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1、Latches, Flip-Flops, and Timers,Zhou Ping School of Biological Science & Medical Engineering,7-1 Latches,The latch is a type of temporary storage device that has two stable states (bi-stable).,The S-R (or R-S) Latch S Set (Q=1) R Reset (Q=0),Active-High input S-R latch,Active-Low input latch,7-1 Lat
2、ches,Assuming that the Q output is High (Q=1),Assuming that the Q output is 0,No-change condition,7-1 Latches,Assuming that the Q output is High (Q=1),Assuming that the Q output is 0,SET operation,7-1 Latches,Assuming that the Q output is High (Q=1),Assuming that the Q output is 0,RESET operation,7-
3、1 Latches,Assuming that the Q output is High (Q=1),Assuming that the Q output is 0,Invalid condition,7-1 Latches,Logic symbols,Active-High input S-R latch,Active-Low input latch,7-1 Latches,7-1 Latches,Application : the latch as a contact-bounce eliminator,7-1 Latches,Application : storage and trans
4、mission of information,7-1 Latches,The gated S-R latch : an enable input, EN was applied.,7-1 Latches,The gated D Latch D latch differs from the S-R latch because it has only one input in addition to EN which is called the D (data) input.,7-1 Latches,Race,hold,transparent,7-1 Latches,Master-slave,sa
5、mple,output,output,sample,7-2 Edge-Triggered Flip-Flops,Flip-flops are synchronous bistable devices.,An edge-triggered flip-flop changes state either at the positive edge (rising edge ) or at the negative edge (falling edge) of the clock pulse and is sensitive to its inputs only at this transition o
6、f the clock.,7-2 Edge-Triggered Flip-Flops,7-2 Edge-Triggered Flip-Flops,A method of edge-triggered,The edge-triggered D flip-flop,7-2 Edge-Triggered Flip-Flops,The edge-triggered D flip-flop,Given the waveforms,7-2 Edge-Triggered Flip-Flops,Delay of D Flip-Flops,The rising-edge of CLK has passed fo
7、r Q2,The rising-edge of CLK has passed for Q3,7-2 Edge-Triggered Flip-Flops,The edge-triggered J-K Flip-Flop The difference is that the J-K FF has no invalid state as does the S-R FF.,7-2 Edge-Triggered Flip-Flops,Assuming Q=0,J=0, K=0,J=1, K=0,J=0, K=1,J=1, K=1,Assuming Q=1,?,?,7-2 Edge-Triggered F
8、lip-Flops,Assuming Q=1,J=0, K=1,J=1, K=1,Assuming Q=1,7-2 Edge-Triggered Flip-Flops,7-2 Edge-Triggered Flip-Flops,7-2 Edge-Triggered Flip-Flops,Asynchronous preset and clear inputs The D, J-K inputs are called synchronous inputs because data on the inputs are transferred to the output only on the tr
9、iggering edge of the clock. Asynchronous inputs are that affect the state of the FF independent of the clock.,7-2 Edge-Triggered Flip-Flops,7-3 FF operating characteristics,Propagation delay times Set-up time Hold time Maximum clock frequency Pulse widths Power dissipation,7-4 FF applications,Parall
10、el Data Storage,7-4 FF applications,Frequency Division,7-4 FF applications,7-4 FF applications,Counting,7-5 One-Shots,The one-shots is a monostable multivibrator, a device with only one stable state. A one-shot is normally in its stable state and will change to its unstable state only when triggered
11、. Once it is triggered, the one-shot remains in its unstable state for a predetermined length of time and then automatically returns to its stable state.,7-5 One-Shots,7-5 One-Shots,Nonretriggerable one-shot,retriggerable one-shot,7-5 One-Shots,Setting the pulse width 74LS121 74LS122,7-6 The 555 Timer,Monostable multivibratorone shot Astable multivibratoroscillates NE555,7-6 The 555 Timer,7-6 The 555 Timer,7-6 The 555 Timer,One shot operation,7-6 The 555 Timer,One shot operation,7-6 The 555 Timer,One shot operation,7-6 The 555 Timer,One shot operation,7-6 The 555 Timer,Astable operation,
