[sdiy] simple TTL programmable counter question (74LS163)

[Colin Hinz]

On Thu, 6 Nov 2003, Ken Stone wrote:

> >> ALSO did one with 161 or 163. The count direction is reversed (i.e. an up
> >> count, so it was pain, and I went back to the 193.
> >
> >The trouble with using '193 counters is that the counter re-load is
> >asynchronous, which means that instead of getting a precise "divide
> >by N" you get a "divide by [ N + Y ], where Y is the total propagation
> >time for the carry to cascade through all the counter chips and
> >through the one-shot (if present). If your load pulse is timed such
> >that it lands within the actual setup/hold window of the input
> >clock, you could find that Y will vary between cycles. You're
> >certainly going to get some jitter caused by variations of when the
> >load pulse takes effect, within a cycle. Scary! You sure you want this?
>
> Actually in real life, a little jitter is not a problem, considering all the
> effort we usually go to add it later - delay lines etc. However, I never
> noticed any such jitter in my prototypes. Also my monostable reload pulse is
> shorter than a single clock cycle for the very reason you mention - to kill
> off that offset. A second mono provides a longer pulse for externally
> conected equipment.

Ach, sorry, I should have qualified my remarks -- they shouldn't have
been about your original design per se, but with respect to the
implementation with an expected 100+ MHz clock rate.

The 50ns or so clock period in your circuit is boatloads of time for
the asynchronous logic to do its stuff, to be sure. As for the jitter
caused by the one-shots, it will be there, though perhaps not in a
quantity that's meaningful in the intended application. (My background
in graphics/video is perhaps getting in my way again -- in the
normal course of my work, 5 ns of clock jitter could be fatal.
I kinda miss working with the slow stuff sometimes. :=)

On a related note (and no apologies for asking this on-list, as
others may appreciate the info) are your PCB shipping prices for
Australia only, or worldwide?

- Colin Hinz
  Toronto, Canada