Digital Circuits/Flip-Flops

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A flip-flop is a device very much like a latch in that it is a bistable multivibrator, having two states and a feedback path that allows it to store a bit of information. The difference between a latch and a flip-flop is that a latch is asynchronous, and the outputs can change as soon as the inputs do (or at least after a small propagation delay). A flip-flop, on the other hand, is edge-triggered and only changes state when a control signal goes from high to low or low to high. This distinction is relatively recent and is not formal, with many authorities still referring to flip-flops as latches and vice versa, but it is a helpful distinction to make for the sake of clarity.

There are several different types of flip-flop each with its own uses and peculiarities. The four main types of flip-flop are : SR, JK, D, and T.

SR Flip-flops[edit | edit source]

Animated interactive SR flip-flop (suggested values: R1, R2 = 1 kΩ R3, R4 = 10 kΩ).

An SR(Set/Reset) flip-flop is perhaps the simplest flip-flop, and is very similar to the SR latch, other than for the fact that it only transitions on clock edges. While as theoretically valid as any flip-flop, synchronous edge-triggered SR flip-flops are extremely uncommon because they retain the illegal state when both S and R are asserted.

Characteristic table
S R Qnext Comment
0 0 0 Hold state
0 1 0 Reset
1 0 1 Set
1 1 Metastable
Excitation table
Q Qnext S R
0 0 0 X
0 1 1 0
1 0 0 1
1 1 X 0

D flip-flop[edit | edit source]

Symbol for a D flip-flop.

The D(DATA) flip-flop is the edge-triggered variant of the transparent latch. On the rising (usually, although negative edge triggering is just as possible) edge of the clock, the output is given the value of the D input at that moment. The output can be only changed at the clock edge, and if the input changes at other times, the output will be unaffected.

Clock D Qnext Comment
Transistion Low-High.svg 0 0 Express D at Q
Transistion Low-High.svg 1 1 Express D at Q
Otherwise X Qprev Hold state

D flip-flops are by far the most common type of flip-flops and some devices (for example some FPGAs) are made entirely from D flip-flops. They are also commonly used for shift-registers and input synchronisation.

JK Flip-flop[edit | edit source]

Symbol for a JK flip-flop

The JK flip-flop is a simple enhancement of the SR flip-flop where the state J=K=1 is not forbidden. It works just like a SR flip-flop where J is serving as set input and K serving as reset. The only difference is that for the formerly "forbidden" combination J=K=1 now performs an action: it inverts its state. As the behavior of the JK flip-flop is completely predictable under all conditions,[citation needed] this is the preferred type of flip-flop for most logic circuit designs.[citation needed] But there is still a problem; i.e. both the outputs are same when one tests the circuit practically. This is because of the internal toggling on every propagation elapse completion. The main remedy is going for master-slave JK flip-flop; this flip-flop overrides the internal recurring toggling through the pulsed clocking feature incorporated.

Characteristic table
J K Qnext Comment
0 0 Qprev Hold state
0 1 0 Reset
1 0 1 Set
1 1 Qprev Toggle
Excitation table
Q Qnext J K Comment
0 0 0 X Hold state
0 1 1 X Set
1 0 X 1 Reset
1 1 X 0 Hold state

T flip-flops[edit | edit source]

A circuit symbol for a T-type flip-flop: T is the toggle input and Q is the stored data output.

A T flip-flop is a device which swaps or "toggles" state every time it is triggered if the T input is asserted, otherwise it holds the current output. This behavior is described by the characteristic equation:

and can be described either of the following tables:

Characteristic table
T Q Qnext Comment
0 0 0 Hold state
0 1 1
1 0 1 Toggle
1 1 0
Excitation table
Q Qnext T
0 0 0
0 1 1
1 0 1
1 1 0

When T is held high, the toggle flip-flop divides the clock frequency by two; that is, if clock frequency is 4 MHz, the output frequency obtained from the flip-flop will be 2 MHz. This 'divide-by' feature has application in various types of digital counters. A T flip-flop can also be built using a JK flip-flop (J & K pins are connected together and act as T) or D flip-flop (T input and Qprev are connected to the D input through an XOR gate). d t