I am using STM205RBT^ & I have noticed that it does not start executing first instruction sometimes. I am using HSI oscillator & used pll to make 120Mhz. My first line in code is to toggle a pin. Only two possibilities I think: (if other plz let me know) 1. Before main there is systeminit() which set PLL to 120Mhz. Systeminit file is created from excel utility provided by ST. I am using HSI. There is one while loop in it. I have still to check this condition by placing toggle code before this while loop. 2. Something is making controller reset on power up (it happens sometimes only). So what could be reason???