Almost working version

AVR/main.c

@@ -3,30 +3,24 @@
 #include <avr/io.h>
 #include <avr/interrupt.h>
 
-#define I2C_ADDR 0x20
+#define I2C_ADDR 0x30
 
 volatile uint8_t challenge = 0; // Use 0 as 'challenge not sent' condition
+uint16_t time_open = 0;
 
 uint8_t check_challenge(uint8_t arg) {
-  return (arg >> 3) ^ (arg << 3);
+  return (arg >> 3);
 }
 
 void I2C_received(uint8_t received_data) {
-  if (challenge != 0) {
-    if (check_challenge(received_data) == check_challenge(challenge)) { // Check validity of response
-      // Open lock
-      PORTC |= 2 << 1;
-      _delay_ms(5000);
-      PORTC = 0;
-    }
+  if (received_data == check_challenge(challenge)) { // Check validity of response
+    // Open lock
+    time_open = 5000;
   }
-  challenge = 0;
 }
 
 void I2C_requested() {
-  challenge = (uint8_t)(TCNT1 & 0xff); // Use content of timer as pseudo-random number
-  if (challenge == 0) // Prevent 'challenge' from being 0, which has a special meaning
-    challenge = 1;
+  challenge = (uint8_t)(TCNT1 & 0xff);
   I2C_transmitByte(challenge);
 }
 
@@ -43,5 +37,13 @@ void setup() {
 
 int main() {
   setup();
-  while (1); // All the work is done in the callbacks
+  while (1) {
+    if (time_open > 0) {
+      PORTC |= 2 << 1;
+      time_open -= 1;
+    } else {
+      PORTC = 0;
+    }
+    _delay_ms(1);
+  }
 }

AVR/makefile

@@ -5,7 +5,7 @@
 # (GNU make, BSD make, SysV make)
 
 
-MCU = atmega328p
+MCU = atmega88
 FORMAT = ihex
 TARGET = main
 SRC = $(TARGET).c \