AK_Interrupts.c

// ----------------------------------
// | AK_Interrupts.c                |
// | ArduinoKit (User Project Side) |
// |                                |
// | Last Update: 2026-07-20        |
// ----------------------------------
//
// Place this file in your S4A project directory and list it in the project's
// .swift4p `files:` section (e.g. `- AK_Interrupts.c`).
//
// WHY C, NOT Swift @interruptHandler?
//   S4A's @interruptHandler currently lowers like avr-libc ISR_NOBLOCK: it
//   emits `sei` at the start of the vector. USART_UDRE / USART_RX do not clear
//   their condition on entry, so early `sei` re-enters the same ISR until the
//   stack overflows (Serial often prints ~2 characters then halts).
//
//   avr-libc's ISR() defaults to "block" / `signal` semantics (no early `sei`),
//   which matches ArduinoCore and is safe for UDRE/RX. See:
//   https://www.nongnu.org/avr-libc/user-manual/group__avr__interrupts.html#ga44569cb914d2aaf8fbb436f8f7c4ca68
//
// Library code cannot own these vectors (they must live in the final linked
// image). ArduinoKit exports the C-callable handlers below; this file only
// registers the AVR vectors and forwards into Swift.

#include <avr/interrupt.h>

/* Implemented in ArduinoKit (InterruptHook.swift) via @_cdecl. */
void AK_timer0Overflow(void);
void AK_usartRxComplete(void);
void AK_usartDataRegisterEmpty(void);

ISR(TIMER0_OVF_vect) {
    AK_timer0Overflow();
}

ISR(USART_RX_vect) {
    AK_usartRxComplete();
}

ISR(USART_UDRE_vect) {
    AK_usartDataRegisterEmpty();
}