a little teaser

rjo__rjo__ Posts: 1,948
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: 5,125
    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.
    "Are we alone in the universe?"
    "Yes," said the Oracle.
    "So there's no other life out there?"
    "There is. They're alone too."

  • 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.
    "Are we alone in the universe?"
    "Yes," said the Oracle.
    "So there's no other life out there?"
    "There is. They're alone too."
  • 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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