As requested in issue 824# by ascillato2, implement split interlock.

=> Split interlock of SONOFF with more than 2 channel into 2 groups
=> This is needed for shutter and similar utilization

sonoff/settings.h

@@ -76,7 +76,7 @@ typedef union {                            // Restricted by MISRA-C Rule 18.4 bu
     uint32_t sleep_normal : 1;             // bit 10 (v6.3.0.15) - SetOption60 - Enable normal sleep instead of dynamic sleep
     uint32_t button_switch_force_local : 1;// bit 11 (v6.3.0.16) - SetOption61 - Force local operation when button/switch topic is set
     uint32_t no_pullup : 1;                // bit 12 (v6.4.1.7)  - SetOption62 - Force no pull-up (0 = (no)pull-up, 1 = no pull-up)
-    uint32_t spare13 : 1;
+    uint32_t split_interlock : 1;          // bit 13 (v6.4.1.8)  - SetOption63 - Split interlock on CH4
     uint32_t spare14 : 1;
     uint32_t spare15 : 1;
     uint32_t spare16 : 1;

sonoff/sonoff.ino

@@ -319,6 +319,23 @@ void SetDevicePower(power_t rpower, int source)
     power = (1 << devices_present) -1;
     rpower = power;
   }
+ if (Settings.flag3.split_interlock) {
+    Settings.flag.interlock = 1; // prevent the situation where interlock is off and split-interlock is on
+    uint8_t mask = 0x01;
+    uint8_t count = 0;
+    byte result1 = 0;
+    byte result2 = 0;
+    for (byte i = 0; i < devices_present; i++) {
+      if (rpower & mask) {
+        if (i <2) { result1++;}//increment if low part is ON
+        if (i >1) { result2++;}//increment if high part is ON
+      }
+       mask <<= 1; // shift the bitmask one left (1,2,4,8) to find out what is on
+    }
+    if ((result1) >1 && (result2 >1)) {power = 0; rpower = 0;} // all 4 switch are on, something is wrong, so we turn all off
+    if ((result1) >1 && (result2 <2)) {power = power & 0x0C; rpower = power;} // 1/2 are both on and 3/4 max one is on
+    if ((result1) <2 && (result2 >1)) {power = power & 0x03; rpower = power;} // 1/2 max one is on and 3/4 both are on
+  } else {
   if (Settings.flag.interlock) {     // Allow only one or no relay set
     power_t mask = 1;
     uint8_t count = 0;
@@ -331,6 +348,7 @@ void SetDevicePower(power_t rpower, int source)
       rpower = 0;
     }
   }
+  }
 
   XdrvMailbox.index = rpower;
   XdrvCall(FUNC_SET_POWER);               // Signal power state
@@ -1389,7 +1407,28 @@ void ExecuteCommandPower(byte device, byte state, int source)
       blink_mask &= (POWER_MASK ^ mask);  // Clear device mask
       MqttPublishPowerBlinkState(device);
     }
-    if (Settings.flag.interlock && !interlock_mutex) {  // Clear all but masked relay
+  if (Settings.flag3.split_interlock && !Settings.flag.interlock ) Settings.flag.interlock=1; // ensure interlock is on, in case split_interlock is on
+  // check if channel 1/2 or 3/4 are to be changed
+  if (device <= 2 && Settings.flag3.split_interlock ) { // channel 1/2 are changed
+    if (Settings.flag3.split_interlock && !interlock_mutex) { // Clear all but masked relay, but only if we are not already doing something
+      interlock_mutex = 1;
+        for (byte i = 0; i < 2; i++) {
+          byte imask = 0x01 << i;
+          if ((power & imask) && (mask != imask)) { ExecuteCommandPower(i +1, POWER_OFF, SRC_IGNORE); delay(50); }// example, first power is ON but the pushed button is not the first, then powerOFF the first one
+        }
+      interlock_mutex = 0; // avoid infinite loop due to recursive requests
+    }
+  } else {  // channel 3/4 are changed
+    if (Settings.flag3.split_interlock && !interlock_mutex) {  // only start if we are on interlock split and have no re-call
+    interlock_mutex = 1;
+      for (byte i = 2; i < devices_present; i++) {
+        byte imask = 0x01 << i;
+        if ((power & imask) && (mask != imask)) ExecuteCommandPower(i +1, POWER_OFF, SRC_IGNORE);
+      }
+      interlock_mutex = 0;
+    }
+  }
+    if ( Settings.flag.interlock && !interlock_mutex && !Settings.flag3.split_interlock) {  //execute regular interlock-mode as interlock-split is off    
       interlock_mutex = 1;
       for (byte i = 0; i < devices_present; i++) {
         power_t imask = 1 << i;