diff --git a/fw/antorcha.c b/fw/antorcha.c
index 320ff5d..2367da7 100644
--- a/fw/antorcha.c
+++ b/fw/antorcha.c
@@ -25,7 +25,6 @@
 
 
 static struct sweep sweep = {
-	.wait_ticks	=	 60000,	/*  60 ms */
 	.pixel_ticks	=	  1000,	/*   1 ms */
 	.left		=	     0,
 	.right		=   MAX_LINES-1,
@@ -128,6 +127,7 @@ sei();
 #if 1
 		if (state == FWD_START_SWEEP && !sweeping) {
 			state = FWD_SWEEP;
+			sweep.start_ticks = uptime()+60000; /* 60 ms */
 			sweep_image(&sweep);
 		}
 #endif
diff --git a/fw/sweep.c b/fw/sweep.c
index 85ef784..1f98f77 100644
--- a/fw/sweep.c
+++ b/fw/sweep.c
@@ -118,6 +118,15 @@ uint32_t uptime(void)
 
 void sweep_image(const struct sweep *sweep)
 {
+	uint32_t t;
+
+	/* calculate start time */
+
+	t = uptime();
+	if (sweep->start_ticks <= t)
+		return;
+	t = sweep->start_ticks-t;
+
 	TCCR1B = 0;	/* stop the timer */
 
 	cli();
@@ -142,9 +151,9 @@ void sweep_image(const struct sweep *sweep)
 	/* timing parameters */
 
 	pixel_ticks = sweep->pixel_ticks;
-	wait_periods = sweep->wait_ticks >> 16;
+	wait_periods = t >> 16;
 	if (wait_periods) {
-	 	wait_short = sweep->wait_ticks;
+	 	wait_short = t;
 		wait_period = 0xffff;
 
 		/*
@@ -157,7 +166,7 @@ void sweep_image(const struct sweep *sweep)
 		}
 		ICR1 = wait_period;
 	} else {
-	 	ICR1 = sweep->wait_ticks;
+	 	ICR1 = t;
 	}
 
 	/* prepare the hardware timer */
diff --git a/fw/sweep.h b/fw/sweep.h
index b0113e4..d9bdcb5 100644
--- a/fw/sweep.h
+++ b/fw/sweep.h
@@ -19,7 +19,7 @@
 
 
 struct sweep {
-	uint32_t wait_ticks;	/* number of ticks to wait before image */
+	uint32_t start_ticks;	/* absolute start time */
 	uint16_t pixel_ticks;	/* number of ticks per pixel */
 	uint8_t left;		/* leftmost line of image */
 	uint8_t right;		/* rightmost line of image */