Add serial test pattern arduino code

Code/SerialTestPattern/README.md

@@ -0,0 +1,7 @@
+
+This is a simple Arduino sketch that can be used to generate a
+serial test pattern (input to the m-bus master for instance).
+
+It will send one byte at the time, changing one bit compared
+to previous at a fixed interval.
+

Code/SerialTestPattern/SerialTestPattern.ino

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+#include <SoftwareSerial.h>
+
+#define RX_PIN 10
+#define TX_PIN 11
+
+SoftwareSerial mySerial(RX_PIN, TX_PIN);
+
+// https://en.wikipedia.org/wiki/Binary_Gray_sequence
+static unsigned int binary_to_gray(unsigned int num)
+{
+    return num ^ (num >> 1);
+}
+
+void setup() {
+	mySerial.begin(9600);
+}
+
+void loop() {
+	static unsigned int i = 0;
+	static unsigned int ch = 0;
+
+	// Change the character more seldom than each loop entry.
+	if (i++ > 20) {
+		i = 0;
+		// 127 => leave the last bit sent always zero.
+		if (++ch > 127) {
+			ch = 0;
+		}
+	}
+
+	mySerial.write(binary_to_gray(ch));
+	
+	// 9600 bps => 8 bit = 8/9600s ~ 0.8ms
+	delay(50);
+}
+