JK Flip-Flop Symbol
Definition: The JK Flip-Flop symbol represents a clocked bistable sequential logic element in circuit diagrams, drawn as a rectangular block with inputs J, CLK, and K on the left and outputs Q and Q̅ (Q-bar) on the right, standardised under IEC 60617-12 and ANSI Y32.14 / IEEE 91-1984, used in digital circuits for toggling, counting, frequency division, and shift-register operations.
Also known as: JKFF, JK bistable, JK master-slave flip-flop, JK latch (informal), toggle flip-flop.
What the JK Flip-Flop symbol means
The JK Flip-Flop symbol denotes a synchronous bistable storage element that samples its J and K inputs on the active clock edge and changes state according to the JK truth table: J=0,K=0 holds state; J=1,K=0 sets Q to 1; J=0,K=1 resets Q to 0; J=1,K=1 toggles Q to the opposite state. The Q̅ output is always the logical complement of Q.
In digital logic diagrams, the JK flip-flop is a fundamental building block for counters, frequency dividers, and state machines. Unlike the SR flip-flop, the J=1,K=1 condition is valid and causes toggling rather than an undefined state, making the JK the most general-purpose edge-triggered bistable. The clock (CLK) input synchronises all state transitions to a common timing reference.
How to identify the JK Flip-Flop symbol
The JK Flip-Flop symbol is a rectangle with five labelled connection points: J and K on the upper and lower left for the data inputs, CLK (or a small triangle on the left edge) in the middle for the clock input, Q on the upper right for the true output, and Q̅ (a Q with an overbar, or Qn) on the lower right for the complementary output. The clock triangle symbol indicates edge-triggering; a bubble (small circle) before the triangle denotes negative-edge (falling-edge) triggering. Optional preset (PRE) and clear (CLR) pins may appear on the top and bottom.
Function in a circuit
The JK Flip-Flop captures the state of inputs J and K at each active clock edge and updates its stored bit accordingly: set (J=1,K=0), reset (J=0,K=1), hold (J=0,K=0), or toggle (J=1,K=1). The Q output represents the stored bit; Q̅ is its complement. By connecting Q̅ back to both J and K inputs, the flip-flop always sees J=1,K=1 and toggles on every clock edge, dividing the clock frequency by two — the basis of binary ripple counters.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-12 (binary logic elements): the JK flip-flop is represented as a rectangular block with the function identifier 'JK' inside, edge-trigger symbol (triangle) on the CLK input, and output pins Q and Q̅. Negated outputs are shown with a small circle (bubble) at the pin. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.14-1973 / IEEE 91-1984 (logic symbols for digital circuits): uses the same rectangular block convention with 'JK' label, triangle for edge-triggered clock, and Q/Q̅ outputs. The IEEE 91 'distinctive shape' style uses the same rectangular block for flip-flops. |
| Key difference | IEC 60617-12 and ANSI Y32.14 / IEEE 91-1984 are functionally identical for the JK flip-flop symbol: both use a rectangle labeled 'JK' with a clock triangle and Q/Q̅ output pins. Minor differences in pin label fonts and placement convention exist between software tools but not in the standards themselves. |
Terminals / pins
| Pin | Name |
|---|---|
| j | J |
| clk | CLK |
| k | K |
| q | Q |
| qn | Q̅ |
Typical values
Supply voltage: 2.0–5.5 V (74HC series), 4.5–5.5 V (74LS series). Propagation delay: 7–25 ns typical. Setup time: 5–20 ns. Hold time: 0–5 ns. Maximum clock frequency: 25–125 MHz depending on technology family. Power: 2 µW static (CMOS) to 40 mW (TTL).
Where the JK Flip-Flop symbol is used
- Binary ripple counters and synchronous counters in frequency dividers and clock generation circuits
- Shift registers and serial-to-parallel data converters in digital communication interfaces
- State machine implementations in digital controllers and sequencers
- Frequency division by two (T flip-flop mode with Q̅ fed back to J and K inputs)
- Synchronising asynchronous input signals to a system clock (metastability reduction with two flip-flops in series)
- PWM waveform generation and duty-cycle halving in motor control and power electronics
Example
In a 4-bit ripple counter schematic, four JK Flip-Flop symbols are cascaded: each flip-flop has J and K tied to logic high (toggle mode), the Q̅ output of each stage connects to the CLK input of the next, and the first CLK input receives the system clock. Each stage divides its input frequency by two, producing Q outputs at f/2, f/4, f/8, and f/16 — the four bits of a binary count.
Key facts
- The JK Flip-Flop symbol is an IEC 60617-12 / ANSI Y32.14 rectangular block labeled 'JK' with inputs J (top-left), CLK (middle-left, shown with an edge-trigger triangle), K (bottom-left), and outputs Q (top-right) and Q̅ (bottom-right).
- The four JK input combinations define unique behaviours: J=0,K=0 (hold), J=1,K=0 (set Q=1), J=0,K=1 (reset Q=0), J=1,K=1 (toggle Q to opposite state).
- With J=1,K=1 (both high), the flip-flop toggles on every active clock edge, acting as a T (toggle) flip-flop that divides the clock frequency by two.
- The JK flip-flop resolves the undefined state of the SR flip-flop: J=1,K=1 is a valid and defined condition (toggle), making the JK the most versatile general-purpose bistable.
- Setup time is the minimum period J and K must be stable before the active clock edge; violating it risks metastability where Q settles to an indeterminate voltage.
- Common ICs: 74HC73 (dual JK, negative-edge), 74HC112 (dual JK with preset/clear, negative-edge), CD4027 (CMOS, positive-edge).
- The designator for a flip-flop IC in a schematic is U (or IC) per IEC 60617 and ANSI Y32.2 / IEEE 315, e.g., U3.
Frequently asked questions
What does the JK Flip-Flop symbol mean in a circuit diagram?
The JK Flip-Flop symbol represents a clocked bistable that samples inputs J and K on the active clock edge and updates its stored bit: J=1,K=0 sets Q to 1; J=0,K=1 resets Q to 0; J=0,K=0 holds; J=1,K=1 toggles Q. It is used wherever a synchronous 1-bit memory element with set, reset, hold, or toggle capability is needed.
What does the JK Flip-Flop symbol look like?
The JK Flip-Flop symbol is a rectangle with the letter labels J (upper-left input), CLK (middle-left, with a small triangle indicating edge-triggering), and K (lower-left input) on the left side, and Q (upper-right) and Q̅ (lower-right, Q with an overbar) on the right side. A bubble on the CLK triangle indicates falling-edge triggering.
What is the difference between a JK flip-flop and a D flip-flop?
A D flip-flop has a single data input D and simply copies D to Q on the active clock edge (no toggle mode). A JK flip-flop has two inputs J and K enabling four distinct behaviours including toggling when both are high. D flip-flops are simpler and preferred for data registers; JK flip-flops are used when toggle or independent set/reset capability is needed.
What does J=1, K=1 mean for a JK flip-flop?
When both J and K are logic high (J=1, K=1), the JK flip-flop toggles on each active clock edge: if Q was 0 it becomes 1, if Q was 1 it becomes 0. This is the toggle (T) mode and is the defining feature that distinguishes the JK from the SR flip-flop, where S=1, R=1 is a forbidden (undefined) condition.
What standard defines the JK flip-flop symbol?
The JK flip-flop symbol is standardised in IEC 60617-12 (binary logic elements) for IEC schematics, and in ANSI Y32.14-1973 / IEEE 91-1984 (logic symbols for digital circuits) for North American schematics. Both standards use a rectangular block labeled 'JK' with an edge-trigger indicator on the clock input.
How is a JK flip-flop used as a frequency divider?
To create a divide-by-two circuit, connect both J and K inputs to logic high (Vcc). The flip-flop is permanently in toggle mode and switches Q on every active clock edge, producing an output at half the clock frequency. Cascading multiple flip-flops in this configuration creates a binary ripple counter where each stage halves the previous frequency.
What is the designator letter for a JK flip-flop in a schematic?
When a JK flip-flop is a standalone IC or part of an integrated circuit, the reference designator is U (or IC) per IEC 60617 and ANSI Y32.2 / IEEE 315, e.g., U1, U2. If drawn as a logic symbol within a larger digital schematic, individual element designators such as FF1, FF2 are sometimes used informally.
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