Alt-BEAM Archive

Message #13105



To: beam@sgiblab.sgi.com
From: "Ben Hitchcock" beh01@uow.edu.au
Date: Fri, 07 Apr 2000 17:09:35 +1000
Subject: Re: microcore question


Hi,

> Yes, yes, newbie question..go ahead and flame me :o)

the nice thing about this email list is that most of us are newbies! So ask
away!

> I made a microcore, with the 74c14 (or whatever the name) chip. I put in
> LEDs as the shown in the schematic (I forget which web site it was, but it
> was a hand-drawn schematic).
>
> When I put power to it, the LEDs just stay constantly lit. Not even using
> the shorting-switches does anything.

Try touching the input of one of the inverters (pin 1,3,5,9,11,13) to the
positive rail. This should kickstart it. Can you remember where you got
the schematic from? Is it just a capacitor connecting pins 2-3, 4-5, 6-8,
9-10, 11-12, 13-1 and a resistor to ground on pins 1,3,5,9,11,13 with a
couple of switches thrown in for good measure? Oh, and pin 14 is +Volts,
and pin 7 is 0 volts.

>
> Anybody know what some possible problems are with this symptom? I need some
> help, 'cause everything I've tried hasn't worked, and it's getting a bit
> discouraging!
>
>
>
> --another question is this: Does the 74c14 chip act as "analog" or as
> "digital"?? I had thought that chips were *always* digital, but since the
> big thing in BEAM is often making analog devices, I'm assuming my original
> assumption was incorrect...

How long is a piece of string? There really isn't much difference between
'digital' and 'analog' circuitry. As far as I'm concerned, digital circuits
are just analog circuits that are good at coping with noise. BEAMers use
digital chips all the time, but in ways that might appear to be analog in
nature. I say everything's analog.

> And if it's not too big a question, what exactly does a "Schmitt inverter"
> *DO*, anyway?

A schmitt inverter is an inverter with positive feedback. In english this
means that the transition from low to high is very fast, as is the
transition from high to low. It's the speed of the transition that makes a
microcore work - hence you need a schmitt trigger.

Another (related) thing about the schmitt trigger is that the voltage on the
input at which the output changes state isn't the same when you're going
from low to high as it is when going from high to low.

Allright, here's a better explanation:

Say we have a normal inverter. We give it a voltage on the input that
starts at 0 volts and ramps up to the supply voltage (call it 5 volts for
the sake of argument). At exactly 2.5 volts the output will switch from
high to low.

Now replace the normal inverter with a schmitt inverter, and we have a
different story. The output will change state at 3 volts or so.

Now ramp the voltage down from 5 volts to 0. The normal inverter will
switch at 2.5 volts, same as before, but the schmitt will switch at about 2
volts.

There is a region between 2 and 3 volts where the schmitt will stay in the
state that it was in previously. This means that a schmitt trigger is very
good at filtering out noise because a voltage of 2.5 volts with a noise of
0.1 volts will wreak havoc with a normal inverter, but a schmitt won't even
see the noise.

hth,

Ben



13106 Fri, 7 Apr 2000 00:18:17 -0700 Re: My head aint turning (beginner stupid q) Elmo
> I have seen this problem too.
>
-snip-
> 2) Checked my wiring again against my circuit wiring diagram and found a
> misplaced wire.
>
> Try both. You can never be 100% sure that your wiring is in the right
> place....unless you have only used one wire that is.
>
> Elmo
>
-snip-

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