clock/crystal: use 64 prescaler

services/clock/clock.c

@@ -60,8 +60,8 @@ clock_init(void)
 #ifdef CLOCK_CRYSTAL_SUPPORT
 	ASSR = _BV(CLOCK_TIMER_AS);
 	CLOCK_TIMER_CNT = 0;
-	/* 128 prescaler to get every second an interrupt */
-	CLOCK_TIMER_TCCR = _BV(CLOCK_SELECT_2) | _BV(CLOCK_SELECT_0);
+	/* 64 prescaler to get every 0.5 second an interrupt */
+	CLOCK_TIMER_TCCR = _BV(CLOCK_SELECT_1) | _BV(CLOCK_SELECT_0);
 
 	/* Wait until the bytes are written */
 #ifdef CLOCK_TIMER_RBUSY
@@ -96,6 +96,15 @@ SIGNAL(CLOCK_SIG)
 	OCR1A = 65536-CLOCK_SECONDS+CLOCK_TICKS;
 #endif
 
+#if defined(CLOCK_CRYSTAL_SUPPORT)
+    /* If we use Crystal Support we have an interrupt every 0.5
+       seconds, so we have to drop every second interrupt */
+    static uint8_t clock_crystal_interrupt_drop = 0;
+    clock_crystal_interrupt_drop ^= 1;
+    if (clock_crystal_interrupt_drop)
+        return;
+#endif
+
 #if defined(NTP_SUPPORT) || defined(DCF77_SUPPORT)
 	if (!sync_timestamp || sync_timestamp == timestamp)
 #endif