avr: added README.

avr/modules/network/README

@@ -0,0 +1,83 @@
+This is the inter-node communication protocol.
+
+Hardware
+========
+
+AqHome uses synchronous serial communication, nodes are connected via 4 lines:
+- 5V
+- GND
+- DATA (0V/3.3V)
+- ATTN (0V/3.3V)
+
+DATA and ATTN are only pulled low by nodes on the bus, pulled high only by a single node in the network via pull-up
+resistor. This allows for multi-master mode. Every node sends data at will (after checking for free line).
+In most cases the pull-up resistors are installed on the node with the 5V power supply.
+
+
+5V, GND
+-------
+Power line 5V, mcu nodes use a voltage regulator to locally provide stable 3.3V.
+
+
+DATA
+----
+Data line, normally high, bits are sampled on the rising edge.
+
+
+CLK
+----
+Clock line, normally high. Pulled down at the beginning of a transmission, held low for a short time to let
+other nodes on the network discover this state. After that data is clocked out on the DATA line at the rising edge
+of the CLK line.
+
+
+
+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 are not repeated on the lowlevel layer. However,
+if a message can't be transmitted because of a busy line 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. The last byte is a CRC8 checksum byte although this
+is for historical reasons not a real CRC8 checksum (due to an implementation error in early versions).
+
+Offset Length Meaning
+---------------------------------------------------------------
+     0      1 destination address
+     1      1 remaining message length (excluding the CRC byte)
+---------------------------------------------------------------
+     2      n payload data
+---------------------------------------------------------------
+     2+n    1 CRC 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 remaining payload data (depending on command code)
+----------------------------------------------------------------
+     4+n    1 CRC byte
+
+
+