Only store last 3 bytes of MAC

Only store last 3 bytes of MAC in EEPROM.

Author: budulinek

arduino-modbus-rtu-tcp-gateway/01-interfaces.ino

@@ -65,15 +65,18 @@ void startEthernet() {
     digitalWrite(ethResetPin, HIGH);
     delay(500);
   }
+  byte mac[6];
+  memcpy(mac, MAC_START, 3); // set first 3 bytes
+  memcpy(mac + 3, localConfig.macEnd, 3); // set last 3 bytes
 #ifdef ENABLE_DHCP
   if (localConfig.enableDhcp) {
-    dhcpSuccess = Ethernet.begin(localConfig.mac);
+    dhcpSuccess = Ethernet.begin(mac);
   }
   if (!localConfig.enableDhcp || dhcpSuccess == false) {
-    Ethernet.begin(localConfig.mac, localConfig.ip, localConfig.dns, localConfig.gateway, localConfig.subnet);
+    Ethernet.begin(mac, localConfig.ip, localConfig.dns, localConfig.gateway, localConfig.subnet);
   }
 #else /* ENABLE_DHCP */
-  Ethernet.begin(localConfig.mac, localConfig.ip, localConfig.dns, localConfig.gateway, localConfig.subnet);
+  Ethernet.begin(mac, localConfig.ip, localConfig.dns, localConfig.gateway, localConfig.subnet);
   localConfig.enableDhcp = false;                 // Make sure Dhcp is disabled in config
 #endif /* ENABLE_DHCP */
   modbusServer = EthernetServer(localConfig.tcpPort);
@@ -143,7 +146,7 @@ void generateMac()
   uint32_t randomBuffer = (seed1 << 16) + seed2;  /* 32-bit random */
 
   for (byte i = 0; i < 3; i++) {
-    localConfig.mac[i + 3] = randomBuffer & 0xFF;
+    localConfig.macEnd[i] = randomBuffer & 0xFF;
     randomBuffer >>= 8;
   }
 }

arduino-modbus-rtu-tcp-gateway/04-webserver.ino

@@ -312,10 +312,10 @@ void processPost(char postParameter[]) {
   switch (action) {
     case FACTORY:
       {
-        byte tempMac[6];
-        memcpy(tempMac, localConfig.mac, 6); // keep current MAC
+        byte tempMac[3];
+        memcpy(tempMac, localConfig.macEnd, 3); // keep current MAC
         localConfig = defaultConfig;
-        memcpy(localConfig.mac, tempMac, 6);
+        memcpy(localConfig.macEnd, tempMac, 3);
         break;
       }
     case MAC:

arduino-modbus-rtu-tcp-gateway/arduino-modbus-rtu-tcp-gateway.ino

@@ -37,10 +37,11 @@
   v3.0 2021-11-07 Improve POST parameters processing, bugfix 404 and 204 error headers. 
   v3.1 2022-01-28 Code optimization, bugfix DHCP settings.
   v3.2 2022-06-04 Reduce program size (so that it fits on Nano), ethernet data counter only available when ENABLE_EXTRA_DIAG.
+  v4.0 2022-11-26 Only store last 3 bytes of MAC in EEPROM,
 
 */
 
-const byte version[] = {3, 2};
+const byte version[] = {4, 0};
 
 #include <SPI.h>
 #include <Ethernet.h>
@@ -84,7 +85,7 @@ const byte scanCommand[] = {0x03, 0x00, 0x00, 0x00, 0x01};  // Command sent duri
 
 typedef struct
 {
-  byte mac[6];
+  byte macEnd[3];
   bool enableDhcp;
   IPAddress ip;
   IPAddress subnet;
@@ -109,7 +110,7 @@ typedef struct
 */
 
 const config_type defaultConfig = {
-  { 0x90, 0xA2, 0xDA },  // mac (bytes 4, 5 and 6 will be generated randomly)
+  {},                    // macEnd (last 3 bytes)
   false,                 // enableDhcp
   {192, 168, 1, 254},    // ip
   {255, 255, 255, 0},    // subnet
@@ -140,6 +141,8 @@ byte maxSockNum = MAX_SOCK_NUM;
 
 bool dhcpSuccess = false;
 
+const byte MAC_START[3] = { 0x90, 0xA2, 0xDA };
+
 EthernetUDP Udp;
 EthernetServer modbusServer(502);
 EthernetServer webServer(80);