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Review S-R flip-flop Clocked S-R flip-flop Edge-triggered flip-flops D, J-K, T flip-flops S Q R Q’ The S-R Flip-flop (H) (H) The output Q is called the state of the flip-flop S=1, R=0 Sets the state to 1 S=0, R=1 Resets (or Clears) the state to 0 S=0, R=0 is the Hold state

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## Review S-R flip-flop Clocked S-R flip-flop Edge-triggered flip-flops D, J-K, T flip-flops

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**Review S-R flip-flop**• Clocked S-R flip-flop • Edge-triggered flip-flops • D, J-K, T flip-flops**S**Q R Q’ The S-R Flip-flop (H) (H) The output Q is called the state of the flip-flop S=1, R=0 Sets the state to 1 S=0, R=1 Resets (or Clears) the state to 0 S=0, R=0 is the Hold state S=1, R=1 is not allowed**current state**next state Truth Tables, Characterisitic Equation S R q Q 0 0 0 0 0 0 1 1 0 1 0 0 0 1 1 0 1 0 0 1 1 0 1 1 1 1 0 x 1 1 1 x HOLD RESET SET not allowed Because the 11 input will not be allowed, the result is a don’t care, and Q = S + R’q This is the characteristic equation for an S-R flip-flop**S-R Flip-flop outputs**R S Q Q(H) or Q’ (L) Q’ (H) or Q(L)**A Clocked S-R Flip-flop**S Q CLK R Q’ When CLK = 0, the flip-flop holds When CLK = 1 It acts like a regular S-R flip-flop**S**R CLK Q Clocked S-R Flip-flop S=1, R=1, can occur as long as the Clock=0**Edge triggered flip-flops**• For clocked flip-flops, the output reflects the input as long as the clock =1. When the clock=0, the flip-flop is in the hold state. • For edge-triggered flip-flops, the output only changes at the edge of the clock • A positive-edge-triggered flip-flop changes state on the low-to-high transition of the clock ( ) • A negative-edge-triggered flip-flop changes state on the high-to-low transition of the clock ( )**S**S S Q Q Q R R R Q’ Q’ Q’ Edge-triggered Symbols A positive edge triggered flip-flop Negative edge triggered flip-flops**CLK**Edge triggered S-R Flip-flops S R Q Positive edge triggered flip-flop Q Negative edge triggered flip-flop**The D Flip-flop**D Q CLK Q’ When CLK = 0, the flip-flop holds When CLK = 1 D q Q 0 0 0 0 1 0 1 0 1 1 1 1 Characteristic Equation: Q = D**J**Q K Q’ J K q Q 0 0 0 0 0 0 1 1 0 1 0 0 0 1 1 0 1 0 0 1 1 0 1 1 1 1 0 1 1 1 1 0 Hold Reset Set The J-K flip-flop Characteristic Equation Q = Jq’ + K’q Invert (or complement)**T**Q J Q Q’ K Q’ The T flip-flop T T q Q 0 0 0 0 1 1 1 0 1 1 1 0 Hold Invert (or complement) Characteristic Equation Q = Tq’ + T’q

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