I'm trying to figure out what's the difference between Fmax and Fclock on an example of 74HC74 D flip-flops. I have two datasheets, one from NXP / Philips and another from TI.

The NXP one is pretty straightforward. It mentions only Fmax (no Fclock), and names it "maximum clock frequency". So my understanding is that it's an estimate of the maximum frequency of the signal on the CLK (a.k.a. CP) port of the flip-flop. My experiments confirmed this. This circuit stoped working properly when the frequency of CLK signal exceeded 59 MHz, while NXP promised a typical Fmax around 69 MHz, depending on Vcc, temperature, and the concrete chip.

The TI datasheet on the other hand has two parameters --- Fmax (for which they give no name or description) and Fclock ("clock frequency"). They say that Fclock can be in 0 Hz - 29 Mhz range and that the minimum value of Fmax is 29 Mhz.

So it appears that min(Fclock) and max(Fclock) are nothing more than 0 and min(Fmax). But in this case, there is no reason to specify Fclock in the datasheet, since I already know it from Fmax. So I'm a little confused. Is Fclock really what I think it is, or it's something else?


1 Answer 1


In the TI datasheet, 'switching charactersitics' table, fmax has a guarranteed minimum value of 31 MHz at 4.5 V, with a typical of 50 MHz. You can expect the part to work to 31 MHz (under conditions of loading, risetimes etc that they also specify) and it may work to 50 MHz and beyond.

In the TI datasheet 'timing requirements' table, tsu and th are defined when measured over a fclock of 0 to 31 MHz. In other words, the full range of clock frequency which they guarrantee will work. There's probably little point in specifying a setup or hold time for use with a clock frequency that's not guarranteed to work.

TI don't make this explicit in the timing requirements table. It looks like it is defining another parameter fclock when it's not. That's just one of the conditions, along with voltage and temperature, for which the tsu and th numbers in that table are valid.

NXP imply the conditions under which they specify the tsu and th timing requirements, by referring you to figure 7, which shows you tw. They probably take the view that if the user cares about setup and hold times, then they'll also be using the part at frequencies that are guarranteed to work, rather than winging it with typical maximum frequencies (as amateurs do).


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