Welcome, @TMurp.3, to the community!
According to the data sheet, the function of the STM32F405 is guaranteed in the supply voltage range 1.8V to 3.6V over the entire temperature range, i.e. from -40°C. However, the data sheet additionally mentions:
The STM32F405xx and STM32F407xx family operates in the -40 to +105°C temperature range from a 1.8 to 3.6 V power supply. The supply voltage can drop to 1.7 V when the device operates in the 0 to 70°C temperature range using an external power supply supervisor.
The correct function of the STM32F405 is no longer guaranteed at a supply voltage below 1.8V and additionally at a temperature below 0°C, so that no statement can therefore be made about the resulting effects.
Depending on your requirements, you could switch to other STM32 whose function is already guaranteed from 1.7V, such as STM32F427, STM32F446 or Cortex-M7 based types like the STM32F722.
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Regards
/Peter
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