[Noisebridge-discuss] getting that goddamned mill to work

[Jonathan Foote]

I don't think jitter is an issue. On the worst machine last night it
was less than 25 us. Mike's borrowed machine got it down to less than
5 (thanks for bringing that in, Mike!)

Seeing as how the actual stepper pulses are generated by 4 Mhz PICs, I
think we are, if anything running too fast.

But we made some progress, we got the spindle motor PWM working, and
found magic enable pins. We can enable the stepper feedback so they
will fight you if you try turning them out of position, and I'm pretty
sure we're giving them drive signals because we can hear them whine.
But they still don't move.

I think the final hurdle is generating true quadrature drive signals
rather than step + direction. This is an easy hardware hack (two
flip-flops and some logic should do it), or we could improve the
open-source EMC2 code to generate the signals we need. The hardware
solution strikes me as much easier than mucking about in the guts of
that Linux port driver, but I could be convinced otherwise, especially
if there's anyone else on the planet with a similar problem. (Is
anyone on the EMC2 list/forum/IRC that could ask?)

I think the next step is testing that hypothesis by feeding one axis
some true quadrature signals from hardware. I have some AVR code to do
just that.


OK, enough hacking, back to the drama.

-J


On Fri, Oct 2, 2009 at 10:13 AM, Andy Isaacson <adi at hexapodia.org> wrote:
> On Thu, Oct 01, 2009 at 11:56:29PM -0700, Jesse Welz wrote:
>> 2009/10/1 dpc <weasel at meer.net>:
>> >Michael Wright <mike at smallip.com> writes:
>> >
>> >> After running the Jitter-Test program on the computer I have the
>> >> sinking suspicion that the machine we're trying to use isn't good
>> >> enough to send step and direction commands.
>> >
>> >hmmm.... i never got around to schleping down the machine for the
>> >reversing stuff but just noticed it has a prallel port. how good is
>> >good enough?
>
>
> The problem is that some low-end computers from about 1998 to 2003 (and
> some up to the current day) use SMI mode (System Management Interrupt)
> to do stuff on the CPU without letting the OS know about it.  This means
> that at any moment your code may stop executing for up to a few hundred
> microseconds -- plenty long enough to totally hose a PWM or stepper
> motor pulse sequence.
>
> The motherboard I last saw being used for the mill was a Via Eden CPU,
> which is known to potentially have such issues.
>
> Most "real" AMD or Intel CPUs' motherboards don't have these issues,
> although it's not guaranteed (it depends on the BIOS and system vendor,
> and laptops are also known for having such problems), so check for a
> better mobo -- I know I saw a few in the E-Waste pile.
>
> -andy
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