Challenge 2 - MorseBit Protocol

This challenge implements a simple communication protocol that transmits data between 2 BBC Micro:bit devices, using one of the GPIO pins.

Contents

1. Overview
2. [Morse:ßit Protocol](##Morse:ßit Protocol)
3. Instructions
4. Prerequisites
5. Authors
6. Extras

Overview

This is an implementation of a custom single-wire protocol designed for the BBC Micro:bits. We'd hope it could be adapted to any single-wire platform though.

In a basic overview of the protocol a TX user inputs data via button presses in Morse Code and the RX user recieves this data. In-between the TX and RX users, the data is converted from button presses into indexes on our morse code lookup table (see Extras) and then encrypted using an XOR cipher. These indexes are then sent to the RX user in binary (uing HI/LO on wire), which the RX user recieves, decrypts and reads.

There are several intricacies in the timing that can be customised in MorseBit.h (basically things like timing for button presses).

Also check out the Extras to see how this protocol reads Morse Code.

Morse:ßit Protocol

The Morse:ßit Protocol is a simple, single-wire communication protocol that trasmits data between 2 devices. This implementation has done it using GPIO pins on BBC Micro:bit devices. It uses Morse Code and a Morse Code lookup table (see Extras) to tx/rx data from user to user.

MorseBit Messages

A MorseBit message is simply a series of MorseBit packets. The first packet in a MorseBit message will always be the number of packets in it's message and the XOR cipher key to decrypting the following packets.

MorseBit Package

A MorseBit packet is a simple 6-bit integer. It's only 6-bits because the lookup table has 63 indexes and 111111 = 63. This could be expanded in the future.

TX over MorseBit:

1. User inputs message via Morse Code presses (set GPIO pin HI whilst this is going on to signal incoming message to the RX user)
2. Morse code is then converted to the Morse Code character's corresponding indexes in the Morse Code lookup table Extras
3. These indexes are then encrypted using an XOR cipher. They key to this cipher wil lbe the number of indexes being encrypted
4. When the RX user is ready, the TX user sends the number of characters to be recieved followed by the indexes over the GPIO pin
   1. Convert the integer to be sent into binary
   2. Set the GPIO pin HI/LO for X ms (HI for 1, LO for 0) (Note: send MSB first)
   3. Then sleep for X ms
   4. Repeat until all packets have been sent

RX over MorseBit:

1. Wait for an incoming message signal to be detected
2. Listen for incoming MorseBit packets:
   1. Read the GPIO pin for X ms
   2. If the GPIO pin is HI push 1 onto the packet integer, else push 0 (Note: recieve MSB first)
   3. After 6 bits, convert to an integer
3. The first packet recieved should always be the number of packets to follow. This will also be used as the key for the XOR cipher if the packet is encrypted
4. Decrypt the packet using an XOR cipher. The key sohuld be the first packet recieved (message size)
5. You should now have indexes to the Morse Code lookup table Extras to convert to characters

Protocol Finite State Machine

Instructions

When micro:bit is flashed, RDY will scroll on the LED to indicate 'Ready'. It will stay in IDLE, waiting for input (either on pin or button). To enter TX mode

• HOLD Button A until a spinning animation plays (away from button A).
• Then release and TX will scroll across the display.
• After TX has scrolled off the screen, start inputting Morse Code using Button A.
• When finished, a TICK symbol will be shown.

To enter RX mode

• wait in IDLE until an incoming message signal is recieved on the GPIO pin
• When this happens a spinning animation will play (towards button A)
• Then RX with scroll across the screen and the device will start recieving data.
• Then a TICK will show to note that it's finished recieving data
• Then the message will scroll across the screen following by a small animation to show it's finished.

Do	Button/Action	Timing
DOT	PRESS Button A	500 ms
DASH	HOLD Button A	1500 ms
SPACE	wait	2000 ms
END	wait	3500 ms

Prerequisites

These instructions assume you're using yotta Set yotta target before building application. Then copy executable to micro:bit. Learn more

yt target bbc-microbit-classic-gcc
yt build

Executable .hex file in directory:

build/bbc-microbit-classic-gcc/source/

Installation

At MorseBit directory, simply build application and flash executable to micro:bit.

cp build/bbc-microbit-classic-gcc/source/morsebit-combined.hex /media/[username]/MICROBIT

Authors

• Alexander Collins
• WeiJun Chai

Extras

Morse Code Chart

** Note: PROSIGNS require spaces inbetween their letters (e.g. SOS goes from ...---... to .../---/...)**

Morse Code Lookup Table

The charts below shows the lookup table and tree structure of morse code. Timing of the dots and dashes are listed under Instructions section.

INDEX	VALUE	MORSE CODE
0
1
2	E	.-
3	T	-..
4	I	..
5	A	.-
6	N	-.
7	M	--
8	S	...
9	U	..-
10	R	.-.
11	W	.--
12	D	-..
13	K	-.-
14	G	--.
15	O	---
16	H	....
17	V	...-
18	F	..-.
19
20	L	.-..
21
22	P	.--.
23	J	.---
24	B	-...
25	X	-..-
26	C	-.-.
27	Y	-.--
28	Z	--..
29	Q	--.-
30
31
32	5	.....
33	4	....-
34
35	3	...--
36
37
38
39	2	..---
40
41
42	+	.-.-.
43
44
45
46
47	1	.----
48	6	-....
49	=	-...-
50	/	-..-.
51
52
53
54
55
56	7	--...
57
58
59
60	8	---..
61
62	9	----.
63	0	-----