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Backup SRAM content lost in VBAT-mode

Question asked by Soderhall.Mats on Feb 28, 2017
Latest reply on Mar 1, 2017 by waclawek.jan

Are VCAP_1 and VCAP_2 used in Vbat-mode?

 

I have an issue with the content of the Backup SRAM is lost when i power down the device, even though the Vbat is active.

If i do a reset the content of backup SRAM and RTC-registers remain. But if i remove Vdd for a few ms the content of the backup SRAM becomes garbage but the RTC-registers are not affected.

I have checked with an oscilloscope that the voltage at Vbat never drops below 3.1 Volts.

I have enabled the backup regulator with a call to "HAL_PWREx_EnableBkUpReg()". I have also checked that the BRE and BRR flags gets set in the PWR->CSR register.

 

My theory that i hope someone can confirm or discard:

Backup SRAM does not work in "BYPASS_REG"-mode.

I use my own 1.2Volt regulators connected to VCAP_1 and VCAP_2 and have pin "BYPASS_REG" hardwired to 3.3 Volts in normal mode.

When i power down Vdd, VCAP_1, VCAP_2 and BYPASS_REG are pulled low.

 

After a power cycle the "Backup regulator enable" in register "PWR power control/status register" is still set, but the "Backup regulator ready" takes some time before it becomes set. I believe this indicates that there is some issue with the power...

 

I use a STM32F429

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