clocking multiple props

nutsonnutson Posts: 237
edited July 2006 in Propeller 1 Vote Up0Vote Down
I have made a setup with a demo board·driving a second·chip·pins P30, P31 and RESn, to try out·the PropellorLoader object, and experiment with prop2prop communication. Comes in the clocking problem, I want one clock for both·props.
1 Can·I drive the second prop XI from the demoboard XI, running the crystal oscillator? I looked at the signal,·it resembles a 2Vp-p sine wave. What kind of buffering would be required?
2 Can·I use·an external oscillator pack, and drive both·props XI inputs from that,·without removing the crystal on the demoboard?
3 ???

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  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 20,594
    edited July 2006 Vote Up0Vote Down
    I doubt that the oscillator signal at XI has a low enough impedance to drive much of anything external. XO might, but it's not really designed for that. It might be better to dedicate a port pin on the chip with the crystal to output a clock for the other chip. This could easily be done using either CTRA or CTRB from a cog that's not using it. It can be either a Spin or assembly cog, since no software intervention is required once the counter is set up.

    The "slave" Propeller would receive this clock on XI. A series resistor may be needed to keep the signal at XI clean, without overshoots. Its _clkmode setting should include the directive XINPUT instead of XTAL1 or the like. This reduces the capacitance on XI to a minimum. It should be possible to include a PLLx directive as well, but the documentation is a little vague on what XINPUT + PLL16, for example, might compile to. (XINPUT appears to affect both the OSCM and CLKSEL fields.) If it can be set up to use the PLL, make sure the clock frequency coming from the "master" chip is between 4 and 8MHz.

    (Warning: This reply contains many assumptions and a lot of supposition. I'm sure one of the Parallax folks could provide a better answer, grounded in some actual experience with this matter.)

    -Phil
    “Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away.

    -Antoine de Saint-Exupery

    We had many discussions at the galley table and there had been many honest attempts to understand each other's thinking. There are several kinds of reception possible. There is the mind which lies in wait with traps for flaws, so set that it may miss, though not grasping it, a soundness. There is a second which is not reception at all, but blind flight because of laziness, or because some pattern is disturbed by the processes of the discussion. The best reception of all is that which is easy and relaxed, which says in effect, "Let me absorb this thing. Let me try to understand it without private barriers. When I have understood what you are saying, only then will I subject it to my own scrutiny and my own criticism."  This is the finest of all critical approaches and the rarest.

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    -John Steinbeck, The Log from the Sea of Cortez
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