11/23 clock issue

Holm Tiffe holm at freibergnet.de
Sat Feb 7 13:30:10 CST 2015

Noel Chiappa wrote:

>     > From: Brent Hilpert
>     >> Well, try removing W1 to isolate the output of the xtal oscillator E15
>     >> from inverter input pin E4.4.
> Sigh, the crystal output is dead as a doornail. Total flat-line. Guess I'm
> going to have to find a new one...
> I actually think there's an open in there, because the input to the buffer
> inverter stays at +2V, whether the crystal is connected or not. If there was
> some sort of internal short in the crystal taking its output to ground, I'd
> have expected it to pull the inverter's input down.
>     > TP4 looks to be an 'input' test point, rather than an output. The
>     > pull-down resistor value (150 or 180 ohms) has been selected such that
>     > it is low enough to allow enough current to flow through the tri-state
>     > control input E4.1 to pull it low normally, but high enough to allow
>     > one to connect TP4 high, to enter tri-state
> Ah, got it. (I tended to assume test points were outputs, but I need to
> remember that they may be inputs.)
>     > What the point of going to tri-state is, is not clear, considering that
>     > W1 is there for an external clock. A conjecture is there may have been
>     > some external test fixture that tri-stated it for some sort of
>     > synchronous single-step clocking.
> Yeah, but couldn't they have lifted the W1 jumper, and fed their test clock in
> that way? Eh, not important.
>     > Could also check the V reading on the open E4.4 input.
> See above - +2V is a floating TTL input, ISTR?
>     > From: Holm Tiffe
>     > Hmm, may be since his hints are standard debugging technique and you
>     > aren't really familiar with debugging??
> I cheerfully admit to being primarily a software person. But I have been
> debugging broken hardware off and on for 30+ years - although not as a
> principal occupation, of course. I think it's more just that my mind does not
> do hardware intuitively (the more-so, the further one gets from the ideal -
> aka digital at the design level - to real hardware) - I have to think about
> it.

There is nothing wrong with your brain :-)

You aren't familiar with that kind of mater, that's all.

>     > You don't ned no pullup for +5. All open TTL inputs are reading High
>     > w/o any pullup.
> That's why all those boards use pullups on unused inputs that need to be 1,
> right? :-) But you're probably correct for a quick test.
>        Noel

Noel, I gave you a hint for the debugging of that circuit, not for
constructing reliable electronics with TTL circuits.
May be there is a difference?

Your answer is exactly what I've expect from one that don't really know
what he is exactly doing inside that electronics.
Take a look to the internal circuit of an 7400, you will find a Transistor
with multible emitters at the input. Inputs are drawing current if you
connect them to GND and excatly NONE if you connect them to VCC.

So if you still want to use smileys while answering me, tell me what do you
think that is happening inside the circuit if you connect the 5V to the

and not it is time for me to smile: :-))



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