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AT91SAM7S XTAL Selection

Discussion in 'Embedded' started by Ian Mcbride, Oct 3, 2005.

  1. Ian Mcbride

    Ian Mcbride Guest

    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.
    Ian Mcbride, Oct 3, 2005
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  2. 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.
    Ulf Samuelsson, Oct 3, 2005
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  3. Ian Mcbride

    Ian Mcbride Guest

    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?
    Ian Mcbride, Oct 4, 2005
  4. If you use an 18.432 Mhz crystal, then the USB can be used to flash the

    Download one of the USB examples...
    Ulf Samuelsson, Oct 4, 2005
  5. Ian Mcbride

    rob Guest

    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.

    rob, Oct 5, 2005
  6. Ian Mcbride

    edaudio2000 Guest

    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?

    edaudio2000, Oct 5, 2005
  7. Ian Mcbride

    Ian Mcbride Guest

    Why not use a 20Mhz xtal instead of a 18.432Mhz one?
    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.
    Ian Mcbride, Oct 6, 2005
  8. Download the schematics from the atmel web page.
    Done ;-)
    Ulf Samuelsson, Oct 6, 2005

  9. No, because the BootROM assumes 18,432 Mhz.
    The SAM-BA USB flashing wont work without it, and you have to use the UART.
    Ulf Samuelsson, Oct 6, 2005
  10. Ian Mcbride

    rob Guest

    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.


    rob, Oct 6, 2005
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