[Coco] What do you use for pull-up and pull-down TTL resistor values? (LS series) (coco related project but possibly OT)

[Joel Rees]

2019年4月23日(火) 4:55 James Ross <jrosslist at outlook.com>:

> Ed Snider wrote:
> > It’s always best to refer to the datasheet for a particular device if
> possible
>
> I am trying to learn how to do that! :)
>
> I am going to go back and re-read some (some rather lengthy) articles on
> pull-ups and pull-downs and try to understand the underlying reason they
> are needed and the formula and numbers they are plugging in.
>

Back when I read datasheets regularly, the manufacturer's datasheets
usually gave a nod to best practices for the device, based on the internal
design and the features the manufacturer was claiming.

If the manufacturers no longer tell things like this, we as an industry are
in serious danger. I hope 2nd source suppliers will at least refer you to
the specs from the original manufacturer.

I gather so far it has to do w/ limiting the current.  Using just enough
> that is needed for a stable circuit w/o using too much.
>

For inputs, right. There will be some dependency on the surrounding
circuitry, on things like frequency, impedance, operating voltage, and what
kind of current is being drawn on the outputs, but, again, the specsheet
should make some indication on that.

Talk about voltage levels is often a shorthand for talking about how the
internal circuitry is constructed.

CMOS had a much wider operating range than TTL, and you really didn't want
to mix the two.

HCMOS had a narrower, and lower operating range than CMOS, and could be
mixed with TTL, carefully.

A lot of things I've read on the internet fail to distinguish between HCMOS
and TTL.

Much of available TTL is no longer truly TTL, and has a broader range of
functionality. MHz circuits impedances and other parameters are pretty
well-defined, and so on. Rules of thumb work pretty well at MHz clock
rates.

If you plan to turn a circuit into a production design, at some point
you'll want an oscilloscope to see what the real results are. And you want
the specsheets from the manufacturer that tell you enough that you can tune
the resistor and capacitor choices to the components. (Circuit board trace
shape and length aren't usually critical at MHz rates, but do become
problematic somewhere around 100 MHz, etc.)