Re: MCF5221x Clock Questions

Hi John

Rich T has used the USB driven from the relaxation oscillator (!!!) - 
see 
http://forums.freescale.com/freescale/board/message?board.id=CFCOMM&thread.id=4734
but as he also comments, the accuracy (and jitter (?)) is not within 
spec., so obviously not recommended....

Regards

Mark

http://www.uTasker.com

> Robert Ritchey wrote:
>> 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.
>
> Your Arrow FAE is correct, you must use a 48 MHz fundamental mode 
> crystal.
>
> The PLL jitter specification is too high to meet the 2500 ppm USB 
> full-speed clock jitter specification.
>
> We intend for the USB clock to be derived directly from the 48 MHz 
> crystal or from the USB_ALTCLK bypass input.
>
> USB will likely work when using the system clock, but whether it does 
> so consistently depends on how tolerant whatever you plug into 
> actually is.
>
> John Bodnar
> Systems Engineer
> Freescale Semiconductor, Inc.
> ---
>
>
>

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