Monday, August 28, 2006


Cerberus emerges...

I grafted the second CPU on the XY controller board to handle 1 KHz PWM signals to run one of the plastic gearmotors. If I don't get some answers back from Microchip that give me some more elegant options the board will get another 16F628A CPU to run the other plastic gearmotor and we will have Cerberus. :-o

UPDATE: The bad news is that Microchip didn't have any good news for me.

The good news is that I've discovered that I have some three-post resonators rated at 4 and 10 MHz. I'm going to pull that 20 MHz crystal and try out the three-poster resonators and see if I can find a usuable PWM frequency. If I do I'm going to see if I can get the board running properly at the lower frequencies.

I hate losing the performance, but hey? What's the choice?

Would you still need the big PIC16F877A with this kind of setup?

If you're going to divy up the work among separate PIC16F628A chips anyway, seems like they'd have plenty of CPU, RAM, and Instruction ROM to handle your needs?
Ah, but a central CPU is still needed to keep the x and y axis motors on track when doing slanted lines and curves.
Why can't you just run one output port on the first axis PIC controller to an input port on the second axis PIC controller, and toggle it every 1/100th of a second? It seems at the rates you're currently moving, this would more than addequately keep the two synchronized.

This seems it was the original idea with RepRap, anyway, for synchronising the various motors. I'm not sure where the clock was generated there, though (perhaps at the comms controller?)
Why I don't is very simple. What you describe is more or less what the mainline RepRap developers are doing now. They don't know if that approach is robust yet. Nor do they know what the limits to that approach are.

What I'm doing is exploring another part of the space of potential solutions to controlling a cartesian reprap. If my way doesn't work and the mainline method does, I can always go back and use the mainline method.

OTOH, if the mainline approach fails for some reason or another we have the possibility of another way of doing the job that will already have been designed and tested that the mainline people can turn, too.

What this does is improve the overall potential for success of the RepRap project.
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