, what will happen with tamper detection when I will bring back the power?

Fully powered processor will work fine according to its specifcations.

Best regards
igor
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[adriengravouill]

Thank you for your answer.

When a low voltage of the coincell condition occurs, Is SNVS block can generate a false tamper detection?

In that case, it will result with an "Activation of the security related hardware (e.g. automatic and immediate erasure of the
Zeroizable Master Key and deny access and erasure of secure memory contents)", so at next start'up software will be bricked?

How to deal with this kind of problem?

[igorpadykov]

Low voltage of the coincell (below values in Table 9. Operating Ranges)

may not guarantee proper SNVS block operaton.