backup SRAM STM32F427 - Vbat

[pavel]

Posted on October 20, 2015 at 22:05

Hello could you please help me with my question about

backup SRAM in STM32F427?

I have design, where I am using voltage regulator for VDD and VDDA - 3V0.

As backup I use batery 3.2V connected to VBAT pin6.

 

My question is, when I save some data to backup registers, and processor will be without main battery, only backup baterry will be connect to VBAT pin, during long time this backup battery will be fall voltage down.

Do you know on which woltage (minimum), will be this data in SRAM backup registers lost? When VBAT falls below 1,65V data will be lost?

because I try to set VBAT voltabe below 1,65V  (cca 1.0V) and data are still stored in SRAM backup register and data was not losted/cleared.

Data I stored in

RTC_BKP_DR0, and when Vbat fall below 1,65V, the data was still in this register. I thinking that under this voltage, will be data from backup SRAM cleared? Is it my assumption corert, or not?

[Tesla DeLorean]

Posted on October 20, 2015 at 22:21

Behaviour below 1.65V is undefined, ie you cant hold ST to functionality outside the specifications.

You will likely see bit-rot at some point rather than clearing. For clearing you are thinking more about TAMPER protection.

[waclawek.jan]

Posted on October 20, 2015 at 22:30

Below the stated minimum voltage, the data *may* be retained, but there is no guarantee for that.

You might want to protect your data by storing redundant information (data duplication, checksums, error correction codes), but there's no method to guarantee 100% data integrity (both copies may get corrupted in the same way, checksum may get  corrupted in a way compensating for data corruptiion).

Or you might want to have hardware provisions to find out whether the battery went under a certain level (periodic check using ADC, using internal comparator in certain models, or using external comparator), but all of these ''cost'' some energy, i.e. shorten battery life.

JW

[pavel]

Posted on October 21, 2015 at 07:38

Hi Jan, our device, may be stored some time in warehouse. In BCKSRAM we have stored data,security key etc. When somebody unauthorized person open the case, BCKSRAM will be cleared by TAMPER, function.

My question was what will happen during aging batteries their voltage drops below 1.65V. If it is not guarantee, that data under this voltage are correct in BC SRAM.

I think that it is information which I need.

thank you

[pavel]

Posted on October 21, 2015 at 09:34

Hi, Clive, may be I create wrong question on voltage level.

I would like to knou, when will be cleared/reset

RTC backup registers RTC_BKPxR), 80bytes.

These registers are reset, when backup domain reset. But when this reset occurs, on tamper detection, it is right function, it is functionall. But when Vbat drop down under 1,65V there is no mechanism, which ensure, that wned this volatge drop bellow 1,65V or some another voltage level will be these registers cleared?

Do you have experience with

RTC backup registers

, which corerct action clear these registers?

[AVI-crak]

Posted on October 22, 2015 at 10:34

Вам нужно убить данные когда к ним утер�?но доверие???

Очень про�?то, провер�?йте дату реального времени на до�?товерно�?ть. Врем�? не может быть ниже порога даты �?оздани�? прошивки, и не может превышать разумный �?рок �?к�?плуатации девай�?а. И �?амое про�?тое - у�?тановки �?о�?то�?ний реги�?тров должны �?овпадать �? вашим ин�?таллом. �?у например: у ва�? не и�?пользует�?�? будильник - а он внезапно взведён.

You need to kill these when they lost the confidence ???

Very simple, check the date on the real-time reliability. Time can not be below the threshold for the creation date of firmware, and may not exceed a reasonable period of operation. And the most simple - install state registers must be the same with your programming. Well, for example: you do not use alarm clock - and he suddenly cocked.