Lecture 11: Sequential Circuit Design

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits2 Outline  Sequencing  Sequencing Element Design  Max and Min-Delay  Clock Skew  Time Borrowing  Two-Phase Clocking

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Sequential Logic 11: Sequential Circuits3

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits4 Sequencing  Combinational logic –output depends on current inputs  Sequential logic –output depends on current and previous inputs –Requires separating previous, current, future –Called state or tokens –Ex: FSM, pipeline

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits5 Sequencing Cont.  If tokens moved through pipeline at constant speed, no sequencing elements would be necessary  Ex: fiber-optic cable –Light pulses (tokens) are sent down cable –Next pulse sent before first reaches end of cable –No need for hardware to separate pulses –But dispersion sets min time between pulses  This is called wave pipelining in circuits  In most circuits, dispersion is high –Delay fast tokens so they don’t catch slow ones.

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits6 Sequencing Overhead  Use flip-flops to delay fast tokens so they move through exactly one stage each cycle.  Inevitably adds some delay to the slow tokens  Makes circuit slower than just the logic delay –Called sequencing overhead  Some people call this clocking overhead –But it applies to asynchronous circuits too –Inevitable side effect of maintaining sequence

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits7 Sequencing Elements  Latch: Level sensitive –a.k.a. transparent latch, D latch  Flip-flop (or Register): edge triggered –A.k.a. master-slave flip-flop, D flip-flop, D register  Timing Diagrams –Transparent –Opaque –Edge-trigger

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Latch versus Register  Latch stores data when clock is low D Clk Q D Q  Register stores data when clock rises Clk D D QQ

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Latches 11: Sequential Circuits9

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Timing Definitions t CLK t D t c 2 q t hold t su t Q DATA STABLE DATA STABLE Register CLK DQ

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Characterizing Timing 11: Sequential Circuits11

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits12 Latch Design  Pass Transistor Latch  Pros +Tiny +Low clock load  Cons –V t drop –nonrestoring –backdriving –output noise sensitivity –dynamic –diffusion input Used in 1970’s

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits13 Latch Design  Transmission gate +No V t drop - Requires inverted clock

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits14 Latch Design  Inverting buffer +Restoring +No backdriving +Fixes either Output noise sensitivity Or diffusion input –Inverted output

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits15 Latch Design  Tristate feedback +Static –Backdriving risk  Static latches are now essential because of leakage

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits16 Latch Design  Buffered input +Fixes diffusion input +Noninverting

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits17 Latch Design  Buffered output +No backdriving  Widely used in standard cells + Very robust (most important) -Rather large -Rather slow (1.5 – 2 FO4 delays) -High clock loading

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits18 Latch Design  Datapath latch +smaller +faster - unbuffered input

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits19 Flip-Flop Design  Flip-flop is built as pair of back-to-back latches

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits20 Enable  Enable: ignore clock when en = 0 –Mux: increase latch D-Q delay –Clock Gating: increase en setup time, skew

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits21 Reset  Force output low when reset asserted  Synchronous vs. asynchronous

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits22 Set / Reset  Set forces output high when enabled  Flip-flop with asynchronous set and reset

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits23 Sequencing Methods  Flip-flops  2-Phase Latches  Pulsed Latches

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits24 Timing Diagrams t pd Logic Prop. Delay t cd Logic Cont. Delay t pcq Latch/Flop Clk->Q Prop. Delay t ccq Latch/Flop Clk->Q Cont. Delay t pdq Latch D->Q Prop. Delay t cdq Latch D->Q Cont. Delay t setup Latch/Flop Setup Time t hold Latch/Flop Hold Time Contamination and Propagation Delays

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits25 Max-Delay: Flip-Flops

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits26 Max Delay: 2-Phase Latches

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits27 Max Delay: Pulsed Latches

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits28 Min-Delay: Flip-Flops

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits29 Min-Delay: 2-Phase Latches Hold time reduced by nonoverlap Paradox: hold applies twice each cycle, vs. only once for flops. But a flop is made of two latches!

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits30 Min-Delay: Pulsed Latches Hold time increased by pulse width

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Itanium 2 ALU 11: Sequential Circuits31

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Combinational Logic Delays ElementPropagation DelayContamination Delay Adder590 ps100 ps Result Mux60 ps35 ps Early Bypass Mux110 ps95 ps Middle Bypass Mux80 ps55 ps Late Bypass Mux70 ps45 ps 2 mm Wire100 ps65 ps 11: Sequential Circuits32

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits33 Time Borrowing  In a flop-based system: –Data launches on one rising edge –Must setup before next rising edge –If it arrives late, system fails –If it arrives early, time is wasted –Flops have hard edges  In a latch-based system –Data can pass through latch while transparent –Long cycle of logic can borrow time into next –As long as each loop completes in one cycle

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits34 Time Borrowing Example

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits35 How Much Borrowing? 2-Phase Latches Pulsed Latches

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits36 Clock Skew  We have assumed zero clock skew  Clocks really have uncertainty in arrival time –Decreases maximum propagation delay –Increases minimum contamination delay –Decreases time borrowing

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits37 Skew: Flip-Flops

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits38 Skew: Latches 2-Phase Latches Pulsed Latches

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits39 Two-Phase Clocking  If setup times are violated, reduce clock speed  If hold times are violated, chip fails at any speed  In this class, working chips are most important –No tools to analyze clock skew  An easy way to guarantee hold times is to use 2- phase latches with big nonoverlap times  Call these clocks  1,  2 (ph1, ph2)

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits40 Safe Flip-Flop  Past years used flip-flop with nonoverlapping clocks –Slow – nonoverlap adds to setup time –But no hold times  In industry, use a better timing analyzer –Add buffers to slow signals if hold time is at risk

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits41 Adaptive Sequencing  Designers include timing margin –Voltage –Temperature –Process variation –Data dependency –Tool inaccuracies  Alternative: run faster and check for near failures –Idea introduced as “Razor” Increase frequency until at the verge of error Can reduce cycle time by ~30%

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Conventional CMOS Latches 11: Sequential Circuits42

CMOS VLSI DesignCMOS VLSI Design 4th Ed. More CMOS Latches 11: Sequential Circuits43

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Clocked CMOS Latches  Sometimes called C 2 MOS  Actually, similar to (d) in that it is tristate 11: Sequential Circuits44

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Conventional CMOS Flip-Flops 11: Sequential Circuits45

CMOS VLSI DesignCMOS VLSI Design 4th Ed. CMOS Flip-Flops  This design has a potential race condition.  Is more likely if there is skew between the two phases of the clock.  One alternative is NORA (NO Race) flip-flop.  The other alternative is to use non-overlapping clocks. 11: Sequential Circuits46

CMOS VLSI DesignCMOS VLSI Design 4th Ed. NORA Flip-flops 11: Sequential Circuits47

CMOS VLSI DesignCMOS VLSI Design 4th Ed. NORA Flip-flops 11: Sequential Circuits48

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Pulse Generators 11: Sequential Circuits49

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Pulsed Latches  The Naffziger pulsed latch is used in Itanium 2 processors. It consists of the latch from (k) and the generator from (b). The pulse width is 1/6 the clock cycle.  The pulse generator of (d) is used in the NEC RISC processor.  Note that pulses are very fast and have to be distributed in the latch.  The Partovi pulsed latch (used on the AMD K6 and Athlon) builds the pulse generator into the latch. 11: Sequential Circuits50

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Pulsed Latches 11: Sequential Circuits51

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Resettable Latches and Flip-flops  There are two types of reset: –Asynchronous –Synchronous  Settable latches and flip-flops force the output high rather than low. 11: Sequential Circuits52

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Resettable Latches and Flip-flops 11: Sequential Circuits53

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Asynchronous Set and Reset 11: Sequential Circuits54

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Enabled Latches and Flip-flops 11: Sequential Circuits55

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Logic in Latches  The sequencing overhead can be reduced by incorporating logic into latches.  The DEC Alpha used a whole assortment of such latches. 11: Sequential Circuits56

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Klass Semidynamic FF (SDFF)  A cross between pulsed latch and a flip-flop.  Used in Sun UltraSparc III along with built in logic. 11: Sequential Circuits57

CMOS VLSI DesignCMOS VLSI Design 4th Ed. SDFF  Similar to the Partovi pulsed latch.  However, it uses a dynamic NAND gate.  Faster than the Partovi pulsed latch.  Worse skew tolerance and time borrowing capability. 11: Sequential Circuits58

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Differential Flip-flops 11: Sequential Circuits59

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Differential Flip-flops  The design in (a) was used in the Alpha  The SR latch formed by the NAND gates is just a salve and can be replaced by inverters if necessary.  The StrongArm 110 processor adds the weak nMOS transistor to reduce the risk when the inputs switch while the clock is high.  The AMD K6 uses the design in (b) at the interface between static and domino logic. 11: Sequential Circuits60

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Dual Edge Triggered FFs  DET flip flops have a similar thoroughput at half the clock frequency. 11: Sequential Circuits61

CMOS VLSI DesignCMOS VLSI Design 4th Ed. DET FFs  Zhao implicitly pulsed DET FF. 11: Sequential Circuits62

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Radiation Hardened FFs 11: Sequential Circuits63

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Some Design Guides  Dynamic latches and registers have been avoided since the 0.35  m technology node. –Use static latches and register.  Include provisions for testing –We will study these later.  Clock distribution, especially multiple phases are problematic. –We will study later.  Unless required performance is at the cutting edge, use registers. 11: Sequential Circuits64

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Some Design Guides  Pulsed latches are best in terms of performance. –Remember that they have long hold times.  Extra circuitry may be necessary for short logic paths. 11: Sequential Circuits65

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Characterizing Delays 11: Sequential Circuits66

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Characterizing Delays  The set-up time cannot be crisply defined.  If the data changes slightly less than a set-up time before the clock edge, the register will still capture the correct value, but its clock-Q delay will be high.  Note that  t DQ has a minimum when the slope of t CQ is -1.  t setup is defined as the t DC at this point.  The propagation delay is the t CQ at this point.  The contamination delay t ccq is the minimum t CQ that occurs when the input arrives early.  The hold time is the minimum delay from clock to D 11: Sequential Circuits67

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Characterizing Delays 11: Sequential Circuits68

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Characterizing Delays  The aperture width, t a is the width of the window around the clock edge during which the data must not transition if the register is to produce the correct output with a propagation delay less than t pcq. 11: Sequential Circuits69

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Characterizing Delays 11: Sequential Circuits70

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Characterizing Delays  If the data arrives before the clock rises (t DCr > 0), it must wait for the clock.  In this region, t CrQ is nearly constant and t DQ increases as the data arrives earlier.  If the data arrives after the clock rises, t DQ is essentially independent of arrival time.  The data must set up before the falling edge of the clock.  If the data arrives too close to the falling edge, t DQ increases. 11: Sequential Circuits71

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Characterizing Delays  Choose the setup time before the knee of the curve, for example 5% greater than its minimum value.  Pulsed latches have different definitions, but they can be converter to ordinary latches by adding or subtracting pulse widths. 11: Sequential Circuits72

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Delay Trade-offs 11: Sequential Circuits73

CMOS VLSI DesignCMOS VLSI Design 4th Ed. State Retention Registers 11: Sequential Circuits74

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Level Conversion 11: Sequential Circuits75

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Design Margins  Designers derate their circuits by about 30% to cope with variations.  Adaptive sequential elements seek to reduce this margin.  Dynamic voltage scaling –Precharacterize the circuit –Canary circuit –Double sampling the input 11: Sequential Circuits76

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Adaptive Sequencing 11: Sequential Circuits77

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Adaptive Sequencing  (a) shows the conceptual diagram of a razor flip-flop, while (b) is the timing diagram.  The razor flip-flop has the drawback that it may become metastable if D changes during the aperture.  An improvement is double sampling with time borrowing (DSTB).  (d) shows the Razor II pulsed latch. 11: Sequential Circuits78

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Synchronizers  A synchronizer is a circuit that accepts an input that can change at arbitrary times and produces an output aligned to its clock.  This is impossible to do in a finite time. 11: Sequential Circuits79

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Metastability 11: Sequential Circuits80

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Metastability 11: Sequential Circuits81

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Metastability 11: Sequential Circuits82

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Metastability  The cross-coupled inverters behave like an amplifier with gain G when A is near the metastable voltage V m.  The delay can be modeled with an RC network.  Let the initial voltage be A and a small offset from the metastable point be a(0). 11: Sequential Circuits83

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Metastability  Assume that the node reaches a legal logic level when  The time to reach this level is  Note that the speed is given by the RC time constant.  High GBW is required. 11: Sequential Circuits84

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Synchronizers 11: Sequential Circuits85

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Synchronizers 11: Sequential Circuits86

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Synchronizers  The probability of a synchronizer failure is  The mean time between failures is 11: Sequential Circuits87

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Degrees of Synchrony ClassificationPeriodic ff Description SynchronousYes00Signal has same frequency and phase as clock. Example: register to register on chip. MesosynchronousYesConstant0Signal has same frequency, but is out of phase with clock. Safe to sample by delaying. Example: chip tp chip where both chips are using the same clock. PlesiosynchronousYesVaries slowly SmallSignal has nearly the same frequency. Phase drifts slowly with time. Safe to sample signal if it is delayed by a variable, but predictable amount. Example: Board to board with same frequency but different crystals. PeriodicYesVaries rapidly LargeSignal is periodic at arbitrary frequency. Board to board with different crystals. AsynchronousNoUnknown Signal changes arbitrarily. 11: Sequential Circuits88

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Asynchronous Domains 11: Sequential Circuits89

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Asynchronous Domains 11: Sequential Circuits90

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Two-Phase Handshake 11: Sequential Circuits91

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Arbiters  An arbiter decides which of the two inputs came first. 11: Sequential Circuits92

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Pipelining 11: Sequential Circuits93

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Wave Pipelining 11: Sequential Circuits94

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Schmitt Triggers 11: Sequential Circuits95

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Noise Suppression 11: Sequential Circuits96

CMOS VLSI DesignCMOS VLSI Design 4th Ed. CMOS Schmitt Trigger 11: Sequential Circuits97

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Schmitt Trigger Simulated VTC 2.5 V X (V) V M2 V M1 V in (V) Voltage-transfer characteristics with hysteresis. The effect of varying the ratio of the PMOS deviceM 4. The width is k* 0.5 m. m V x (V) k = 2 k = 3 k = 4 k = 1 V in (V)

CMOS VLSI DesignCMOS VLSI Design 4th Ed. CMOS Schmitt Trigger 2 11: Sequential Circuits99

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Multivibrator Circuits

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Transition-Triggered Monostable

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Monostable Trigger (RC-based)

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Astable Multivibrators (Oscillators) 11: Sequential Circuits103

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Relaxation Oscillator

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Voltage Controller Oscillator (VCO)

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Differential Delay Element and VCO in 2 two stage VCO v 1 v 2 v 3 v 4 V ctrl V o 2 V o 1 in 1 delay cell simulated waveforms of 2-stage VCO

CMOS VLSI DesignCMOS VLSI Design 4th Ed. Pitfalls and Fallacies  Incompletely reporting flip-flop delay.  Failing to check hold times.  Choosing a sequencing method too late in the design.  Failing to synchronize asynchronous inputs.  Building faulty synchronizers. 11: Sequential Circuits107

CMOS VLSI DesignCMOS VLSI Design 4th Ed. 11: Sequential Circuits108 Summary  Flip-Flops: –Very easy to use, supported by all tools  2-Phase Transparent Latches: –Lots of skew tolerance and time borrowing  Pulsed Latches: –Fast, some skew tol & borrow, hold time risk