Re: MCF5221x Clock Questions

Bob

The USB can be clocked from the system clock (controlled in USB_CTRL), 
so a 4MHz input multiplied up to 48MHz is possible. Ensure that the 
source is 2500ppm accurate to achieve the required precision for full 
speed USB. I think that the only disadvantage of this is that the system 
clock can not be used at maximum speed (up to 80MHz otherwise depending 
on the exact device used).
By clocking directly from the crystal a 48MHz crystal is needed, but 
this then doesn't restrict the PLL speed - it could also be reduced to 
save power in this case.

Regards

Mark Butcher

www.uTasker.com



>
> Hi,
> I am just switching into the Coldfire line and am using the MCF51QE32 and
> the MCF52213 in new designs.  The details for using a crystal or 
> resonator
> with the MCF51QE32 are detailed very nicely in the data sheet.  On the 
> other
> hand there is virtually no information in the MCF5221x data sheet or 
> reference
> manual about how to hook up a crystal or resonator.  I would like to 
> use a
> 4MHz resonator with this part and the PLL to boost this to 48MHz 
> internal.
>
> A second question about the MCF5221x parts regarding the 48MHz for the
> USB.  Do I have to use a 48MHz crystal for some reason or can I use the
> PLL output at 48MHz?  The diagram looks like I can use the PLL but my
> Arrow FAE for Coldfire said he was told I needed a 48MHz crystal.
>
> Thanks for any help in advance.
>
> -Bob
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> Robert Ritchey
> Quest Engineering & Development, Inc.
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