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Can I use 8Mhz instead of 5 for an external oscillator? — Parallax Forums

Can I use 8Mhz instead of 5 for an external oscillator?

tad4shitad4shi Posts: 3
in Propeller 1
What's up y'all,

I'm trying to make a Propeller 1 circuit on a perfboard, and I only have 8mhz crystals. Does it matter if I use an 8mhz crystal as the external oscillator instead of 5? I see everyone using 5mhz but I can't find the reason why. I read somewhere that anything between 4mhz and 10mhz works.

Would my propeller go faster with an 8mhz crystal? Can I set the frequency multiplier to x10 instead of x16 to manage 80mhz?

Thanks!

Comments

  • Wuerfel_21Wuerfel_21 Posts: 5,107
    Yes, the manual says that crystals from 4 to 60 MHz are supported (with different _CLKMODE loading settings).

    The reason most people use 5 MHz crystals is that 5x16 = 80 MHz is the official max frequency across the entire rated temperature range. (6 MHz for 96 MHz and 6.5 MHz for 104 MHz are also fairly common and will work at any reasonable temperature)

    The only available multipliers are 1x, 2x, 4x, 8x and 16x, so with 8 MHz XTAL you can get either 64 MHz or 128 MHz core clock. The former is a bit slower than usual and the latter would be a pretty big overclock, which may or may not work reliably.
  • Capt. QuirkCapt. Quirk Posts: 872
    edited 2020-03-30 22:01
    tad4shi wrote: »
    Can I set the frequency multiplier to x10 instead of x16 to manage 80mhz?

    Unfortunately PLL10x doesn't exist. XTAL1 + PLL8X


    Don't forget the 6.25 mhz crystal available from Parallax
    for 100mhz (10ns) using the 16x multiplier.


    Bill M.
  • Wuerfel_21Wuerfel_21 Posts: 5,107
    It is also notable that a too fast clock from what I've heard generally won't cause damage to your chip - it just won't work (reliably).
  • RaymanRayman Posts: 14,768
    edited 2020-03-30 19:52
    You can actually run one cog at 160 MHz.
    But, it locks up if you try to start another...

    Some of the very first P1 boards had 10 MHz crystals.
    But, I think we mostly switched to 5 MHz to avoid issues with wrong setting...
  • Wuerfel_21Wuerfel_21 Posts: 5,107
    Rayman wrote: »
    But, I think we mostly switched to 5 MHz to avoid issues with wrong setting...

    I think the bigger issue was that the 10 MHz crystals were/are less precise or something to that extent.
    When I find old Spin code that has _CLKMODE = XTAL2 + PLL8X, there's usually a comment telling you to tweak _XINFREQ if the timing is off (manifesting in monochrome video output)
  • yetiyeti Posts: 818
    edited 2020-03-30 21:41
    If you have a FT232R, ENC28J60 or other hardware at hand that has a clock output in a reasonable range, you can use them with the XINPUT setting.

    A while ago I used an ENC28J60 module as 6.25MHz source for a propeller on a breadboard without any problems.
  • Cluso99Cluso99 Posts: 18,069
    edited 2020-03-30 22:58
    You will need to use PLLx8 which then runs at 64MHz. You will not be able to get reliable operation at 128MHz!!!

    There is a limitation governing the max xtal range because the xtal is multiplied x16 first - ie PLLx8 is actually x16 then /2.

    The max reliable overclocking was found to be 15MHz xtal and PLLx8 giving 120MHz using the DIP40 prop and proper decoupling. The PLCC44 doesn't seem to work reliably above about 14.3MHz and PLLx8 or 7.3MHz and PLLx16.
    IIRC Chip said the expected PLL ceiling was around 7.3MHz *16 which is what we found in practice.
  • MAElektronikMAElektronik Posts: 96
    I use a small 10MHz xtal. I switched to 10MHz cause I wanted a small surface munted device. It works very well.
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