qmk_firmware/keyboards/sol/serial.h

#ifndef MY_SERIAL_H
#define MY_SERIAL_H

#include <stdbool.h>

/* TODO:  some defines for interrupt setup */
#define SERIAL_PIN_DDR DDRD
#define SERIAL_PIN_PORT PORTD
#define SERIAL_PIN_INPUT PIND
#define SERIAL_PIN_MASK _BV(PD3) //SErial pin goes here, D0-D3
#define SERIAL_PIN_INTERRUPT INT3_vect //"INT#" of your serial pin


#define SERIAL_SLAVE_BUFFER_LENGTH MATRIX_ROWS/2
#define SERIAL_MASTER_BUFFER_LENGTH MATRIX_ROWS/2

// Buffers for master - slave communication
extern volatile uint8_t serial_slave_buffer[SERIAL_SLAVE_BUFFER_LENGTH];
extern volatile uint8_t serial_master_buffer[SERIAL_MASTER_BUFFER_LENGTH];

void serial_master_init(void);
void serial_slave_init(void);
int serial_update_buffers(void);
bool serial_slave_data_corrupt(void);

#endif