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Lecture #31 Flip-Flops, Clocks, Timing

Lecture #31 Flip-Flops, Clocks, Timing. Last lecture: Finite State Machines This lecture: Digital circuits with feedback Clocks Flip-Flops. Clocked Logic.

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Lecture #31 Flip-Flops, Clocks, Timing

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  1. Lecture #31 Flip-Flops, Clocks, Timing • Last lecture: • Finite State Machines • This lecture: • Digital circuits with feedback • Clocks • Flip-Flops EE 42 fall 2004 lecture 31

  2. Clocked Logic • In the last few lectures, we have been discussing the implementation of circuits which can break a problem down into a sequence of events, as contrasted with evaluation of a single Boolean expression (Combinatorial Logic) EE 42 fall 2004 lecture 31

  3. Definition: Combinatorial logic Combinatorial logic is a set of digital gates which produces an output based solely on its current inputs. A combinatorial logic circuit can be described using a truth table EE 42 fall 2004 lecture 31

  4. Definition: Truth table • A Truth table is a description of a digital circuit which is a tabulation of all possible inputs, and the outputs which will result from those inputs. • Two combinatorial logic circuits are considered logically equivalent if they have the same truth table EE 42 fall 2004 lecture 31

  5. Sequential Logic • In order to solve more complex problems using a sequence of steps, we looked at the concept of feedback of the output of intermediate results back into the circuit for additional processing. In solving the problems caused by this, we arrived at the finite state machine with latched and clocked feedback. EE 42 fall 2004 lecture 31

  6. In the next two lectures, we will discuss the implementation of sequential logic, latches, clocks, and the various problems which can occur in dynamic digital logic circuits. EE 42 fall 2004 lecture 31

  7. Q R S R Q Q' S Q S' S' R' Q Q' R' Memory with Cross-coupled Gates • Cross-coupled NOR gates • Similar to inverter pair, with capability to force output to 0 (reset=1) or 1 (set=1) • Cross-coupled NAND gates • Similar to inverter pair, with capability to force output to 0 (reset=0) or 1 (set=0) EE 42 fall 2004 lecture 31

  8. R Q Q' S Timing Behavior Hold Race Reset Set Reset Set 100 R S Q \Q EE 42 fall 2004 lecture 31

  9. Race condition on falling edge of S/R • If both set and reset are high, then the value latched will be whichever falling edge happens last. If this is controlled by delays in the logic, then the outcome of which is first might be random, erratic, or dependent on other parameters. EE 42 fall 2004 lecture 31

  10. Definition: Race, or Race condition • A Race condition is when a device's output depends on two [or more] nearly simultaneous events to occur, and where which signal arrives first will change the output of the circuit. If the race condition is so close in time, the output may be unpredictable. When the circuit is manufactured, slight differences can cause a change in the operation of the circuit. A race condition can be a logic hazard, or can result in a random value being held in a latch. EE 42 fall 2004 lecture 31

  11. Definition: Glitch, Hazard • A Glitch is a momentary output of a digital circuit of an incorrect value • A Static Hazard is when a single variable change at the input causes a momentary change in the output. • A Dynamic Hazard occurs when a change in the input causes multiple changes in the output. Latches can remove glitches by allowing the output to progress only after the logic has had adequate time to stabilize on the correct output. It is desirable to design out hazard in the logic, because the extra transitions consume power and produce excess noise.If Static Hazards are removed from the design, Dynamic Hazards will not occur. If not removed by latches or Flip-Flops, timing hazards will develop as random or intermittent circuit failures. EE 42 fall 2004 lecture 31

  12. Definition: Latch • A latch is a digital circuit which will hold a value when the level of a latching signal is at a certain level. For example, while the reset signal is low, the SR latch will hold the value of Q, (and set it if Set goes high). The alternative is to set the output only at an rising or falling edge, which is referred to as being edge triggered. EE 42 fall 2004 lecture 31

  13. Level Trigger • A level trigger refers to the capture of a value while a signal (clock, for example, is high (or low). The data must be held valid and stable during the entire time it is being sampled Data Clock EE 42 fall 2004 lecture 31

  14. R R' Q enable' Q' S' S Reset Set 100 S' R' enable' Q Q' Gated R-S Latch • Control when R and S inputs matter • Otherwise, the slightest glitch on R or S while enable is low could cause change in value stored EE 42 fall 2004 lecture 31

  15. R R' Q clock' Q' S' S stable changing stable changing stable R' and S' clock R-S latch controlled with clock • Controlling an R-S latch with a clock • Can't let R and S change while clock is active (allowing R and S to pass) • Only have half of clock period for signal changes to propagate • Signals must be stable for the other half of clock period EE 42 fall 2004 lecture 31 active low

  16. Edge Trigger • Edge Trigger refers to the capture of a value at a rising or falling edge of a signal. For example, the data from a memory might be held valid and sampled at a rising edge of a clock Data Clock EE 42 fall 2004 lecture 31

  17. Definition: Flip-Flop • A flip flop is a digital circuit which will capture a value at a rising (or falling) edge, and will hold that value. It will only change the value held at an edge, and will not pass on transitions from the inputs while the clock or latch signal is either high or low. EE 42 fall 2004 lecture 31

  18. R R Q' R Q' S S Q S Q clock Cascading Latches • Connect output of one latch to input of another • How to stop changes from racing through chain? • Need to control flow of data from one latch to the next • Advance from one latch per clock period • Must worry about logic between latches (arrows) that is too fast EE 42 fall 2004 lecture 31

  19. slave stage master stage R Q' R Q' P' R S Q S Q S P CLK Master-Slave Structure • Break flow by alternating clocks (like an air-lock) • Use positive clock to latch inputs into one R-S latch • Use negative clock to change outputs with another R-S latch • View pair as one basic unit • master-slave flip-flop • twice as much logic • output changes a few gate delays after the falling edge of clock but does not affect any cascaded flip-flops EE 42 fall 2004 lecture 31

  20. 1s catch Set Reset slave stage master stage R Q' R Q' S R CLK P P' Q Q' P' R S Q S Q MasterOutputs S P SlaveOutputs CLK The 1s Catching Problem • In first R-S stage of master-slave FF • 0-1-0 glitch on R or S while clock is high "caught" by master stage • Leads to constraints on logic to be hazard-free EE 42 fall 2004 lecture 31

  21. slave stage master stage P' Q' R Q' R Q' Q D S Q S Q P CLK D Flip-Flop • Make S and R complements of each other • Eliminates 1s catching problem • Can't just hold previous value (must have new value ready every clock period) • Value of D just before clock goes low is what is stored in flip-flop • Can make R-S flip-flop by adding logic to make D = S + R' Q • The value at the output (Q, Q’) only changes based on the value of the falling edge of the clock of the master stage 10 gates EE 42 fall 2004 lecture 31

  22. Edge triggering • The proceeding slide showed how a flip flop could be designed by using two latches which are cascaded in a master-slave relationship. • Another way of creating an edge triggered flip flop is to use logic with feedback, as in the following slide. EE 42 fall 2004 lecture 31

  23. Edge-Triggered Flip-Flops • More efficient solution: only 6 gates • sensitive to inputs only near edge of clock signal (not while high) holds D' when clock goes low negative edge-triggered D flip-flop (D-FF) 4-5 gate delays must respect setup and hold time constraints to successfullycapture input R Q Clk=1 Q’ S holds D whenclock goes low characteristic equationQ(t+1) = D D EE 42 fall 2004 lecture 31

  24. D’ D’ D D D’ D’ R R Q Q Clk=0 Clk=0 S S D D D’ D’ D new D Edge-Triggered Flip-Flops (cont’d) • Step-by-step analysis when clock is low data is held new D  old D when clock goes high-to-low data is latched EE 42 fall 2004 lecture 31

  25. Edge-Triggered Flip-Flops (cont’d) • Positive edge-triggered • Inputs sampled on rising edge; outputs change after rising edge • Negative edge-triggered flip-flops • Inputs sampled on falling edge; outputs change after falling edge 100 D CLK Qpos Qpos' Qneg Qneg' positive edge-triggered FF negative edge-triggered FF EE 42 fall 2004 lecture 31

  26. Timing Methodologies • Rules for interconnecting components and clocks • Guarantee proper operation of system when strictly followed • Approach depends on building blocks used for memory elements • Focus on systems with edge-triggered flip-flops • Found in programmable logic devices • Many custom integrated circuits focus on level-sensitive latches • Basic rules for correct timing: • (1) Correct inputs, with respect to time, are provided to the flip-flops • (2) No flip-flop changes state more than once per clocking event EE 42 fall 2004 lecture 31

  27. D D Q Q Tsu Th input clock Timing Methodologies (cont’d) • Definition of terms • clock: periodic event, causes state of memory element to change; can be rising or falling edge, or high or low level • setup time: minimum time before the clocking event by which the input must be stable (Tsu) • hold time: minimum time after the clocking event until which the input must remain stable (Th) data clock there is a timing "window" around the clocking event during which the input must remain stable and unchanged in order to be recognized stable changing data clock EE 42 fall 2004 lecture 31

  28. D Q D Q G Comparison of Latches and Flip-Flops D CLK Qedge Qlatch CLK positiveedge-triggeredflip-flop CLK transparent(level-sensitive)latch behavior is the same unless input changes while the clock is high EE 42 fall 2004 lecture 31

  29. Comparison of Latches and Flip-Flops (cont’d) TypeWhen inputs are sampledWhen output is valid unclocked always propagation delay from input changelatch level-sensitive clock high propagation delay from input changelatch (Tsu/Th around falling or clock edge (whichever is later) edge of clock) master-slave clock high propagation delay from falling edgeflip-flop (Tsu/Th around falling of clock edge of clock) negative clock hi-to-lo transition propagation delay from falling edgeedge-triggered (Tsu/Th around falling of clockflip-flop edge of clock) EE 42 fall 2004 lecture 31

  30. Tsu20ns Th5ns D Tsu20ns Th5ns CLK Tw 25ns Tplh25ns13ns Tphl40ns25ns Q Typical Timing Specifications • Positive edge-triggered D flip-flop • Setup and hold times • Minimum clock width • Propagation delays (low to high, high to low, max and typical) all measurements are made from the clocking event that is, the rising edge of the clock EE 42 fall 2004 lecture 31

  31. Definition: Metastability • Metastability is a condition in which a latch or a Flip-Flop is exactly balanced between the logic high and logic low states. This can be caused by an asynchronous data signal input to a clocked Flip Flop. The resulting output may stay undefined for some time. EE 42 fall 2004 lecture 31

  32. Q0 Q1 D D Q Q IN OUT CLK Cascading Edge-triggered Flip-Flops • Shift register • New value goes into first stage • While previous value of first stage goes into second stage • Consider setup/hold/propagation delays (prop must be > hold) 100 IN Q0 Q1 CLK EE 42 fall 2004 lecture 31

  33. Cascading Edge-triggered Flip-Flops (cont’d) • Why this works • Propagation delays exceed hold times • Clock width constraint exceeds setup time • This guarantees following stage will latch current value before it changes to new value In Q0 Q1 CLK Tsu 4ns Tsu 4ns timing constraints guarantee proper operation of cascaded components Tp 3ns Tp 3ns assumes infinitely fast distribution of the clock Th 2ns Th 2ns EE 42 fall 2004 lecture 31

  34. Clock Skew • The problem • Correct behavior assumes next state of all storage elementsdetermined by all storage elements at the same time • tThis is difficult in high-performance systems because time for clock to arrive at flip-flop is comparable to delays through logic • Effect of skew on cascaded flip-flops: In Q0 Q1 CLK0 CLK1 100 CLK1 is a delayed version of CLK0 original state: IN = 0, Q0 = 1, Q1 = 1 due to skew, next state becomes: Q0 = 0, Q1 = 0, and not Q0 = 0, Q1 = 1 EE 42 fall 2004 lecture 31

  35. Summary of Latches and Flip-Flops • Development of D-Flip-Flop • Level-sensitive used in custom integrated circuits • can be made with 4 switches • Edge-triggered used in programmable logic devices • Good choice for data storage register • Historically J-K Flip Flop was popular but now never used • Similar to R-S but with 1-1 being used to toggle output (complement state) • Can always be implemented using D-FF • Preset and clear inputs are highly desirable on flip-flops • Used at start-up or to reset system to a known state EE 42 fall 2004 lecture 31

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