Em 22/3/2012 20:44, IVP escreveu:
>> I have been shooting for 115200 with out any luck
> Hi Bill,
>
> Section 20.1 of DS39623B outlines how to select a baud rate
>
> (it's been a little while since I did this, apologies for any errors)
>
> BRGH =3D 0
>
> ((Fosc/baud rate)/64)-1
>
> ((20000000/9600)/64)-1
>
> BRG =3D 31.55
>
> ((20000000/115200)/64)-1
>
> BRG =3D 1.71
>
> With BRGH =3D 1
>
> ((20000000/115200)/16)-1
>
> BRG =3D 9.85 (call it 10)
>
> The actual baud rate is
>
> 20000000/(16(10+1))
>
> =3D 113636
>
> with an error of (113636 -115200)/115200 =3D 1.36% slow. This would
> probably not be a problem if each frame is synched. For long streams
> with no reference the receiver will drift away from the incoming data
>
> A 'hexadecimal' crystal is more suitable for common baud rates, but
> whether you can or do use one depends on the functional priorities. For
> 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
>
> Joe


The problem is that the USB peripheral needs a 48MHz clock to operate.
The main clock is constrained to values that after being multiplied by
the PLL and divided by the pre/postscalers can result in an USB clock of
48MHz.


Isaac

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