AT91SAM7S XTAL Selection

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Is there a "standard" crystal value for AT91SAM7S designs?

It seems that the boot program would expect something specific for proper
configuration of the PLL, serial, and USB devices.  However, I can't find
any guidance in the documentation.



Re: AT91SAM7S XTAL Selection
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If you use an 18.432 Mhz crystal, then the USB can be used to flash the part
at production.
If you use anything else, then the USB will not work, but the UART will.


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Ulf Samuelsson
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Re: AT91SAM7S XTAL Selection

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part

Thanks, Ulf.

The 18.432/48 ratio seems a bit wide for this PLL -- divide by 48, multiply
by 125?  This works OK with this part?





Re: AT91SAM7S XTAL Selection
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Download one of the USB examples...

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Best Regards,
Ulf Samuelsson
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Re: AT91SAM7S XTAL Selection
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If I use a 18.432 MHz crystal, then what values do I use for
the caps and resistors connected to PLLRC?

The datasheet contains very little information, but hints that
there is a "tool" available to calculate the values.  I searched
Atmel's website, and all I could find was an Excel spreadsheet
application.  I downloaded it, and selected the part I am using,
AT91SAM7256.  The first inputs it asked for was the "Input
Divider Ratio" and the "Loop Divider Ratio" with no explaination
for what these numbers mean, or what criteria I should use to
select them.  Furthermore, it appears to be impossible to use
the spreadsheet to select any values that result in an output
frequency lower than 88MHz.  Yet, according to the datasheet,
the max frequency for the chip is 55MHz.

The next number it asks for is the "PLL Frequency Bandwidth",
with no explaination of what that means.  The phrase "PLL
Frequency Bandwidth" occurs nowhere in the 519 page datasheet.
If I Google for "PLL Frequency Bandwidth" I get only one hit,
for a non-relevant webpage on a server in Argentina.

Sorry if I sound a little frustrated.

      -bob


Re: AT91SAM7S XTAL Selection
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Download the schematics from the atmel web page.
Cut
Paste
Done ;-)



--
Best Regards,
Ulf Samuelsson
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Re: AT91SAM7S XTAL Selection
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Hi Ulf,
Can you please provide a URL?  I went to www.atmel.com
and searched for "schematic AT91SAM7S", and some other
combinations, but didn't find anything useful.

Thanks,

    -bob


Re: AT91SAM7S XTAL Selection

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Why not use a 20Mhz xtal instead of a 18.432Mhz one?
18.432Mhz divided by 5 and multiplied by 26 gives 95.846MHz, whereas
20MHz divided by 5 multiplied by 24 gives exactly 96MHz, a better value
for USB

Is it just because 18.432xtals are cheaper than 20Mhz ones?

ted


Re: AT91SAM7S XTAL Selection
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18.432MHz divided by 48, multiplied by 125 gives 48MHz.

However, this would drive 384 KHz into the PLL, which is lower than the 1MHz
specified minimum in the datasheet.

BUT, the minimum output of the PLL is 80MHz and the data sheet talks about
the PLL generating a 48MHz clock in quite a few places.. so I'm wondering if
the PLL is a little wider than is specified.





Re: AT91SAM7S XTAL Selection
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No, because the BootROM assumes 18,432 Mhz.
The SAM-BA USB flashing wont work without it, and you have to use the UART.
--
Best Regards,
Ulf Samuelsson
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