qmk-firmware/keyboards/clueboard/17/17.c

#include "17.h"

int pwm_level;

void matrix_init_kb(void) {
    // put your keyboard start-up code here
    // runs once when the firmware starts up
    matrix_init_user();

    // JTAG disable for PORT F. write JTD bit twice within four cycles.
    MCUCR |= (1<<JTD);
    MCUCR |= (1<<JTD);
};

void led_set_kb(uint8_t usb_led) {
    print("led_set\n");
}

void backlight_init_ports(void) {
    // Set C7 to output
    DDRC |= (1<<7);

    // Initialize the timer
    TC4H = 0x03;
    OCR4C = 0xFF;
    TCCR4A = 0b10000010;
    TCCR4B = 0b00000001;
}

void backlight_set(uint8_t level) {
    // Determine the PWM level
    switch (level)
    {
        case 0:
            // 33%
            pwm_level = 0x54;
            break;
        case 1:
            // 66%
            pwm_level = 0xA8;
            break;
        case 2:
            // 100%
            pwm_level = 0xFF;
            break;
        case 3:
            // 0%
            pwm_level = 0x00;
            break;
        default:
            xprintf("Unknown level: %d\n", level);
    }

    // Write the PWM level to the timer
    TC4H = pwm_level >> 8;
    OCR4A = 0xFF & pwm_level;
}