RAM data retention in Standby mode

[HF.Wang]

Hi

when the STM32F103 enter the standby mode, it has the lowest power consumption. And it said the voltage regulator is switched off. And all of the data in the RAM will be lost except the backup ram and register of the RTC.

But I tested it, the data will still keep the value (the standby mode could be entered successfully that the consumption will be reduced to several micro-amperes). the different thing is that all the supplies are still supply to the MCU (not only the VBAT).

So that means the LDO is not actually switched off as it doesn't detect VDD to be lost or lower than the threshold.

Is it right?

[Sarra.S]

Hello @HF.Wang 

Thank you for your questions, 

By entering standby mode, the main VDD supply is powered off ( the 1.8 V domain is consequently powered off), and RTC and backup registers are retained because they are powered from the VBAT as shown in the figure:

>>So that means the LDO is not actually switched off as it doesn't detect VDD to be lost or lower than the threshold.

Switching to the VBAT supply is controlled by the power-down reset (PDR) circuitry embedded in the reset block, which is always active. When VDD drops below a specified threshold, the device remains in reset mode (check datasheet and AN2629) 

Could you please specify the test you held to conclude that all the power supplies are running?

Thank you! 

[HF.Wang]

My test is keep all the supplies on. And let the MCU enter standby mode. for a long time, wakeup from the wake-up pin. all the data in the RAM all still keep the values.

>>Switching to the VBAT supply is controlled by the power-down reset (PDR) circuitry embedded in the reset block, which is always active. When VDD drops below a specified threshold, the device remains in reset mode 

So if the VDD is active, the switch of the VBAT will not occurs, and the LDO is continue to supply the RAM, is it right?

[Sarra.S · Accepted solution]

Hello again, 

>>So if the VDD is active, the switch of the VBAT will not occurs, and the LDO is continue to supply the RAM

Yes, there is no need to retain contents of backup registers (switch to VBAT) when VDD is ON

>>All the data in the RAM all still keep the values.

What you're seeing is a latched state of RAM prior to VDD loss, however, this is not reliable as these values can vary with temperature, different environment... So, we cannot guarantee the state of RAM data.