I want to make a circuit for triggering the STM32 microcontroller into the Bootloader mode. For normal operation of MCU using flash memory the BOOT0 pin should be low and for entering the bootloader mode the BOOT0 pin should be pulled high. Presently I am using Bistable multivibrator using 555 timer. The output of bistable multivibrator is connected to BOOT0 pin.

Initially on power up, the output of the bistable multivibrator should be low but in my circuit it is coming high because of which my MCU by default is entering in bootloader mode.

Why on power up the output of circuit is coming high and what can I do for output to be low initially?

 

Well it's not a multivibrator because there are no timing components, it's just a set/reset latch. But that aside it all depends on the relative times of trig and reset. You need to keep reset low while power is applied. A small capacitor, 10nF, from reset to ground might help.