power supplyst-linkstm32
I have made my own board with an stm32g474 on it. Unfortunately I have some problem with connecting to it with ST-link. The board has it's own power supply.
If I connect the SWDIO, SWCLK, NRST and GND pins , then the ST-link cannot connect to it.
However it works, if I disconnect the board's power supply and connect the ST-link's 3,3V.
Do you have any idea, why is that?
The resulting error message indicates, that your PC is unable to find the ST Link altogether, can you try to disconnect your board from the CN2 connector and try again?
You should get something like "No MCU found" then, if you still get "Cannot connect to ST-Link" it would seem like your ST-Link is damaged or your USB cable is broken (that happens sometimes).
Your connections seem fine - check if you have no solder bridges on your board, if there is a short somewhere maybe that prevents the ST-Link from working correctly.
The GPIO information you're looking for is in the reference manual. Here's the structure of a regular, non-5V-tolerant† GPIO pin on the STM32F103 series (used in the STLink/V2-1 on Nucleo boards):
Note that the top protection diode shunts voltages above the microcontroller's VDD into the supply rail to protect the IO pin. When the microcontroller is unpowered, that means that the IO pin might inadvertently sink current through the protection diode to power the microcontroller.
This parasitic power draw may be enough to overcome the internal/external pullup resistors on NRST and hold your main microcontroller in reset.
Unfortunately, without changing the design of the debug hardware, I don't see a good way to prevent NRST from being pulled to ground without disconnecting it. You might be able to find a series resistor value that allows normal reset operation (pulled to ground through the NMOS of the IO pin) while limiting the voltage drop when the debugger is powered down (pulled to ground through the protection diode).
Equivalent circuit for normal operation:
simulate this circuit – Schematic created using CircuitLab
Equivalent circuit for unpowered operation:
Best Answer
That is because the ST-Link can work with a range of different voltages, it needs the voltage reference what to use from the target. If it for example always used 3.3V, it would damage something if connected to a 2.7V target.