max operation frequency of a gate

Well, 1 / 2*Tpd would be closer.

I agree, and this depends dramatically on the circuitry within the gate. But in general, even for unloaded outputs, I'd say the maximum frequency is below that predicted from the formulas above.

Im not so much concerned about the shape of the waveform ...what im trying to figure out is whether the input sampling window for a gate is the same as the rise/fall time ..or is it lesser?

Gates don't sample, but flip flops do. They have a very small sample time indeed, compared to the propagation delay.

The positive delay can be different than the negative delay. Therefore, the max freq is 1/(Tpdp +Tpdn). This also assumes that the input is a square wave and not a ramp which it probably is.

You should also consider the impact of rise and fall times on waveform shape.

You can apply any frequency you want to the input- just be sure you do not exceed the min/max voltage levels.

I read in sci.electronics.design that Rich Grise wrote (in ) about 'max operation frequency of a gate', on Sun, 27 Feb 2005:

Yes, even I refrained from pointing out that you could shine a blue laser on the input pin. I decided that the OP didn't even know what question to ask, so I left it to others to enlighten, if possible.

The data sheet generally gives the answer ( for reliable operation ). Clock gates too fast continually and you're likely to overheat them ( like overclocking cpus ).

Graham

why is it that a flip flop has a sample time , but a gate doesnt???

Is it coz of the feedback used in a flip flop?

An edge-triggered flip-flop (like a 74LS74) will "read" the state of the D input _at the time of the positive clock transition_ - that is the "sample time".

A simple logic gate just passes the state of its inputs through to its output (doing whatever logic is required along the way). It does not "sample" the inputs at discrete times - it continuously examines them.

"Continuously", AKA Analog ;-)

...Jim Thompson

At the fringes, the FFs are analog too- if you want it to sample the input- in general you have to "set up" the input to a valid logic level before the clock edge, the clock rise or fall time may have to be shorter than some maximum, and there is, in general, a "hold time" following the clock edge.

Violate any of those (analog) timing constraints and the behavior is not guaranteed.

Best regards, Spehro Pefhany

You are probably going to have "complex" structures such as flip-flops in your system that will limit your fastest speed.

In ASIC designs I evaluate the performance of a new process by setting up a long ring oscillator in PSpice, with worst-case devices, supply voltages, and temperature.

Then I usually back off by a fudge factor of (typically) 4X as insurance ;-)

...Jim Thompson

Not quite true- a FF will read the state of the gate which conforms to its setup and hold times. It's worth noting that one or both of these can be negative- i.e. the time the state is actually 'read' could be slightly before or after the clock edge.

It's also worth noting that inputs that DON'T conform can produce odd effects, like a transient pulse at the output- I remember falling over this in one of my first designs, had to change to a 74S74 instead of LS!

Similarly, with logic gates, if it's not Schmitty, you can't say what the output will be if the Tr, Tf and state time are less than the propogation delay.

Sure. At the noise floor level :-)

...Jim Thompson

But electronics relies on the movement of discrete (countable) charge units (electrons), AKA digital ;-))

At some point isn't *everything*?

Is time granular?

Best regards, Spehro Pefhany