Custom board Design using STM32F446RE and STM32F103CBT6

[Ellu1213]

I am designing a custom board using STM32F446RE as main MCU and STM32F103CBT6 for ST link. I am using MB1136 board schematic as reference. I have the following questions:

1) Should I connect NRST of pin of main MCU to ST link MCU? But in reference schematic, I can't see both are connected together, instead on NRST pin of STM32F103CBT6 a capacitor and pullup resistor are connected. Please clarify the connection. I can't see any netlabel or port of same name for NRST. Please find attachment of reference design.

2) What is the use of PA0 pin. Why is it connected to +3V3 with two 4.7K resistor? Should I connect those when I design the custom board? Please clarify.

Thanks in advance

[TDK]

The ST-LINK firmware isn't open source so we can only make educated guesses here. If you're dead set on using the STM32F103 chip and making to a part of the board, I would copy the reference design exactly to future-proof yourself to firmware changes.

Looks like PA0 is being used as an ADC input to verify the ST_LINK +3V3 voltage.

Consider instead using the STLINK-V3MODS and soldering that directly to your board. That is the supported design direction. Or an external debugger.

https://www.st.com/en/development-tools/stlink-v3mods.html