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Five BOR thresholds are available through option bytes, starting from 1.8 V to 3 V. Toreduce the power consumption in Stop mode, it is possible to automatically switch off theinternal reference voltage (VREFINT) in Stop mode. The device remains in reset mode whenVDD is below a specified threshold, VPOR/PDR or VBOR, without the need for any externalreset circuit.Note:The start-up time at power-on is typically 3.3 ms when BOR is active at power-up, the start-up time at power-on can be decreased down to 1 ms typically for devices with BOR inactiveat power-up.The device features an embedded programmable voltage detector (PVD) that monitors theVDD/VDDA power supply and compares it to the VPVD threshold. This PVD offers 7 differentlevels between 1.85 V and 3.05 V, chosen by software, with a step around 200 mV. Aninterrupt can be generated when VDD/VDDA drops below the VPVD threshold and/or whenVDD/VDDA is higher than the VPVD threshold. The interrupt service routine can then generatea warning message and/or put the MCU into a safe state. The PVD is enabled by software.3.3.3Voltage regulatorThe regulator has three operation modes: main (MR), low power (LPR) and power down.●MR is used in Run mode (nominal regulation)●LPR is used in the Low-power run, Low-power sleep and Stop modes●Power down is used in Standby mode. The regulator output is high impedance, thekernel circuitry is powered down, inducing zero consumption but the contents of theregisters and RAM are lost are lost except for the standby circuitry (wakeup logic,IWDG, RTC, LSI, LSE crystal 32K osc, RCC_CSR).3.3.4Boot modesAt startup, boot pins are used to select one of three boot options:●Boot from Flash memory●Boot from System Memory●Boot from embedded RAMThe boot loader is located in System Memory. It is used to reprogram the Flash memory byusing USART1 or USART2. See STM32™ microcontroller system memory boot modeAN2606 for detail