Exchange serial messages via UART between 3.3V µController and 5V µController concrete (STM32F4 and ATMega 2560)
You only need wires. The STM32F4 has 5V-tolerant inputs and 3.3V is above the '1' threshold for the AVR. Connect ground and one or both pairs of TX/RX depending on whether you want bidirectional comms.