# Sequential Circuits: How Are They Stored?

• Tactified
In summary, the clock is not what stores the information in a flip flop, but rather the feedback loop. A pulse from a clock tells the flip flop to change its state, but without the clock, the flip flop would stay in the same state forever.f

#### Tactified

Hello all,

I'm new to this forum, but I've browsed this forum quite a bit and it has helped me numerous times throughout my years of college. I have a question that I need clarification on with sequential circuits.

Synchronous Circuits store "information" using a clock and the added signal from the input, usually from some Combinational circuit, but what I still don't understand is how. I guess physically its not making sense to me.

How does an changing clock pulse (what looks like to be oscillating in my book) help store a 1 or 0 after a signal has been input into the flip flop?

Welcome to PF! We're glad it has helped you in the past.

The clock is actually unrelated. A flip-flop will store its value indefinitely, even if its clock signal never toggles. The clock is what tells the flip-flop to change its state; without the clock, it will stay in the same state forever.

Flip-flops hold their state because they use feedback. Take a look at the schematic here:

http://en.wikipedia.org/wiki/Flip-flop_(electronics)#Simple_set-reset_latches

Can you see how the NOR gates are connected in a 'loop,' where the output of one drives the input of the other, and vice versa? That feedback loop is what allows the flip-flop to maintain its state indefinitely.

- Warren

Ok, I think I understand this now, so essentially the feedback loop is what the "current state" in the flip flop is, and the clock is the pulse that is sent into the other input terminals of both NOR gates which will change the state that is held in that feedback loop. Is that the way to look at it?

So how do you determine the timing of clock pulses? Are they always periodic, or are the pulses determined by some other circuitry (thus allowing someone to control WHEN the pulses happen and when to change the information stored)?

And thank you Chroot, that was really helpful.

Clock pulses can be periodic and at a very precise frequency or they may not.

One application of flip-flops is the production of a precise lower frequency from a known high frequency one.
I have a Rubidium oscillator that gives an output at 10 MHz which is correct to 1/600 th of 1 Hz.

I could use a lot of flip flops to get this divided down to 1 Hz to produce 1 second pulses to drive a clock which would be accurate to 1 second in 190 years.

This is possible because each two clock pulses into a flip flop (connected as a divider) produce one pulse out. So, the frequency is divided by two. Take that to another flip flop and the frequency will be divided by 4 and so on.

There are single chips which can divide by 16384. There is one that will divide by 224 or 16,777,216 (the CD4521). There are others that will divide by 10 or 128 or 256. Or you can make dividers to suit yourself.

So, it depends on what you want to do.

You could have one clock pulse come from a sensor that was activated by the front wheels of a car at the start of a race, and the next from the wheels when the car finished the race. So you could get an output that depended on the time a car took to finish the race.
This pulse might turn a clock on and off to measure how long a car took to complete a measured kilometer of track.