diff --git a/fw/sweep.c b/fw/sweep.c
index 059c286..3aac954 100644
--- a/fw/sweep.c
+++ b/fw/sweep.c
@@ -84,6 +84,17 @@ ISR(TIMER1_OVF_vect)
 }
 
 
+uint32_t uptime_irq(void)
+{
+	uint32_t t;
+
+	t = t_up+TCNT1;
+	if (TIFR1 & TOV1)
+		t += ICR1;
+	return t;
+}
+
+
 uint32_t uptime(void)
 {
 	uint32_t a, b;
@@ -92,7 +103,8 @@ uint32_t uptime(void)
 	do {
 		cli();
 		a = t_up;
-		d = ICR1;
+		d = TCNT1;
+		cli();
 		b = t_up;
 		sei();
 	}
diff --git a/fw/sweep.h b/fw/sweep.h
index 08b325e..b0113e4 100644
--- a/fw/sweep.h
+++ b/fw/sweep.h
@@ -29,7 +29,9 @@ struct sweep {
 
 extern volatile bool sweeping;
 
+uint32_t uptime_irq(void);
 uint32_t uptime(void);
+
 void sweep_image(const struct sweep *sweep);
 void sweep_init(void);