Flip-Flop in LTSpice, set/reset assertion?

[Joerg]

On 2019-09-22 18:36, Gerhard Hoffmann wrote:

Am 22.09.19 um 16:21 schrieb Joerg:
On 2019-09-21 13:53, Gerhard Hoffmann wrote:

Old TTL didn't pull very high. 3.5V to 4V in some cases so noise
immunity was indeed worse, until CMOS came. That made things
cumbersome because you could not use OC structures to operate it from
external.

TTL levels have nothing to do with 5V. That is just the supply voltage.
The switching threshold is somewhere near 1V8, anything below 0V6 is
definitely low and anything above 2V4 is definitely high.

And sometimes the drive signal didn't quite get there. Or not all the
time. The threshold in TTL is lower though, but occasionally it wasn't
low enough. Unless you used 244 bus drivers this stuff just didn't have
any oomph.

CMOS has its threshold at 1/2 VCC, but the historic importance of
TTL required the extraneous 74HCT family with the lower TTL input
threshold, implemented by playing games with the width/length ratio
of the FETs.

That is one reason I was never much of a fan of HCT. In my youth I built
a lot of circuits with CD4000 logic because it didn't have such
problems. It had other problems but the main upside was a vastly lower
power consumption.

And no, modern digital design has nothing to do with deploying 74xxx.
You formulate your system in VHDL, Verilog or Matlab and that's it.
Nobody cares about flipflops, let alone their reset pin polarity.


Nobody? Really nobody? Way north in France, in the province of the
Gauloises ...

Deploying a few gates is not digital design.

That part fulfills logic funtions. If this and that happens at the same
time tug on an alert rail, otherwise not. Preferably while consuming
less than 1uA. What's not digital about that?

For example, right now I have to design a circuit for a device that
replaces a uC function because the uC can't be trusted to do the job
reliably enough. I think it could be made reliable but a client's wish
is a client's wish. It'll need several 74LVC chips.

Consultant's creed:
It's our policy to give the customer what he wants.
That is very strong medicine, and usually only required once.

Once is the goal, of course. After that I (and the client) expect this
to run for the next few decades.

--
Regards, Joerg

http://www.analogconsultants.com/