[time-nuts] Frequency counter questions

Orin Eman orin.eman at gmail.com
Tue Apr 25 01:24:32 EDT 2017

On Mon, Apr 24, 2017 at 5:16 PM, Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
> > On Apr 24, 2017, at 6:54 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
> >
> >
> > kb8tq at n1k.org said:
> >> You have a “gate” from the GPSDO and a “signal” from somewhere else. If
> you
> >> want the STM to do the whole thing, the “gate” pin needs to get the job
> done
> >> in  X +/-  1 cycles of the “signal” pin. Delay X (if it’s consistent)
> isn’t
> >> a problem. Having a
> >> +/- 1 cycle delay *is* a problem. The interrupt servicing structure in
> the
> >> MCU may or may not be able to hit things +/- 10 ns or even +/- 100 ns.
> >> Sometimes a “lower power” MCU with simple code is better at this than a
> >> multi core gizmo running a high level operating system.
> >
> > Some of the counter/timer hardware has another register where the
> hardware
> > will save a copy of the main counter when another signal happens.  If
> your
> > gate time is the time between two pulses rather than the width of a
> single
> > pulse, then all the software has to do is read that copy-register before
> the
> > next pulse happens.
> Works a bit better if the input is edge sensing rather than level sensing
> ….
> (copy on positive edge vs keep copying the whole time the input is high)

They are in my experience.  You get the choice of positive edge, negative
edge and if you're lucky, both.  I used the capture feature on PIC timers
back in the late 90s to capture automotive engine timing signals and log
engine timing etc., but that's another story.

But the big gotcha is that the external signal is going to be synchronized
to an internal clock, often by a couple of D flip-flops.  This introduces a
delay of two to three internal clocks (it can be three if the D flip-flop
setup or hold time is violated and the first flip-flop goes metastable).
It was looking for where/how the synchronization was done on the STM32
timers that led me to the application note (AN4776) that I posted earlier.

FWIW, it's equally entertaining to work out when a write to a pin that's
designated as output actually appears on the pin.  Immediately (subject to
propagation delays) never seems to be the answer for the ARM based chips
I've asked about.  The answers tend to be of the form mumble mumble
pipelining mumble mumble...


More information about the time-nuts mailing list