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ST-Link

ST-Link is a programmer and debugger for STM32 microcontrollers.

In the context of simple devices, it is used to:

  • flash an STM32 board;
  • recover a board after a failed flash;
  • connect to the chip through SWD;
  • erase memory;
  • verify the microcontroller is visible;
  • sometimes debug the program.

ST-Link is not needed for all boards. ESP32, RP2040 and Arduino usually flash a different way.

What is SWD

SWD is Serial Wire Debug.

It is a debug and flash interface for ARM microcontrollers, commonly used with STM32.

For basic connection you usually need:

  • SWDIO;
  • SWCLK;
  • GND;
  • sometimes NRST;
  • sometimes 3.3V or VTref as target reference voltage.

Important: 3.3V on ST-Link doesn't always mean "power the board from here". Often ST-Link needs to see the target board voltage to understand the signal levels.

Minimal connection

Usually you connect:

ST-Link SWDIO -> board SWDIO
ST-Link SWCLK -> board SWCLK
ST-Link GND   -> board GND
ST-Link NRST  -> board NRST if reset is needed
ST-Link 3.3V/VTref -> 3.3V target if required by specific ST-Link

The STM32 board is often powered from its normal power or USB.

Before connecting, check the documentation for your specific ST-Link and board.

Nucleo boards for STM32 with built-in ST-Link debugger

Source: Wikimedia Commons, Avandalen, CC BY-SA 4.0

Board power

The most common confusion: is the board powered by ST-Link or separately?

Options:

  1. Board is powered separately, ST-Link only connects to SWD and GND.
  2. ST-Link provides 3.3V to a small target board, if it is actually designed for this.
  3. ST-Link only reads VTref but should not power the board.

You cannot blindly connect multiple power sources.

If the board is already powered by USB or a power supply, don't connect power from ST-Link until you are sure the schematic allows it.

DFU is a flash mode through a USB bootloader, if available.

ST-Link works through SWD directly with the microcontroller.

ST-Link is useful when:

  • DFU is not available;
  • the bootloader is corrupted or not used;
  • flashing disabled USB;
  • the board won't enter normal flash mode;
  • you need to erase memory;
  • you need to recover STM32 access.

But if a board flashes normally through USB/DFU or SD card, ST-Link may not be needed.

Connect under reset

Sometimes firmware breaks SWD access.

For example:

  • SWD pins are used as normal GPIO;
  • firmware quickly sleeps the chip;
  • code hangs right after startup;
  • clock/power modes are set incorrectly.

In such cases, Connect under reset mode helps.

The idea: ST-Link holds reset and tries to connect before bad firmware executes.

This often requires connecting NRST.

In STM32CubeProgrammer or similar tool you usually see:

  • whether ST-Link is connected;
  • whether the target is found;
  • what chip is detected;
  • whether you can read memory;
  • whether you can erase flash.

If ST-Link is visible as a USB device but the target is not found, the problem is often the board connection:

  • target board has no power;
  • no common GND;
  • SWDIO and SWCLK are swapped;
  • VTref is not connected if needed;
  • SWD frequency too high;
  • chip is locked;
  • firmware prevents connection;
  • Connect under reset is needed.

SWD frequency

If communication is unstable, try reducing the SWD frequency.

On long wires, poor Dupont jumpers or unstable boards, high frequency can interfere with connection.

For board recovery, slow and reliable beats fast.

Common errors

  • forgot GND;
  • swapped SWDIO and SWCLK;
  • connected power from ST-Link and USB at the same time without understanding the schematic;
  • didn't power the target board;
  • didn't connect NRST when Connect under reset is needed;
  • set SWD frequency too high;
  • trying to flash ESP32 or RP2040 with ST-Link;
  • trying to connect to a locked chip without understanding readout protection;
  • using long poor quality wires;
  • didn't update ST-Link firmware or didn't install driver.

What you cannot do

You cannot:

  • apply 5V to a 3.3V STM32 board without checking;
  • connect multiple power sources blindly;
  • connect ST-Link to a board under mains voltage without safe isolation and enclosure;
  • change SWD wires with power on if there is risk of shorting;
  • assume a clone ST-Link has the same pinout as the original.

Clone ST-Link pinouts may differ. Always check the markings on your specific adapter.

The essentials

  • ST-Link is needed for STM32 through SWD.
  • Minimum: SWDIO, SWCLK, GND, sometimes NRST and VTref.
  • The board should be powered correctly, but not necessarily from ST-Link.
  • DFU and ST-Link are different flashing methods.
  • Connect under reset helps recover access after bad firmware.
  • If the target is not found, first check power, GND, SWDIO/SWCLK, NRST and SWD frequency.

Reference materials