Michael Rigby-Jones wrote:
> Looking at
> (e.g.) an (8,4) hamming code, an 8 bit code can represent 16 discrete
> values (same as manchester) but with the advantage of being able to
> correct 2 bit errors.

Not quite the same.  The UART sends an additional start bit and stop bit, so
you are sending 4 bits of data for each 10 slice times.  With manchester you
get 4 bits per 8 slice times.

> The downside is that 2 of the 16 resulting codes
> are totally unbalanced (i.e. 0x00 and 0xFF).  I could just ignore those
> codes, using the other 14 and only geting ~3.8 bits encoded per octet,
> and this would actualy be ok as 4 encoded bytes would hold a ~15bit
> value which is plenty for my application.  However, if there is a
> reasonably simple scheme that would allow the full range of codes to be
> used it would be usefull.

How about using the synchronous serial port to send the manchester data.
This gets you 1 free bit for every 4 compared to using the UART.  Then use
those extra bits separately for a CRC checksum or error correcting codes.  I
doubt you'd need an extra 25% more bits to reasonable protection, so this
probably takes less bit slice times and has better DC average over shorter
intervals, and is reasonably tolerant of clock inaccuracies.  There is a
reason manchester is used a lot in low cost RF data transmission.


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