Protocol Definition between IHU and TTX
=======================================


Bitstream
---------

IHU and TTX communicate using a simple serial synchronous bus (SDA,SCL).
The IHU is the master and drives the clock. On idle the SCL and SDA are both 
at logic 0. When the IHU want's to communicate it has to generate a wakeup pulse 
with a minimun length of 1µs. After this it has to wait at least 3ms for the TTX
to wake up and switch to a higher oscillator frequency. The data is transferred 
MSB first in groups of 8bits (1 Octet). SDA must be stable before the clock goes 
high. A Clock pulse must be at least 4µs long and the data has to stay valid for 
at least 17µs. After each octet there must be a break of 50µs.

     | wake up |               1st octet                        | break  |  2nd octet
               _________________               _________________
SDA           |                 |             |                 |
-----------//-+                 +---------//--+                 +----//------------
      _         ____              ____          ____                      ____
SCL  | |       |    |            |    |        |    |                    |    |
-----+ +---//--+    +------------+    +---//---+    +----------------//--+    +----
     
     |  3ms    | 4µs|     13µs   |                              |  50µs  |  


Frames
------

Every frame starts with 4 bit command code and a 4 bit length field. After this follow
len octets of data. When all the data has been transferred the master (IHU) switches
SDA to input and clocks in on byte as acknowledge. The aknowledge is a one byte XOR
checksum over all octets (including the command/len octet).

Field-length:    1           LEN           1
            +---------+----------------+--------+
Name        | CMD/LEN |      DATA      |   CS   |
            +---------+----------------+--------+

Commands
--------

 Hex '0'
 ~~~~~~~

 This tells the TTX to shut up. The TTX will send out one aknowledge over cw and after
 that disables any further broadcasts. (Safe mode)

 Hex '1'
 ~~~~~~~
 
 This is the reverse of the '0' command. The TTX will send out an aknowledge over cw
 and reenables broadcasting of beacons. 

 Hex '2'
 ~~~~~~~
 
 The command sets a new beacon interval. The data is one octet containing the new beacon 
 interval in seconds.

 Hex '3'
 ~~~~~~~

 This sets a new cw speed. It is followed by three octets representing the dot, dash
 and word space times.

 Hex 'A'
 ~~~~~~~ 
 
 This instructs the TTX to send out an aknowledge frame over cw. The command
 has 1 octet of data containing the ASCII Code of the aknowledge to send out.

 Hex 'B'
 ~~~~~~~

 This inscructs the TTX to send out a beacon immediately. It also resets the timer
 so that the next beacon will follow after the normal beacon period. This command has
 no data.

 Hex 'F'
 ~~~~~~~

 This transmits a new format string for the standard beacon. The data is a variable
 length string containing ASCII characters each standing for a beacon element to send
 out.