diff --git a/tornado/fw/Makefile b/tornado/fw/Makefile
index 20ad483..2a02cdb 100644
--- a/tornado/fw/Makefile
+++ b/tornado/fw/Makefile
@@ -31,7 +31,7 @@ OBJCOPY = $(AVR_PREFIX)objcopy
 #OBJDUMP = $(AVR_PREFIX)objdump
 SIZE = $(AVR_PREFIX)size
 
-OBJS = $(NAME).o led.o $(COMMON_OBJS)
+OBJS = $(NAME).o accel.o led.o $(COMMON_OBJS)
 BOOT_OBJS = boot.o $(COMMON_OBJS)
 COMMON_OBJS =
 
diff --git a/tornado/fw/accel.c b/tornado/fw/accel.c
new file mode 100644
index 0000000..2a75644
--- /dev/null
+++ b/tornado/fw/accel.c
@@ -0,0 +1,109 @@
+/*
+ * fw/accel.c - Acceleration sensor
+ *
+ * Written 2012 by Werner Almesberger
+ * Copyright 2012 Werner Almesberger
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ */
+
+
+#include <stddef.h>
+#include <stdbool.h>
+#include <stdint.h>
+
+#include <avr/io.h>
+#include <avr/interrupt.h>
+#define F_CPU   8000000UL
+#include <util/delay.h>
+
+#include "io.h"
+#include "accel.h"
+
+
+void (*sample)(uint16_t x, uint16_t y) = NULL;
+
+
+static bool chan_x;
+
+
+static inline void admux(bool x)
+{
+	ADMUX =
+	    1 << REFS0 |	/* Vref is AVcc */
+	    (x ? ADC_X : ADC_Y);
+}
+
+
+static inline void adcsra(bool start)
+{
+	/*
+	 * The ADC needs to run at clkADC <= 200 kHz for full resolution.
+	 * At clkADC = 125 kHz, a conversion takes about 110 us.
+	 */
+	ADCSRA =
+	    1 << ADEN |		/* enable ADC */
+	    (start ? 1 << ADSC : 0) |
+	    1 << ADIE |		/* enable ADC interrupts */
+	    6;			/* clkADC = clk/64 -> 125 kHz */
+}
+
+
+uint16_t measure_ref(void)
+{
+	while (ADCSRA & (1 << ADSC));
+	adcsra(0);
+	ADMUX =
+	    1 << REFS0 |	/* Vref is AVcc */
+	    14;			/* Vbg (1.1 V) */
+	_delay_us(100);
+	adcsra(1);
+	while (ADCSRA & (1 << ADSC));
+	return ADC;
+}
+
+
+ISR(ADC_vect)
+{
+	static uint16_t last_x;
+	uint16_t v;
+
+	v = ADC;
+
+	if (chan_x) {
+		last_x = v;
+		chan_x = 0;
+		admux(0);
+		adcsra(1);
+	} else {
+		sample(last_x, v);
+	}
+}
+
+
+ISR(TIMER0_OVF_vect)
+{
+	chan_x = 1;
+	admux(1);
+	adcsra(1);
+}
+
+
+void accel_start(void)
+{
+	adcsra(0);
+
+	TCNT0 = 0;
+	OCR0A = 62;		/* 8 MHz/64/62 = 2.02 kHz */
+	TCCR0A =
+	    1 << WGM01 |	/* WG Mode 7 (Fast PWM to OCR0A) */
+	    1 << WGM00;
+	TCCR0B =
+	    1 << WGM02 |	/* WG Mode 7, continued */
+	    1 << CS01 |		/* clkIO/64 */
+	    1 << CS00;
+	TIMSK0 = 1 << TOIE0;	/* interrupt on overflow */
+}
diff --git a/tornado/fw/accel.h b/tornado/fw/accel.h
new file mode 100644
index 0000000..2791352
--- /dev/null
+++ b/tornado/fw/accel.h
@@ -0,0 +1,26 @@
+/*
+ * fw/accel.h - Acceleration sensor
+ *
+ * Written 2012 by Werner Almesberger
+ * Copyright 2012 Werner Almesberger
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ */
+
+
+#ifndef ACCEL_H
+#define ACCEL_H
+
+#include <stdint.h>
+
+
+extern void (*sample)(uint16_t x, uint16_t y);
+
+
+uint16_t measure_ref(void);
+void accel_start(void);
+
+#endif /* !ACCEL_H */