[time-nuts] precision timing pulse

Thu Nov 17 23:21:07 UTC 2016

Tom describes the PIC as a 'poor man's FPGA'. And I'm aware of how they're
programmed - I agree they're a good choice for this sort of device.

However, even poor men can use FPGAs now. I'm following with interest the
open-source toolchain available for the Lattice ice40,
http://www.clifford.at/icestorm/

There are, of course, other cheap FPGAs and free-to-use vendor tools. I've
always found the vendor tools horrendous, with huge overheads and
complicated licensing, so something a bit more manageable is attractive.

On Thu, Nov 17, 2016 at 10:07 PM, Bob Camp <kb8tq at n1k.org> wrote:

> Hi
>
> There are a number of us on the list who code on ARM MCU’s. Doing the
> same thing on one of them is *not* a trivial undertaking. Making sure that
> it does what it should simply is not worth the effort. The PIC 12 is a low
> cost
> solution and has been extensively tested to show that it does what it
> should
> under a range of conditions ….
>
> Bob
>
>
>
> > On Nov 17, 2016, at 4:26 PM, Bryan _ <bpl521 at outlook.com> wrote:
> >
> > Tom:
> >
> >
> > As you were gracious to release the source code for these excellent
> little dividers, I would suspect someone who is somewhat fluent in assembly
> could just modify so it counts a specific number of pulses and then toggles
> a output off. The ARM could be used to restart etc. Although I think the OP
> wanted to know the time taken between a run of a number of cycles.
> >
> >
> > Been playing around with the PD15 to replace 2 -14 pin clock dividers
> and it seems to work like a charm, although I can't seem to find any
> information if the inputs on a 12F675 are Schmitt trigger inputs. Can't
> seem to find anything in the datasheet.
> >
> >
> > -=Bryan=-
> >
> >
> > ________________________________
> > From: time-nuts <time-nuts-bounces at febo.com> on behalf of Tom Van Baak
> <tvb at LeapSecond.com>
> > Sent: November 16, 2016 3:27 PM
> > To: Discussion of precise time and frequency measurement
> > Subject: Re: [time-nuts] precision timing pulse
> >
> > Chris & Chris,
> >
> > Yes the "TVB dividers" are instruction loops where every cycle is
> counted by hand, and then verified with a simulator, and then checked with
> a time interval counter. All the code is out there at
> www.leapsecond.com/pic/<http://www.leapsecond.com/pic/>
> >
> >
> > I use PIC's for historical and technical reasons. Modern CPU's and MCU's
> have so many layers of optimization that they are no longer deterministic
> at the cycle level. This makes them worthless for precise time
> applications. In addition many of them use clock edge or PLL tricks to
> increase speed, but this introduces jitter. Or they enable interrupts which
> kills precise timing. The PIC's are old-school, fully synchronous and use
> only one clock edge so the jitter is down at the picosecond level. My code
> is isochronous and doesn't use interrupts for timing. It acts like a poor
> man's FPGA.
> >
> > Arduino probably uses compiled code, external libraries, and interrupts
> so that also is a no-no for precise time. I'm not sure about bare metal
> AVR. I hope someone can do measurements on an AVR divider one day so we can
> compare PIC and AVR jitter. I worry about how AVR implements the clock. So
> this is an open question. But I know how PIC's work so that's what I use.
> BTW, these are "ancient" PIC 12F parts. It's possible modern 16- and 32-bit
> PIC's also go to the dark side and optimize for performance rather than
> picosecond determinism. Another open question.
> >
> > Realize that using a MCU for a frequency divider is a weird niche use of
> the chip, so none of what I say applies if you plan to use any of these as
> a general purpose computing system.
> >
> > Also, we don't know if OP is looking for microsecond, nanosecond, or
> picosecond precision in his timing pulses.
> >
> > /tvb
> >
> > Chris Albertson:
> >> I'm wondering why everyone seems to be assuming a PIC is the right
> >> processor.   Of course it could work for this but I'd select whatever is
> >> the easiest to program and debug.   Those tend to the they little single
> >> board development systems that sell for under $10.  They typically have
> USB
> >> connections to a computer and don't require any specialized equipment to
> >> program.   The Arduino is the prime example of these although I've
> moved to
> >> ARM based uP because they can be less expensive with an order of
> magnitude
> >> better performance and can use the same Arduino IDE and run Arduino
> >> sketches.    The PIC is not so beginner friendly and requires some study
> >> before it can be used.    But if you as said of course a PIC could work,
> >> this is a pretty simple application
> >
> > Chris Caudle:
> >> You could use AVR (arduino), but once you get to most of the ARM
> >> processors it is harder to make them deterministic because of caches.
> You
> >> want a processor where the number of clocks is constant for all or
> almost
> >> all instructions, and is easy enough that you can code the loops by
> hand.
> >> Code like TVB's divider probably doesn't even use interrupts, it's just
> a
> >> tight loop where the number of instructions is counted to give the
> >> behavior wanted.
> >
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