MCU resets after couple hours

[Adistrict]

Hi, everyone

I have a weird problem, my stm32f103c8 chip goes to reset mode(nRST pin voltage goes below 1.5 v) after working for couple hours. The MCU is switching the GPIOB1(frequency is 500KHz). Do you have any suggestion about this issue? Is it a power up problem(could this be because of wrong voltage rise sequence in VDD,VDDA and nRST pines)?

Thanks in advance,

Amir

[Adistrict]

What should I do with the reset pin? (nRST pin is currently open circuit and not pulled down)

Do you have any suggestions about the program or hardware?

What can cause the microcontroller to reset after working for hours?

I already had a serious problem and this problem(going to reset mode) was happening after 15 minutes or less. I added a 10uF capacitor to the VDD 3 pin and now, it's happening after a couple of hours(not after 15 minutes).