D Latch Delay (D) latch:a) logic symbolb) NAND implementationc) NOR implementation

D Latch The D latch is “transparent” –As long as C=1, changes in D are propagated to the output D latch timing diagram

D Latch D latch timing constraints Latches undesirable in a sequential machine –Why?

D latch Logic function Q* = f(C,D,Q) = ? Q* = DC + D’’Q + C’Q = CD + C’Q + DQ = CD + C’Q Undesirable in sequential machines, because transparent –What does it imply for the sequential machine function?

Flip-flops Flip-flops are pulse-triggered Master-slave gated SR flip-flops Symbol signifies rising pulse edge triggered

Master-slave gated SR flip-flop Timing diagram Master and slave alternate

Master-slave gated SR flip-flop Timing diagram Setup and hold times defined for the external inputs to the master latch S and R must be stable around the clock edge that puts master on hold

Master-slave gated SR flip-flop Excitation table (a) and Timing diagram (b) Needs both the 0-1 and 1-0 transitions on C in order to operate properly Logic function Q* = f(S,R,Q,C) = ?

Master-slave D flip-flop Symbol signifies rising pulse edge triggered

Master-slave D flip-flop Timing diagram Needs both the 0-1 and 1-0 transitions on C in order to operate properly Logic function Q* = f(D,Q,C) = ? Q = D

Master-slave JK flip-flop Same as SR flip-flop with J=S and K=R When J=K=1, “toggles” state: Q*=Q’ Why connect Q* to D of the same FF? Why not cascade 2 D Flip-flops? Symbol signifies falling pulse edge triggered Logic function Q* = f(D,Q,C) = ?

K Master-slave JK flip-flop Has don’t cares Q* = K’Q + JQ’

Master – Slave configuration One way to avoid race-around conditions and transient oscillations Needs both rising and falling clock edges to function correctly –Therefore known as “pulse triggered” circuits Another way to avoid race-around and transients? –Design circuit that is sensitive to its excitation inputs only during rising or falling clock edge transitions –Called edge-triggered

G4 G3 G2 G1 G5 G6 Edge- triggered D flip-flop Rising edge triggered

Edge-triggered D flip-flop Operation on Set –PRE’ = 0; CLR’ = 1  G5 = Q = 1; G1 = 1 –CLK = 0  G3 = 1  G6 = Q’ = 0 –CLK = 1  G2 = 0  G3 = 1  G6 = Q’ = 0 G4 G3 G2 G1 G5 G

Edge-triggered D flip-flop Operation on Reset –PRE’ = 1; CLR’ = 0  G6 = Q’ = 1; G2 = 1; G4 = 1  G5 = Q = 0 –CLR’ alone sets the outputs G4 G3 G2 G1 G5 G

G5 G6 Edge-triggered D flip-flop Operation when both Set and Reset asserted –PRE’ = 0  G5 = Q = 1; G1 = 1 –CLR’ = 0  G6 = Q’ = 1; G2 = 1; G4 = 1 –Both Q and Q’ are 1 at the same time –No oscillation G4 G3 G2 G

Edge-triggered D flip-flop Normal operation: PRE’ and CLR’ = 1 –Equivalent circuit without them –CLK = 0  G2 = G3 = 1 –G5 and G6 form a stable pair of cross-coupled inverters –Stable (hold) state G4 G3 G2 G1 G5 G

Edge-triggered D flip-flop Normal operation: PRE’ and CLR’ = 1; latch 0 –CLK = 0  1;D = 0 –G3 becomes 0 and sets the G5 G6 pair to 0 1 –CLK = 1; D = 0  1;  G3 is still 0, G4 cannot change: no change. G4 G3 G2 G1 G5 G6 0101 100100 010 11 00 111111 111111

Edge-triggered D flip-flop Normal operation: PRE’ and CLR’ = 1; latch 1 –CLK = 0  1;D = 1 –G2 becomes 0 and sets the G5 G6 pair to 1 0 –CLK = 1; D = 0  1;  G2 is still 0, G3 cannot change; no change. G4 G3 G2 G1 G5 G6 0101 111111 100100 1010001001 00 11 0101 001001 111111 001001

Edge-triggered D flip-flop Timing specifications of SN7474, edge-triggered D FF –Delays from C to output are small –Value from D is almost instantly transferred to Q

Edge- triggered JK flip-flops SN74LS73 is negative edge triggered

Edge-triggered JK flip-flops SN74111 is a master-slave JK flip-flop with a positive edge- triggered master and negative edge pulse-triggered slave –Master flip-flop latches data on the rising edge of the clock and ignores it at other times –Slave transfers new value from master on falling clock edge

Edge-triggered T flip-flops T (or Toggle, or Trigger) flip-flop –JK flip-flop in toggle mode –Negative edge-triggered: –Excitation table and state diagram:

Clocked T flip-flops Clocked T flip-flop –Changes on clock when T –Negative edge-triggered: –Excitation table, logic symbol and timing diagram:

Edge-triggered versus pulse-triggered Pulse triggered –Denoted by  Q or  Q on the output pin –Requires both edges of the clock before input values are transferred to and held at the output –Example: master-slave configuration Edge triggered –Denoted by ¨ or o ¨ on the clock pin –Input value transferred and held in response to a single rising or falling edge –Example: edge triggered D and JK flip-flops

Latches and flip-flops summary Latches: –When data is to be captured from a signal line and stored Flip-flops: –Used to remember state in sequential circuit design