RE: [CAD_CAM_EDM_DRO] Just How Accurate Is CNC?

Hi Les,

The PIC family divides the oscillator clock by 4 to get the
instruction cycle time. So a 20MHz PIC runs a 1/5MHz
instruction cycle or 200ns per instruction.

The new PICs (18F series) have a PLL multiplier (mostly to get
rid of EMI) so that you can use a 10MHz crystal which during
programming can be configured to be stepped up to 40MHz and then
that is applied to the processor and divided by 4 for an
instruction clock cycle of 100nS.

All but the branch instructions operate in one clock cycle.

John Dammeyer


Wireless CAN with the CANRF module now available.
http://www.autoartisans.com/products
Automation Artisans Inc.
Ph. 1 250 544 4950

> -----Original Message-----
> From: Les Newell [mailto:lesnewell@...]
> Sent: Tuesday, November 09, 2004 3:43 PM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: Re: [CAD_CAM_EDM_DRO] Just How Accurate Is CNC?
>
>
>
> Hi Chuck,
>
> I am afraid you have got your units slightly mixed up. At
> 20MHz the pic
> is executing each instruction in 50ns. We need a resolution
> of 0.000150 ns.
>
> Light is just too damn fast :-)
>
> Les
>
>
> Chuck Rice wrote:
>
> >
> > So 150 fs = 0.000150 ns
> >
> > A crystal 20 MHz PIC can run at 5,000,000 instructions per

second.

> > Doesn't that give a headway of 75 instructions per required
> resolution?
> > Perhaps my math is off, but if not, with a PIC chip, you can

do a

> > lot in 75 instructions. If you wanted to drop it a notch

tighter,

> > 15 fs would give you 7.5 instructions per second. That might

be

> > pushing it.
> >
> > I doubt that I have enough knowledge to do it, but I think

that

> > someone will be doing it soon. Too many of the pieces are

becoming

> > available at low price points.
> >
> > Seems, even if you wanted to laser engrave a scale on the
> bottom of a
> > table, you could read it much the way we read a micrometer.

You

> > could have a set of LEDs that read the course scale and

another set

> > matched the fine scale. The two readings together would give

you

> > a pretty fine resolution.
> >
>
>
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