clocking multiple props

nutson

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 ???

Phil Pilgrim (PhiPi)

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