# CLOCK SPEED

#### hero22222222222

Joined Dec 22, 2019
71
Hi, im currently working on a digital ciruit schematic and i need help to determine the fastest clock speed that i can use.

i read in a book that the minimum clock perod = flip flop set up time + clock to output( flip flop) + longest propagation from flipflop output to flipflop input

i intially used the result of the equation however the functionality of the circuit still works if i use a faster clock speed

I was wondering if someone could explain to me why the result of the equation i found in the book is not correct.

Any help will be much appreciated

#### Papabravo

Joined Feb 24, 2006
14,460
Different technologies react differently to setup and hold violations and individual units may be better or worse than average. Specifications are supposed to be valid over extended ranges of temperature and other factors. Just cranking up the clock on one unit doesn't allow you to draw any conclusion except that it worked once on that unit.

#### pmd34

Joined Feb 22, 2014
518
The Data sheets provide you with information about the "worse case" components and the range which they are guaranteed to work in, due to tiny changes in production between the ICs some will be better than others.

#### atferrari

Joined Jan 6, 2004
4,095
The concept is clear: minimum period means maximum frequency. Whoever is telling you this, won't accept any claims if for faster clocks something does not work right.

People doing overclocking (a monumental waste of time) should keep always in mind: could not work at any moment.

Enjoy while it lasts.

#### cmartinez

Joined Jan 17, 2007
7,100
What the manufacturer is telling you is the minimum clock period at which the circuit will be guaranteed to work. It doesn't mean it's the fastest speed at which you can actually run it. But it's always best practice to respect the stated limits. Otherwise your circuit might work, but it won't do so reliably. As atferrari has implied, pushing those limits is very bad practice.

#### OBW0549

Joined Mar 2, 2015
3,551
i read in a book that the minimum clock perod = flip flop set up time + clock to output( flip flop) + longest propagation from flipflop output to flipflop input

i intially used the result of the equation however the functionality of the circuit still works if i use a faster clock speed

I was wondering if someone could explain to me why the result of the equation i found in the book is not correct.
The equation is correct; the inference you're drawing from it is NOT correct.

As @cmartinez said,
What the manufacturer is telling you is the minimum clock period at which the circuit will be guaranteed to work.
An important correlary to that is, the manufacturer is NOT telling you that if you go below that minimum clock period the circuit is guaranteed to NOT work. It may very well work (otherwise we wouldn't have people going around overclocking their CPUs), since specifications such as minimum clock period usually have some safety margin built in.

So the fact that your circuit still worked at a faster clock speed than the formula allows, doesn't mean the formula is incorrect.