Lpc2000

In the USB chapter we can read this:

The following registers are located in the AHB clock domain. The minimum 
AHB clock frequency should be 18 MHz. They can be accessed directly by 
the CPU. All registers are 32 bit wide and aligned in the word address 
boundaries.

If the USB is in the VPB range why does the AHB clock matter ?
Since the lowest xtal freq that can be used with USB is 12Mhz, then the 
minimum clock for the CPU is 24Mhz. is this correct ?

Mauricio

> Since the lowest xtal freq that can be used with USB is 12Mhz, then the
> minimum clock for the CPU is 24Mhz. is this correct ?

That's the way I read it.  I found things work fine running core at 12MHz,
although I'm running at 24MHz.  I wonder what the context is for the 18MHz
minimum is. 

Joel

> If the USB is in the VPB range why does the AHB clock matter ?
> Since the lowest xtal freq that can be used with USB is 12Mhz, then the
> minimum clock for the CPU is 24Mhz. is this correct ?
>
> Mauricio

Yes and no.

Yes: With a 12 MHz xtal the minimum CPU frequency is 24 MHz.
No:  You could use a 18 MHz xtal, then the CPU frequency is 18 MHz.

Rob

>
> Yes: With a 12 MHz xtal the minimum CPU frequency is 24 MHz.
> No:  You could use a 18 MHz xtal, then the CPU frequency is 18 MHz.


How do you get 48MHz on the USB PLL with an 18MHz crystal?

When using USB, PLL1 opposed to PLL0 has to be set for 48MHz.


Joel


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