Re: MCF5221x Clock Questions

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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