| Nucleo board pin | HC-SR04 pin |
|---|---|
| 5V | Vcc |
| GND | GND |
| PA8 | Echo |
| PB4 | Trig |
NOTE: Though most IO pins on STM32 microcontrollers are 5V tolerant, it's safer to add a voltage divider between the Echo pin and the PA8 pin. A 5K / 3.3K divider should do the job just fine and the voltage going to pin PA8 should not exceed 3V.