Odp: [sdiy] Frequecy Divisor Question (TTL pulse stretcher needed)

[Roman Sowa]

use 5ns carry pulse to set RS toggle, and one of the outputs
of first counter to reset it. So if you use QD, the pulse would be
16x5ns=80ns.
For RS use 74F02 (is that quad NOR?)

Roman

----- Original Message -----
From: Michael Buchstaller <buchi at takeonetech.de>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Saturday, November 01, 2003 1:40 AM
Subject: [sdiy] Frequecy Divisor Question (TTL pulse stretcher needed)


> Hallo friends,
>
> my Journey with the Oscillator continues...
> After my fruitless efforts with the 74LS193, i built Ken Stones
> 74LS193 based design, which worked from the 1st try.
>
> Now, i want to use a fast clock to gain higher accuracy.
> So i decided to use 74F193 counters, which are (according to
> the data sheet) good for up to 133 MHz.
>
> I cannot directly measure here, because my 20 MHz scope
> does not trigger such fast signals. But when i use one of the counters
> free-running, i can see the outputs work as expected (QD: 6.25 MHz,
> QC: 12.5 MHz, QB: 25 MHz (barely visible), QD: no trace, but will be 50 MHz)
> So, the counter chips do work.
>
> The carry signal from the chip is only 1 clock cycle long - that gives a pulse
> of 5 ns width (assuming a 50% duty cycle from the oscillator).
> It popagates properly over my board; even the last counter gets it and reacts
> correctly.
>
> But there is the monostable that is fed by the last counter´s carry bit. It is
used to stretch
> the pulse duration, so all counters will be reliably reloaded with their
respective values.
> If i insert a 25 MHz or slower oscillator, everything works fine. 33 MHz and
faster does not
> reload the counters. No output pulse from the 74LS123 monostable. I tried
several brands,
> and some of the ´123s seem to work with 33 MHz.
> So, i assume the pulse is too short for the monostable to fire. 20 ns with the
25 MHz oscillator
> does work, but if i can get the signal to stretch to 20 ns, i could eliminate
the 123 completely and
> feed the LOAD input of the counters directly with that, correct ?
> I tried to simply use the carry output for that (directly and inverted), but
this does not work.
>
> So, how do i stretch the 5 ns pulse to (say) 20 ns ?
>
>
> -Michael Buchstaller
>