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a little teaser

rjo__rjo__ Posts: 1,940
edited November 2017 in Propeller 2 Vote Up0Vote Down
If you have been following Rayman's thread you know that we have a teeny tiny technical issue... sure we could just use the right crystal to generate 24MHz... but what fun is that?

Let's say we want to cover the spectrum from 8 to 48MHz with integer steps, how many and which crystals do we need in our bank of switchable crystal?

I'm not sure I could solve this problem. Hell, I'm not sure I can even define it correctlhy... jitter? What is this "jitter" you guys keep talking about. Does it go well will scotch?



  • 5 Comments sorted by Date Added Votes
  • evanhevanh Posts: 4,875
    edited November 2017 Vote Up0Vote Down
    ​NCO​ ​frequency mode will be okay with much lower target frequencies where the dynamic range isn't being stretched. EDIT: And PWM mode works here too.

    But for MHz ranges the answer is PLL, or DLL. I don't have any experience with it but the Prop1 has a PLL in each Cog counter I think. Phil advocated for this to be included in the Prop2 as well if I'm not mistaken.
    “What car did you learn to drive stick shift on?” == “What was the make and model of your first car?” -> Password reset! --

  • Too late for that... or Ken is going to kill someone:)
    AND I'm not (at all) in favor of Chip trying to solve this in Verilog just to make the five of us happy:)

    On the P1, we just switch out the crystal to the one with the sweet spot in it.

    I am just assuming that we are going to be doing that for the P2... EXCEPT that for testing purposes, I don't want to be switching between boards or switching crystals.
    Ergo, a little board with switchable crystals... but which ones? And how many?

    Sounds like a good product for someone else to make.

    If I'm not understanding you, you have to tell me:)



  • Well, a little board with a programmable PLL would be a way. There will be ICs on the market for this very purpose.
    “What car did you learn to drive stick shift on?” == “What was the make and model of your first car?” -> Password reset! --
  • You don't even need a programmable PLL, just an analog-tunable oscillator. The P2 can provide the analog output, to be fed back, wherein it's kept adjusted via a DPLL program.

    But a PLL/DPLL option may not help if the 24 MHz clock edges have to align somehow with the system clock for pixel read-in from the video chip.

    “Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away. -Antoine de Saint-Exupery
  • :) get to work.
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