Logic families

Jorge Nuno · Post by **Jorge Nuno** » Sat Oct 06, 2007 6:34 pm

What is the fastest logic families you know?

I am trying to make a oscilator with a crystal and a gate of these families:
74LS;
74F;
74HC;
74HCT;
but the output overshoots like crazy, the rise/fall times are bad, and using coils or caps will worsen the rise and fall times, though the shoots are weaken...

Crap, I wanna uber-over-clock the 68000 but it has to be a square wave, not $hity...

What family of logic to use besides the ones I listed? (something that can handle frequencies to 20MHz approx.)

TmEE co.(TM) · Post by **TmEE co.(TM)** » Sat Oct 06, 2007 6:41 pm

AC, ACT ?

Shiru · Post by **Shiru** » Sat Oct 06, 2007 6:44 pm

You'd got answer faster if you ask google. Check here for example. HC, HCT, LS must work fine on 20 MHz. You can also use generator instead of oscillator.

Jorge Nuno · Post by **Jorge Nuno** » Sat Oct 06, 2007 6:47 pm

Ok... I will give them a chance... but I don't have any AC now

I'm going to look into some AC quadruple nand sheets...

Well it seems mine don't go well with +10MHz.

generator?? your meaning metal can pre-made osc? I've only saw oscilators that outputs a sine-wave (like my MD osc does too...)

EDIT: seems that a 74AC00 is great for what I need, with a max. Tpd of 9ns, typical is 5.5ns.

TmEE co.(TM) · Post by **TmEE co.(TM)** » Sat Oct 06, 2007 7:15 pm

68K has a schmitt triger on CLK pin... so distorted signals aren't too much of a problem.... I guess...

Jorge Nuno · Post by **Jorge Nuno** » Sat Oct 06, 2007 7:22 pm

Some of them are, specially the signals that have harmonics of high amplitude and frequency superimposed on them (the ones I were getting), and because it's an out of spec working condition I want the cleanest clock I can get.

The clock I was getting out of the gate was like this:

http://www.newport.com/images/webclickt ... 233658.gif

TmEE co.(TM) · Post by **TmEE co.(TM)** » Sat Oct 06, 2007 7:25 pm

Why not use a pulse generator ?

Jorge Nuno · Post by **Jorge Nuno** » Sat Oct 06, 2007 7:39 pm

Pulse generator? what is that? I only know the 4-pin metal oscilators and the 555 timer IC..., besides the oscilators that can be made with discrete transistors.

You are not meaning the function generators are you? those big, heavy and expensive equipment.

I want something really small and a crystal+74AC00 seems to be a excellent/cheap choice, chaper than those 4-pin metal ones.

TmEE co.(TM) · Post by **TmEE co.(TM)** » Sat Oct 06, 2007 7:51 pm

I cut off the can of a metal can osc, and put my own crystal into its own crystals place... works well

Jorge Nuno · Post by **Jorge Nuno** » Sat Oct 06, 2007 8:01 pm

What!? can you please elaborate on that?

TmEE co.(TM) · Post by **TmEE co.(TM)** » Sat Oct 06, 2007 8:04 pm

I took a metal can oscillator, went to garage, turned on the knife sharpener, and carefully filed off the edges of the can, then I jacked off the top, broke the crystal disc, soldered my own crystal in place and voila !!!

A pic of one in action :

Jorge Nuno · Post by **Jorge Nuno** » Sat Oct 06, 2007 8:15 pm

AHhhhh!! now I understood, LOL but the 74AC00 + crystal is even cheaper, HEHE

Nahhh, I'm not risking an oscillator for this, I only own 2: a 53.2304MHz from a broken MD2 (315-5660 was ripped off) and a 50.0MHz from a network pci card

Now for the experts there is the variable frequency oscillator controlled by voltage (read varicap), you loose precision but who needs precise clock on the cpu? The faster the better.

TmEE co.(TM) · Post by **TmEE co.(TM)** » Sat Oct 06, 2007 8:20 pm

How the 74AC00 + Crystal thing works (schematic please) ?

Jorge Nuno · Post by **Jorge Nuno** » Sat Oct 06, 2007 8:31 pm

Easiest thing. You only need an inverter, 2 caps with a few picofarads (10pF ~ 33pF) and a 1MEG resistor and the crystal.

http://www.jmargolin.com/project2/image036.jpg

It can be made with a transistor too but the circuit is different (very simple too), bu it needs to switch very fast.

If you want to know the concept behind it I can explain it...

TmEE co.(TM) · Post by **TmEE co.(TM)** » Sat Oct 06, 2007 8:36 pm

I figured out how it works, I've build similiar things, but these used big cap instead of crystal... for very low freqs... a door buzzer