qmk_firmware/keyboards/handwired/splittest/teensy_2/teensy_2.c

#include QMK_KEYBOARD_H

bool is_keyboard_master(void) {
    // TODO: replace this override once USB host detection is implemented
    // SPLIT_HAND_PIN Combined with MASTER_LEFT or MASTER_RIGHT, gives a crude
    // way to force teensy to run as slave/master
    setPinInput(SPLIT_HAND_PIN);

#if defined(MASTER_RIGHT)
    return !readPin(SPLIT_HAND_PIN);
#else
    return readPin(SPLIT_HAND_PIN);
#endif
}