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Edge triggered JK flip-flop circuit

Short answer

A narrow pulse from an edge detector enables J and K input gates at the rising clock edge. For J=K=1, the output toggles once because the pulse width is less than the feedback delay, preventing race-around. The latch holds state otherwise.

Draw the circuit diagram of edge triggered JK flip flop and explain it.

An edge-triggered J-K flip-flop changes state only at one edge of the clock (rising edge for positive edge triggering). It consists of an edge (pulse-transition) detector and a J-K NAND circuit.

Circuit diagram

 CLK --+---------------->+-----+
       |                 | AND |---> CLK*
       +--->[ NOT ]----->+-----+
            (delay dt)
 CLK* = CLK . CLK'(delayed): a spike of width
 about dt at every rising edge of CLK

          +------+  S*  +------+
 J -------|      |----->|      |
 CLK* ----| NAND |      | NAND |-----> Q
 Q' ------|  G1  |  Q'->|  G3  |
          +------+      +------+
          +------+  R*  +------+
 K -------|      |----->|      |
 CLK* ----| NAND |      | NAND |-----> Q'
 Q  ------|  G2  |  Q-->|  G4  |
          +------+      +------+
 G3 and G4 are cross-coupled (NAND latch).
 Q' is fed back to G1, Q is fed back to G2.

Explanation

  • Edge detector: the inverter delays CLK by its propagation delay Δt\Delta t. For this short time after a rising edge both AND inputs are 1, so CLK* is a narrow positive spike. This spike enables G1 and G2 only at the edge.
  • Input gates: G1 receives J, CLK*, Q'; G2 receives K, CLK*, Q. Their outputs drive the active-low inputs of the G3-G4 latch.
JKAt rising edgeQ+
00G1 = G2 = 1, latch holdsQ (no change)
01G2 = 0 if Q = 10 (reset)
10G1 = 0 if Q = 01 (set)
11G1 fires if Q = 0, G2 if Q = 1Q' (toggle)

Characteristic equation: Qn+1=J Qn‾+K‾ QnQ_{n+1} = J\,\overline{Q_n} + \overline{K}\,Q_n.

The CLK* pulse ends before the change of Q can travel back to G1/G2, so for J = K = 1 the output toggles only once per clock edge. This removes the race-around problem of the level-triggered J-K flip-flop.

✓ Answer checked by Claude Opus 5.5 (high effort)

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