What are Flip Flop Clocks?

Thread Starter


Joined Apr 4, 2016
This may be a really dumb question but I am researching flip flops and I am getting confused with one part of the circuit; the "clock". What is the source of the clock? Is it something that I manually change? or do I connect a timer circuit like a 555? Any help is greatly appreciated.


Joined Feb 24, 2006
The flip-flop clock is any periodic signal with a defined period and duty cycle. Typically a 555 doesn't have a stable frequency and duty cycle. A typical clock would come from a crystal controlled oscillator, like one of these:

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Joined Mar 14, 2008
A clock is just a digital pulse signal that controls when the FF changes state (usually on the rising or falling edge of the pulse).
Most digital systems use a continuous clock pulse to synchronize the operations so that all the functions occur in lock-step.
Typically, digital system perform a series of distinct operations in a rapid sequence and the clock regulates that sequence. Otherwise the steps would be jumbled together and the output would be garbage.
For example, the frequency quoted for various microprocessors is their internal clock frequency which controls how fast they can perform the steps of the program they are executing.
The clock duty-cycle is not normally critical but it's often near 50%.
The frequency accuracy of the clock depends upon the application. For many applications, an accurate frequency is not needed.
If the system can operate with a low and less than exact clock frequency, then a 555 operating as an astable will work fine.


Joined Oct 2, 2009
There are two basic classes of digital circuits:
  1. Combinational
  2. Sequential
The result of a combinational circuit depends exclusively on static input presented to the inputs of the circuit.

The outcome of sequential circuits, in addition to inputs of the circuit, also depends on the current state of the circuit and the element of time. Time is introduced to delineated the previous state and the new state. This element of time is defined by the transition of a logic signal which is labelled CLOCK. The precise moment in time is identified by either the low-to-high transition of the CLOCK or high-to-low transition depending on the design of the specific digital circuit.


Joined Apr 24, 2011
The term "clock" has an Alice in wonderland quality about it: "clock" means exactly what I say it means, no more, no less.

The meaning varies depending on the context, what means can and does change depending on what you are doing.

For a simple flip flop the clock input designates the pin that loads or changes the state of the flip flop.

For a flip flop used as a memory storage location the clock will occur at only those times when the data needs to be stored, otherwise no clock.

For say an alarm clock the clock input to step to the next time is very regular, so regular a '555 is not accurate enough to be suitable.


Joined Sep 24, 2015
A flip flop sets the output of Q to whatever digital state the input D is at. The clock pulse is the signal for the circuit to perform this function. If Q is low and D is low then with every clock pulse Q will be set to D. If the digital state of D goes high Q remains low until the next clock pulse. Then with the rising edge (usually) of the clock pulse Q gets set to the state of D. So at that clock pulse Q goes high because D was high at that moment.

The FF has a complementary output called NOT Q (symbolized by a Q with a bar over it). NOT Q is always opposite of Q. Which is why its called "NOT Q" Because whatever Q is NOT Q is - well, it's "Not" Q.

A FF can be used as a "Divide by 2" if you feed the output of NOT Q directly to D. With each clock pulse Q will be set to D. Since D is directly connected to NOT Q then Q will take on the digital state of NOT Q and NOT Q will take the opposite state. With the next clock pulse Q is set to D and NOT Q takes the opposite state - over and over. So if you have 100 clock pulses per second then Q will have an output equal to 50 clock pulses (divide by 2). Since many FF's come in "Dual" formats you can use the first FF to clock the second and achieve a divide by 4 circuit.

There are different types of FF's. In general I'm referring to the "Dual D Flip Flop" chip. There's another, called the "J K Flip Flop" and it has two inputs, one on J and one on K. I haven't messed with those so I really am not sure on how they work, but if you google it I'm sure you'll find a wealth of information on it.

So to answer your question about needing a clock signal (a 555 timer) - the answer is "Yes, typically". But you don't need a 555 for a clock pulse, you can set up many different types of digital oscillators made from many different chips. NOT gates, NAND gates and even OR gates. I even made a clock pulse generator out of a Dual D FF. A bit more tricky, but doable if you use the Set and Reset inputs. I hate having more chips on a board than is necessary.

Interesting note: It's possible to drive both Q and NOT Q to the same digital state using set and reset. But you have to HOLD the Set and Reset in order to force the two Q's to the same state.


Joined Nov 23, 2015
This may be a really dumb question but I am researching flip flops and I am getting confused with one part of the circuit; the "clock". What is the source of the clock? Is it something that I manually change? or do I connect a timer circuit like a 555? Any help is greatly appreciated.
Clock is just the synchronizing input to the chip. The synchronous inputs (J, K or D) don't have an influence until Clock happens.


Joined May 4, 2016
Clocks are used where precision timing is needed. For example, assume a memory cell inputs one binary digit on the rising edge of a clock. The binary digit must be at the memory cell input before the rising edge of the clock appears. If the rising edge of the clock appears before the binary digit is present, that binary digit will not be stored in the memory cell. Hence, whatever binary digit was at the input when the rising edge of the clock appeared will be stored in that memory cell.