[time-nuts] 1pps signal measurement

[Gerhard Hoffmann]

Am 29.06.20 um 19:35 schrieb Mark Spencer:
> I am also wondering a bit about the possible impact of my 5370B's having a maximum trigger setting of approx 2.18 volts vis a vis the typical specs for 5 volt TTL signals that typically define a logic 1 as having a slightly higher voltage ?  Would enabling the 10:1 attenuator on the input to the 5370B make sense ?
TTL has nothing to do with 5V, other than that 5V is the intended supply
voltage. An input at less than 0.8V is guaranteed to be low, anything
higher than 2.4V is guaranteed high, the nominal switching level is
about 1V8. The difference between 1V8 and 0V8/2V4 is to provide
a minimum noise immunity.

It's different for CMOS. CMOS outputs are close to either VDD or GND,
and the inputs switch nominally at 1/2 VDD. That went on for a little
eternity, with CMOS being so slow that interoperability with CMOS
was not an advantage. So people used pullup resistors and were happy.

I still hear the voice of the professor saying "Ladies & Gents, now you
understand why CMOS will never be fast!". Oh, he was wrong!
Then the CMOS steamroller accelerated. Nobody escapes the CMOS 
steamroller, as they used to say.  The 74HC series was as fast as
74LS and switched at 1/2 VDD. The functions were like the TTL
series. Then there was the 74HCT series with a switching threshold
at 1V8, like TTL. That was implemented by massaging the width/length
ratio of the FETs and the intention was to get 74LS sockets.

Replacements were a no-brainer, only open inputs
would no longer default to "High". With CMOS, they have to be
definitely driven to a valid value, bad things happen if not.

74AC / 74ACT is like 74HC/HCT on steroids. Faster but essentially the same.

3.3V CMOS came later with a switching threshold at 1/2 * 3V3, so
it is TTL compatible by coincidence.  74LV, 74LVC... etc, depending on
who makes them. 74LVC may operate at less than 3V3, then the
switching thresholds are lower in proportion. These chips may not like
input voltages higher than their 2V5 or 3V3 supply, such as 5V.
Some families are designed to accept that abuse, some will die.
Caveat emptor.


To make it short, you have no problem with your counter.
The magic voltage for TTL is 1.8V.

Cheers, Gerhard