Re: [avr-gcc-list] howto: accurate fast usart with IntRC clock andOSCCAL

[Graham Davies]

David McNab wrote (in part):

> I've dropped this whole notion, and switched to external crystals.

Good idea (although none of this belongs on this list).

> Seems one of my suppliers offers a 13.56MHz crystal. With UBRR set to 7,
> this allows for 115.2kb/s serial with < 4.2% error - very much good 
> enough.

Actually, this is only just good enough.  I would recommend you try to get 
within 2%.  If you assume perfect transmitter timing and no distortion 
during transmission, then after detecting the onset of the start bit the 
receiver has to reach the stop bit, ten bits later, with no more than half a 
bit error.  This requires 5% accuracy in timing.  By the time you allow for 
transmitter timing error, distortion during transmission and the delay in 
detecting the start bit you need better receiver timing accuracy to make 
sure things hold together.  I seem to recall that 2% is recommended, but 
4.2% is surely right on the edge.  Can you not get 14.7456MHz?  Both Mouser 
and Digi-Key carry many choices at this frequency.

Graham.