diff --git a/avr/main.c b/avr/main.c
index 0d30910..f94cf27 100644
--- a/avr/main.c
+++ b/avr/main.c
@@ -4,26 +4,28 @@
 #include <avr/sleep.h>
 
 #include <tabby/clock.h>
+#include <tabby/packet.h>
+
 #include <tabby/avr/uart.h>
+#include <tabby/avr/buffer.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 */
+static volatile uint8_t value_out = 0x00; /* Data going out to Game Boy */
 
-uint16_t timer_counter_intervals[4] = {
+static const uint16_t timer_counter_intervals[4] = {
     1953, 977, 61, 31
 };
 
+static volatile tabby_avr_buffer buffer = {
+    .len = 0,
+    .cur = 0
+};
+
 /*
  * 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)) {
@@ -41,13 +43,38 @@ ISR(TIM0_COMPB_vect) {
     if (--bits == 0) {
         uart_putchar(value_in, NULL);
 
-        value_out = uart_getchar(NULL);
+        if (buffer.cur < buffer.len) {
+            value_out = buffer.data[++buffer.cur];
+        } else {
+            buffer.len = 0;
+            buffer.cur = 0;
+        }
 
         bits = 8;
     }
+
+    /*
+     * Strobe the SCK pin
+     */
+    PORTB |=  (1 << PORTB1);
+    PORTB &= ~(1 << PORTB1);
 }
 
-static void clock_internal_setup(tabby_clock_speed speed) {
+/*
+ * SPI byte receipt interrupt vector
+ */
+ISR(SPI_STC_vect) {
+    uart_putchar(SPDR);
+
+    if (buffer.cur < buffer.len) {
+        SPDR = buffer.data[++buffer.cur];
+    } else {
+        buffer.len = 0;
+        buffer.cur = 0;
+    }
+}
+
+static void setup_clock_internal(tabby_clock_speed speed) {
     /*
      * Configure MISO as output
      */
@@ -84,13 +111,41 @@ static void clock_internal_setup(tabby_clock_speed speed) {
     TCCR1B = (1 << CS10);
 }
 
-static void clock_external_setup() {
+static void setup_clock_external() {
     /*
      * Disable internal timer interrupts
      */
     TCCR1B = 0;
     OCR1A  = 0;
     TIMSK  = 0;
+
+    /*
+     * Configure MISO as output
+     */
+    DDB3 |= (1 << PORTB3);
+
+    /*
+     * Configure MOSI as input
+     */
+    DDB2 &= ~(1 << PORTB2);
+
+    /*
+     * Set SPI slave mode, and shift in/out most significant bit first
+     */
+    SPCR &= ~((1 << MSTR) | (1 << DORD));
+
+    /*
+     * Enable SPI in Mode 3 with interrupts
+     */
+    SPCR |= (1 << CPOL) | (1 << CPHA) | (1 << SPIE) | (1 << SPE);
+
+    /*
+     * Initialize the SPI Data Register and serial buffer
+     */
+    SPDR = 0;
+
+    buf.len = 0;
+    buf.cur = 0;
 }
 
 static void snooze() {
@@ -101,6 +156,12 @@ static void snooze() {
 }
 
 int main() {
+    tabby_command state = TABBY_COMMAND_NONE;
+
+    uint8_t last = 0x00;
+
+    int received = 0;
+
     /*
      * Best turn on the serial UART!
      */
@@ -110,7 +171,7 @@ 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();
+    setup_clock_external();
 
     /*
      * We do actually want to globally enable interrupts here, so here's that
@@ -119,7 +180,51 @@ int main() {
     sei();
 
     while (1) {
-        snooze();
+        uint8_t c = uart_getchar(NULL);
+
+        received++;
+
+        switch (state) {
+            case TABBY_COMMAND_NONE: {
+                if (last == 0 && c == TABBY_PACKET_START) {
+                    continue;
+                } else if (last == TABBY_PACKET_START) {
+                    state = c;
+                }
+
+                break;
+            }
+
+            case TABBY_COMMAND_SEND: {
+                if (received == 3 || received == 4) {
+                    buffer.len = (buffer.len >> 8) | (c << 8);
+                } else {
+                    buffer.data[buffer.cur++] = c;
+                }
+
+                break;
+            }
+
+            case TABBY_COMMAND_CLOCK_SOURCE:
+            case TABBY_COMMAND_CLOCK_SPEED:
+
+            default:
+                goto error_invalid_packet;
+        }
+
+        last = c;
+
+        received++;
+
+        continue;
+
+error_invalid_packet:
+        state    = TABBY_COMMAND_NONE;
+        last     = 0;
+        received = 0;
+
+        buffer.cur = 0;
+        buffer.len = 0;
     }
 
     return 0;
diff --git a/include/tabby/command.h b/include/tabby/command.h
index 40e2a0d..a146c9d 100644
--- a/include/tabby/command.h
+++ b/include/tabby/command.h
@@ -2,6 +2,7 @@
 #define _TABBY_COMMAND_H
 
 typedef enum {
+    TABBY_COMMAND_NONE         = 0x00,
     TABBY_COMMAND_SEND         = 0x01,
     TABBY_COMMAND_CLOCK_SOURCE = 0x02,
     TABBY_COMMAND_CLOCK_SPEED  = 0x03
diff --git a/include/tabby/packet.h b/include/tabby/packet.h
index f94c2ae..cc9be2e 100644
--- a/include/tabby/packet.h
+++ b/include/tabby/packet.h
@@ -1,13 +1,16 @@
 #ifndef _TABBY_PACKET_H
 #define _TABBY_PACKET_H
 
-#define TABBY_PACKET_MAX_LEN 65535
+#define TABBY_PACKET_START   0xff
+#define TABBY_PACKET_MAX_LEN 1024
 
 #include <tabby/command.h>
 
 typedef struct _tabby_packet {
-    uint16_t type,
-             command;
+    uint8_t start,
+            command;
+
+    uint16_t len;
 } tabby_packet;
 
 #endif /* _TABBY_PACKET_H */