# What is the method used to find the output frequency of sequential circuits?

#### Devika B S

Joined Mar 8, 2017
144
Given the input frequency of a sequential circuit, what is the method used to find its output frequency?
For example: the input frequency of SR flip flop is 10 kHz, the output frequency is 5 kHz. This I know because its simple. Output (q) toggles at every half of the time period T, so fo = fin/2. But when it comes to harder circuits (such as mod N counters), is there a generic method or formula to calculate the output frequency. Can anyone please send a relevant link from where I can know more about this.

#### crutschow

Joined Mar 14, 2008
31,128
For a binary counter, each FF stage reduces the frequency by 1/2 or 2^N where N is the number of stages.

• Devika B S

#### MrChips

Joined Oct 2, 2009
27,693
It is not so easy with a mod N counter.

The only way is to write out the binary sequence and to examine the output bit stream of each bit. Be aware that the output will not necessarily have a 50% duty cycle.

You may also be interested in looking up binary rate multiplier.

#### WBahn

Joined Mar 31, 2012
27,900
Given the input frequency of a sequential circuit, what is the method used to find its output frequency?
For example: the input frequency of SR flip flop is 10 kHz, the output frequency is 5 kHz. This I know because its simple. Output (q) toggles at every half of the time period T, so fo = fin/2. But when it comes to harder circuits (such as mod N counters), is there a generic method or formula to calculate the output frequency. Can anyone please send a relevant link from where I can know more about this.
There is no way to do so for a general sequential circuit because the very notion of "output frequency" may not even have any meaning for a particular circuit. So, for a given circuit, you must first define what the measure of the output frequency is that is meaningful to your needs in the context of what that circuit does and then analyze the circuit accordingly.

#### ArakelTheDragon

Joined Nov 18, 2016
1,353

#### Devika B S

Joined Mar 8, 2017
144
Give the circuit.
I was just asking whether there is a general method to find. I am not asking this because I found a particular circuit that I couldn't calculate the output frequency.

From this discussion and further reading, i understand that for asynchronous ripple counters, f0 divides by 2^n where n is the number of flip flops. For remaining, I have to find f0 by figuring out the number of states involved.

#### ArakelTheDragon

Joined Nov 18, 2016
1,353
The idea of counters is this:
Since counters are made with t triggers, the trigger turns to 1 on signal input and to 0 on second signal input. The first trigger is given a signal input and then its output becomes 0, but on the second clock the output becomes 1, whic is taken by the second trigger and changes the second trigger's output to 0. This way we have 4 clocks for a change on the second trigger instead of 2 clock on the first. The third clock makes the output of trigger 1 to 0, the forth clock makes the output of trigger 1 to 1 again which changes the second triggers output to 1. Like this we have divided the frequency by 4 already. The first trigger divided it by 2, the second one by another 2.

The idea is that it takes 2 impulses to change the trigger's output to 1. The second trigger requires another 2 impulses and so on.

• Devika B S

#### Devika B S

Joined Mar 8, 2017
144
The idea of counters is this:
Since counters are made with t triggers, the trigger turns to 1 on signal input and to 0 on second signal input. The first trigger is given a signal input and then its output becomes 0, but on the second clock the output becomes 1, whic is taken by the second trigger and changes the second trigger's output to 0. This way we have 4 clocks for a change on the second trigger instead of 2 clock on the first. The third clock makes the output of trigger 1 to 0, the forth clock makes the output of trigger 1 to 1 again which changes the second triggers output to 1. Like this we have divided the frequency by 4 already. The first trigger divided it by 2, the second one by another 2.

The idea is that it takes 2 impulses to change the trigger's output to 1. The second trigger requires another 2 impulses and so on.
But this is only for asynchronous counters, right? For synchronous, there is only one clock pulse for all flip flops. And output frequency remains same at every flip flop.

#### AnalogKid

Joined Aug 1, 2013
10,176
For remaining, I have to find f0 by figuring out the number of states involved.
The number of states AND the internal connections. Using old part numbers, the 7490 and 7492 both are ripple counters with 4 flip flops. But the 7490 divides the input by 10 and the 7492 divides the input by 12. The CMOS CD4017 is a 5-stage part that divides the input by 10 by a very different way, because internally it is a shift register rather than a divider.

If you have nothing but the internal schematic, figuring out what a multi-stage counter is doing can be difficult.

ak

#### ArakelTheDragon

Joined Nov 18, 2016
1,353
No! You have the same at any clock, all digital clocks are realized like this. The synchronous part makes no difference.

#### AnalogKid

Joined Aug 1, 2013
10,176
Please use the "Reply" function to capture all or part of what you are responding to so the rest of us can follow the conversation. See post #9 for an example.

ak

#### ArakelTheDragon

Joined Nov 18, 2016
1,353
Sorry for that. Your post was published while I was typing .

Marry Christmas and happy new year to everyone!

#### ElectricSpidey

Joined Dec 2, 2017
2,182
The difference between asynchronous and synchronous is this ….with a synchronous counter all of the outputs change at the same time, but with a asynchronous counter the outputs change one after another in a ripple effect.

But the counter only uses a single clock pulse to change.

• Devika B S

#### ArakelTheDragon

Joined Nov 18, 2016
1,353
The difference between asynchronous and synchronous is this ….with a synchronous counter all of the outputs change at the same time, but with a asynchronous counter the outputs change one after another in a ripple effect.

But the counter only uses a single clock pulse to change.
That makes no difference with frequency dividers.

#### Devika B S

Joined Mar 8, 2017
144
That makes no difference with frequency dividers.
Ok. Thanks for the clarification.

Sorry for that. Your post was published while I was typing .