qmk_firmware/keyboards/clueboard1/clueboard1.c

#include "clueboard1.h"

void led_set_kb(uint8_t usb_led) {
    DDRF |= (1<<0);
    if (usb_led & (1<<USB_LED_CAPS_LOCK)) {
        // Turn capslock on
        PORTF |= (1<<0);
    } else {
        // Turn capslock off
        PORTF &= ~(1<<0);
    }
}