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

[Michael Buchstaller]

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