Hello everyone. I have a custom board with STM32F103C8T6. It was made for debugging so it doesn't have any unusal peripherals (only oscillator, filtering capacitors etc.). But there is problem: voltage on NRST pin equals about 1 V.

[Axcvn.1]

It is strange because I did not program the chip, the flash should be pure. Also I can't connect to the MCU with ST-LINK. What's wrong with NRST?

[Tesla DeLorean]

Double check part orientation and pin designations.

[Axcvn.1]

I checked them repeatedly, there is nothing wrong related with pinout.

[Tesla DeLorean]

What kind of current draw are you seeing on the 3.3V supply side?

[Axcvn.1]

About 1.5 mA.

[Tesla DeLorean]

Power pins look ok, is the 1V a static DC level, or is the pin oscillating?

What does it look like on a scope, What does VDD look like?

[Axcvn.1]

VDD is stable, it has 3.3 V with small pulsations about 5 mV, so it seems that the problem isn't related with power supply. 1V on NRST is not static, but it is not oscillating too. The voltage is floating randomly from 0.4 V to 2 V, but mostly is staying beside 1 V. I'll attach oscillogram lately, I do not have possibility to make it right now. My assumption is that MCU can't start properly and trying to reset by itself but I do not see any reasons for that.

[Piranha]

What is the purpose of R4? Copy-paste from the Internet without thinking? There is no pull-up resistor on NRST line in AN2586!

An again VBAT is not connected because of not reading the documentation.

Is C7 value really 1 mF, which is equal to 1000 uF, or is it just incorrect unit prefix?

[Axcvn.1]

Documentation really says that pullup resistance on NRST is not required but I decided to leave possibility to solder it. I checked board with external pullup and without, it does not make any changes, MCU is still not working correctly.

AN2586 only recommends to connect VBAT to VDD, it does not say that I must connect them. VBAT are used only for powering RTC and backup registers, I don't use them because I even can't load any program to MCU.

C7 capacitance equals 1uF in fact, it is the wrong value on schematic.