Hello,
Im confused on how to utilize the VBAT pin when 0 ohm resistor is desoldered. I am aware that SRAM, RTC and some registers are powered. I havent gone into understanding of the STM32 MCU. Can you help with some examples on how to use the feature.
Regards,
Prudhvi Sagar
@prusaga, providing power to VBAT allows the STM32 to maintin power to its Real Time Clock and a portion (4KB) or RAM (retained variables) when the processor is powered off or in Deep Sleep. Without power to VBAT, neither of these is maintained.
Having the zero-ohm resistor removed with an external battery (3v) allows your application to store variables in retained memory which could be helpful for your application to store important values during deep sleep or when no power is provided to the STM32. Those values would be “instantly” available to your application instead of storing them to EEPROM for example.
Having the RTC running means it maintains its time through deep sleep or power down, which might be important when connection to the cloud is not done regularly.
For most users, it’s not necessary to remove the 0 ohm resistor that ties VBAT to 3V3. The reason is that 3V3 will be powered by the LiPo battery (or USB or VIN) all the time, so on the Electron you don’t need an external power source on VBAT.
You can, however, remove the 0 ohm resistor and connect an external lithium coin cell battery which will keep the retained memory and RTC active when in deep sleep or powered off. You might do this if you use the Electron without the LiPo battery and instead use a power supply connected to VIN that’s not always powered, but you still want to preserve retained memory and the RTC when it’s not powered.