AT91SAM7S oscillator?

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I'm trying to figure out how the oscillator is working in the AT91SAM7S from
Atmel. The data sheet lists that an up to 20 Mhz crystal can be connected
and the summary says that the oscillator runs at up to 55 MHz - is the
crystal frequency multiplied by 2.75 or what?

The system I'm developing needs a 6.0MHz and a 5.12MHz oscillator - will it
be configure the PCK0-2 outputs to these frequencies while running the
controller at near maximum speed? I have read the data sheet of the
AT91SAM7S but I'll need some help on this, which is highly appreciated.

Thanks,
orc



Re: AT91SAM7S oscillator?
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It might be a good idea to read the section of the processor's
specification entitled "Clock Generator". Look at the description of PLL.

Re: AT91SAM7S oscillator?
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Thanks.
Yes, that's the section i read (twice actually :-) ) but still having
several qustions to it - the most important is the one I mentioned:
is it possible to have a 6.0MHz and a 5.12MHz oscillator output to other
devices like an external ADC'es etc. while running the processor at a speed
near it's max (55 MHz)? (I do not need to know how yet, because I'm only
designing the hardware and need to pass the design to our PCB-layouter for
layout very soon - just need to know if it is possible at all or if I need
to have additional X-oscillators for the ADC's etc.).

As far as i understand the crystal is the input to the Main Oscillator which
runs at the same frequency as the crystal. The Main oscillator frequency can
be divided by a factor 1 to 255 and the output from the divider goes to a
multiplier in the PLL that can be set to a factor 1 to 2047 which determines
the PLL frequency. The data sheet lists that the input frequency to the PLL
must be between 1 MHz and 32 MHz and that the output frequency must be
between 80 MHz and 220 MHz - how is that possible - this means that the
multiplier can only be used in the range 3 (3 * 26.7 MHz = 80 Mhz) to 220
(220 * 1 MHz = 220 MHz) and not 1 to 2047 ???? This is just an example of my
confusion.

Thank you for your help.

Best regards,
orc



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