This is the inter-node communication protocol.

Hardware
========

AqHome uses asynchronous serial communication, nodes are connected via a 4 lines:
- 5V
- GND
- DATA (0V/3.3V)
- ATTN (0V/3.3V)

DATA and ATTN are only pulled low by nodes on the bus, never pulled high, this allows for multi-master
mode. Every node sends data at will.


5V, GND
-------
Power line 5V, mcu nodes use a voltage regulator to locally provide stable 3.3V.


DATA
----
TTL-UART communication line (3.3V, 19200 Baud, 1 startbit, 8 databits, one stopbit, no parity bit->19200N1).


ATTN
----
Whenever a node wants to send data it has to first check whether this line is HIGH. If it is, the sending
node must pull down this line to GND to inform other nodes about the intent to send. This is used by mcu nodes
as an interrupt source thus waking each node as soon as a message is to appear on the bus.
After sending data via the DATA line the sending node has to release this line which is then automatically pulled
HIGH by the logic on the power supply board.



Protocol
========

The protocol is simple to reduce overhead and to keep the code size inside the mcu nodes small.
There is no generic request-reply protocol. Messages a not repeated on the lowlevel layer. However,
if a message can't be transmitted it might get retried later by the sending node (e.g. when reporting
sensor data). Also, some operations consist of a request and a reply, but that's on a higher level (e.g. remotely setting a
specific value in a node). The maximum size of a message is 24 bytes (max. 21 bytes payload).


Basic Message Format
--------------------

Nodes communicate via messages. The first byte of every message is the destination node address, so every node can determine
after the first byte whether it is interested in the following message. All nodes but the destination can immediately go back
to sleep after receiving the first byte (they will be awoken again by ATTN going low for the next message).
The next byte is the length of the remainder of the message (if any). The last byte is a CRC8 checksum byte.

Offset Length Meaning
---------------------------------------------------------
     0      1 destination address
     1      1 remaining message length
---------------------------------------------------------
     2      n payload data
---------------------------------------------------------
     2+n    1 CRC8 byte


Command Messages
----------------

Command messages are an extension to the Basic Message Format specifying the first two bytes of the payload.
The first byte of the payload is a command code (e.g. "100" for report of sensor data), followed by the
bus address of the sender. The remainder of the payload is defined by the type of message(i.e. the message code).


Offset Length Meaning
---------------------------------------------------------
     0      1 destination address
     1      1 remaining message length
---------------------------------------------------------
     2      1 command code
     3      1 source address
---------------------------------------------------------
     4      n payload data (depending on command code)
---------------------------------------------------------
     4+n    1 CRC8 byte




