revert second tick calculus for external sync

src/uClock.cpp

@@ -316,11 +316,15 @@ void uClockClass::handleExternalClock()
                 ext_interval_idx = 0;
             ext_interval_buffer[ext_interval_idx] = last_interval;
 
-            // calculate sync interval
-            ext_interval = (((uint32_t)ext_interval * (uint32_t)PLL_X) + (uint32_t)(256 - PLL_X) * (uint32_t)last_interval) >> 8;
-
             // external clock tick me!
             ext_clock_tick++;
+
+            // calculate sync interval
+            if (ext_clock_tick == 1) {
+                ext_interval = last_interval;
+            } else {
+                ext_interval = (((uint32_t)ext_interval * (uint32_t)PLL_X) + (uint32_t)(256 - PLL_X) * (uint32_t)last_interval) >> 8;
+            }
             break;
 
         case PAUSED: