I am powering my Nucleo Board (Nucleo-F767ZI) externally with 12V. The external powersupply is connected with the GND and VIN Pins of the Nucleo Board and Jumper JP3 is set to VIN. Unfortunatelly, the Microcontroller needs a reset Signal to start. A reset signal earlier than 2 seconds after power has been applied does not have any effect. I would like the microcontroller to start instantly when powered externally. What is the Problem here?
ANSWER SHORT
When powered externally, the crystal oscillator X3 (HSE) has to be used.
Edit: Or the internal oscillator has to be used (HSI)
ANSWER LONG
After some investigation it seems, that the clock for the STM32 is provided by the on board debugger. This clock signal is generated about 2 seconds after the board is power, after which a reset takes effect. This Problem can be solved by using an external oscillator. It is explained in the User Manual of the Nucleo Board, that the Clock is provided by the ST-LINK. Furthermore, it is explained that the HSE on-board oscillator from X3 crystal is not provided. Buying an appropriate oscillator and capacitors and connecting it according to the User Manual, will solve the issue. Attention has to be paid, when soldering and desoldering solder-bridges. Do not forget to change the setting of RCC from Bypass Clock to Crystal/Ceramic Resonator.