Jan-Erik,
You've cleared up one bit of confusion I had... I didn't understand how the
register size was the size of that register. It seems obvious now :\

I did mean 8 bit registers.

Thanks
Bill

On Fri, Mar 23, 2012 at 9:45 AM, Jan-Erik Soderholm <
jan-erik.soderholm@telia.com> wrote:

>
>
> Bill Stoddart wrote 2012-03-23 13:30:
> > Thanks to everyone who chimed in, it got me thinking through the proble=
m
> > better.
> >
> > I think I am doing the calculations correctly but some of the baud rate=
s
> > I'd chosen must have had too much of an error.
> >
> > I am able to run at 9600 with 8 bit, BRGH=3D0, BRG=3D78 and at 115200 w=
ith
> 8bit
> > BRGH=3D1, BRG=3D25.
> >
> > I tried to run 9600 with 8bit BRGH=3D1, BRG=3D311 with no luck.
> >
>
> I think you must be a bit more carefull with what you write.
>
> There is no "BRG" register at all in the 18F4550!
>
> There is a SPBRG register (in 8 bit baudrate mode) and a
> SPBRGH/SPBRG register pair (in 16 bit baudrate mode).
>
> Then there is control bit BRG16 that selects between 8 (default
> at POR) and 16 bit mode.
>
> I'm not sure what the "with 8bit" stands for.
> 8 bit characters or 8 bit baudrate register.
>
> Of course one can't set BRG=3D311 if in 8 bit baudrate register mode.
>
> Jan-Erik.
>
>
> > This is all still using 48MHz.
> >
> > Thanks again
> > Bill
> >
> > On Fri, Mar 23, 2012 at 7:26 AM,<alan.b.pearce@stfc.ac.uk>  wrote:
> >
> >>
> >>> A 'hexadecimal' crystal is more suitable for common baud rates, but
> >>> whether you can or do use one depends on the functional priorities. F=
or
> >>> example if a microsecond timer is more important than comms, then a
> >>> 20MHz would be preferable (for the 200ns cycles). You might have to
> >>> compromise somewhere
> >>>
> >>> An 18.432MHz crystal in the calculations
> >>>
> >>> ((18432000/115200)/16)-1
> >>>
> >>> =3D 9.000000, no error
> >>>
> >>
> >> There is also a 19.something that gives exact baud rates (19.66088? or
> >> something like that). The other option would be to 'overclock' slightl=
y
> at
> >> twice 12.288 which is also an exact baud rate frequency (hence it
> tended to
> >> be used as the standard frequency for 8080 CPUs).
> >>
> >>
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