qmk_firmware/keyboards/helix/rev2/serial_config.h

#ifndef SOFT_SERIAL_CONFIG_H
#define SOFT_SERIAL_CONFIG_H

/* Soft Serial defines */
#define SERIAL_PIN_DDR DDRD
#define SERIAL_PIN_PORT PORTD
#define SERIAL_PIN_INPUT PIND
#define SERIAL_PIN_MASK _BV(PD2)
#define SERIAL_PIN_INTERRUPT INT2_vect

#define SERIAL_USE_MULTI_TRANSACTION

//// #error rev2 serial config

#endif /* SOFT_SERIAL_CONFIG_H */