diff --git a/tornado/fw/sim/Makefile b/tornado/fw/sim/Makefile
new file mode 100644
index 0000000..1cffd56
--- /dev/null
+++ b/tornado/fw/sim/Makefile
@@ -0,0 +1,4 @@
+CFLAGS = -g -Wall
+LDLIBS = -lm
+
+all:	alg
diff --git a/tornado/fw/sim/alg.c b/tornado/fw/sim/alg.c
new file mode 100755
index 0000000..dedb519
--- /dev/null
+++ b/tornado/fw/sim/alg.c
@@ -0,0 +1,71 @@
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <math.h>
+
+
+#define	G	28	/* 1 Earth gravity in accelerometer units */
+#define	MID	512	/* accelerometer middle value */
+
+#define	S	1000	/* sample rate (samples/s) */
+#define	F	2	/* rotational speed in Hz */
+#define	R	0.46	/* radius of accelerometer orbit */
+
+#define	H	(G/2)	/* hysteresis, in accelerometer units */
+
+
+/* noise pattern: frequency, amplitude pairs */
+
+static struct noise {
+	int f;	/* frequency (in samples) */
+	int a;	/* amplitude (in accelerometer units) */
+} noise[] = {
+	{ 1, G/3 },	/* general noise */
+	{ S/F/10, 5*G }, /* hits caused by the chain "jumping" */
+	{ 0, }
+};
+
+
+static uint16_t sample(double t)
+{
+	int fz = R*4*M_PI*M_PI*F*F/9.81*G;
+	int fg = cos(2*M_PI*F*t)*G;
+	int f = fz+fg;
+	const struct noise *n;
+
+	for (n = noise; n->f; n++)
+		if (!(random() % n->f))
+			f += random() % (2*n->a) - n->a;
+	return f+MID;
+}
+
+
+#define	E_SHIFT	3	/* ~ 0.1 */
+#define	M_SHIFT	10	/* ~ 1/S */
+
+int main(void)
+{
+	uint16_t e = MID << E_SHIFT;
+	uint32_t m = MID << M_SHIFT;
+	int d;
+	int i;
+	bool up = 0;
+
+	for (i = 0; i != 10*S; i++) {
+		unsigned v = sample((double) i/S);
+		e = v+(e-(e >> E_SHIFT));
+		m = v+(m-(m >> M_SHIFT));
+		d = (e >> E_SHIFT)-(m >> M_SHIFT); 
+		if (up) {
+			if (d < -H)
+				up = 0;
+		} else {
+			if (d > H)
+				up = 1;
+		}
+		printf("%d %d %d %d %d\n",
+		    v, e >> E_SHIFT, m >> M_SHIFT, d, up);
+	}
+	return 0;
+}
diff --git a/tornado/fw/sim/p b/tornado/fw/sim/p
new file mode 100755
index 0000000..0e2da1e
--- /dev/null
+++ b/tornado/fw/sim/p
@@ -0,0 +1,7 @@
+#!/bin/sh
+./alg >_out
+gnuplot -persist <<EOF
+set style data lines
+plot "_out" using 1, "_out" using 2, "_out" using 3, "_out" using 4, \
+  "_out" using (100*\$5)
+EOF