Coming along, I suppose

avr/main.c

@@ -1,12 +1,67 @@
+#include <stdio.h>
+
 #include <avr/interrupt.h>
+#include <avr/sleep.h>
 
 #include <tabby/clock.h>
 
+static volatile uint8_t bits      = 0;
+static volatile uint8_t value_in  = 0x00; /* Data coming in from Game Boy */
+static volatile uint8_t value_out = 0x00; /* Data going out from host */
+
 uint16_t timer_counter_intervals[4] = {
     1953, 977, 61, 31
 };
 
-void setup_internal_clock(tabby_clock_speed speed) {
+/*
+ * Internal clock source interrupt vector
+ */
+ISR(TIM0_COMPB_vect) {
+    /*
+     * Strobe the SCK pin
+     */
+    PORTB |=  (1 << PORTB1);
+    PORTB &= ~(1 << PORTB1);
+
+    value_in >>= 1;
+
+    if (PORTB & (1 << PORTB2)) {
+        value_in |= 0x80;
+    }
+
+    if (value_out & 0x80) {
+        PORTB |=  (1 << PORTB3);
+    } else {
+        PORTB &= ~(1 << PORTB3);
+    }
+
+    value_out <<= 1;
+
+    if (--bits == 0) {
+        putchar(value_in);
+
+        value_out = getchar();
+
+        bits = 8;
+    }
+}
+
+static void clock_internal_setup(tabby_clock_speed speed) {
+    /*
+     * Configure MISO as output
+     */
+    DDB3 |= (1 << PORTB3);
+
+    /*
+     * Configure MOSI as input
+     */
+    DDB2 &= ~(1 << PORTB2);
+
+    /*
+     * Configure SCK pin as output
+     */
+    DDB1 |= (1 << PORTB1);
+
     /*
      * Enable timer interrupt vector
      */
@@ -28,7 +83,7 @@ void setup_internal_clock(tabby_clock_speed speed) {
     TCCR1B = (1 << CS10);
 }
 
-void setup_external_clock() {
+static void clock_external_setup() {
     /*
      * Disable internal timer interrupts
      */
@@ -37,6 +92,29 @@ void setup_external_clock() {
     TIMSK  = 0;
 }
 
+static void snooze() {
+    set_sleep_mode(mode);
+    sleep_enable();
+    sleep_mode();
+    sleep_disable();
+}
+
 int main() {
+    /*
+     * By default, the Game Boy link port is configured to monitor for external
+     * clock pulses, so we'll go ahead and do that in this case.
+     */
+    clock_external_setup();
+
+    /*
+     * We do actually want to globally enable interrupts here, so here's that
+     * one assembly instruction to do so.
+     */
+    sei();
+
+    while (1) {
+        snooze();
+    }
+
     return 0;
 }

include/tabby/avr.h

@@ -0,0 +1,6 @@
+#ifndef _TABBY_AVR_H
+#define _TABBY_AVR_H
+
+#define TABBY_AVR_SERIAL_BAUD 115200
+
+#endif /* _TABBY_AVR_H */