In addition to the mtbf summary and synchronizer summary reports, the timing analyzer tool reports additional statistics in a report for each synchronizer chain. Modeling and characterization of metastability in single. Clock domain crossing design 3 part series verilog pro. The timing analyzer views input ports as asynchronous signals unless they are associated correctly with a. Metastability events are common in digital circuits, and synchronizers are a necessity to protect us from their fatal effects. Digital logic metastability index output waveforms output waveforms due to signal timing da, db, dc da produces a normal output, as the data does not violate the setup or hold time of the device in relation to the clock. Occurrence of metastability if the system uses a synchronizer output while the output is still in the metastable state causes synchronizer failure. Pdf onchip measurement of deep metastability in synchronizers.
A tutorial ran ginosar technion israel institute of technology metastability events are common in digital circuits, and synchronizers are a necessity to protect us from their fatal effects. Synchronization and metastability has remained a fertile and sometimes. Metastability is a phenomenon that cannot be neglected when using timing speculation. Identifying synchronizers for metastability analysis 3 qii51018 2014. This is the first segment of a discussion of the metastability problem that occurs in synchronizers in clock systems, and some of the same analysis techniques and issues and ideas arise in talking about arbiters in asynchronous systems. Metastability arises at the moment the latch samples. Next, we perform spice circuit simulations to generate the metastability model parameters for several commercial fpga devices. The different paths through the combinational logic will inevitably have different delays, the likelihood of an inconsistent result is even more greater. Designing reliable synchronizers requires estimating and evaluating synchronizer parameters. They cannot be synthesized, they are hard to verify, and often what has been good in the past.
In clock domain crossing techniques part 2, i will discuss the difficulties with passing multiple control signals, and some possible solutions. A timing violation does occur, and the output oscillates between the valid states for a long time, or until its needed. However, in most of the design, the data is asynchronous w. Synchronizers must be designed to reduce the chances system failure due to metastability synchronizer requirements nreliable high mtbf nlow latency works as quickly as possible nlow powerarea impact single signal synchronizer traditional synchronizer n sig is asynchronous, and meta might go metastable from time to time. Passing a single control signal across a clock domain crossing cdc isnt very exciting. Synchronizers, in the form of latcbes, bave been used to provide the synchronous system with conditioned inputs baving the proper setup and bold times. And that has just been pointed out in a d converters. Asynchronous inputs d clk q asyncin syncin clock asynchronous input system clock synchroniser synchronous system clock asyncin syncin. Metastability synchronizer although circuits show extremely quick metastable settling time, the dsigner has to still improve the mtbf by using its unique internal feedback configuration. Characterizing and optimizing for metastability in fpgas, acm international. Synchronization and metastability trilobyte systems.
Understanding metastability in fpgas, white paper, altera corporation july 2009 d. If the input is a stable high or low voltage when the latch samples, then it will work properly. In a 2ff synchronizer, the first flipflop samples the asynchronous input signal into the destination clock domain and waits for a full destination clock cycle to permit any metastability on the stage1 output signal to decay, then the stage1 signal is sampled by the same clock into a second stage flipflop, with the intended. Jan 06, 2012 usb design house metastability 11 an asynchronous input driving two synchronizers through combinational logic. To validate our models, we then develop a hardware characterization methodology. Onchip measurement of deep metastability in synchronizers article pdf available in ieee journal of solidstate circuits 432. Apr 20, 2017 questa clockdomain crossing datasheet datasheet pdf, 510kb how to avoid metastability on reset signal networks, aka reset check is the new cdc blog post power aware cdc verification of dynamic frequency and voltage scaling dvfs artifacts. As we have seen that whenever setup and hold violation time occurs, metastability occurs, so we have to see when signals violate this timing requirement. Statistical modeling of metastability in adcbased serial io receivers shengchang cai, ayman shafik, shiva kiran, ehsan zhian tabasy, sebastian hoyos, and samuel palermo department of electrical and computer engineering. Metastability and synchronizers a tutorial ran ginosar. Statistical modeling of metastability in adcbased serial.
This allows thursday section to have a lab section to. This paper analyzes metastability in sfq circuits and then quanti. Metastability in electronics is the ability of a digital electronics system to persist for an unbounded time in an unstable equilibrium or metastable state. To address this problem, a test circuit for synchronizers was implemented in 2m and 1. Mar 28, 2016 passing a single control signal across a clock domain crossing cdc isnt very exciting. Mechanical metastability in flipflops, metastability means indecision of whether the output should be. An incremental encoder employs a quadrature encoder to generate its a and b output signals. In physics, metastability is a stable state of a dynamical system other than the systems state of least energy.
These synchronizers bave, bowever, been known to fail througb a pbenomenon known as metastability. Characterizing and optimizing for metastability in fpgas, acm. Synchronizer circuits, which guard against metastability, are becoming ubiquitous with the proliferation of timing domains on a chip. Synchronization in digital logic circuits synchronization. And that has just been pointed out in ad converters. Metastability in digital systems occurs when two asynchronous signals combine in such a way that their resulting output goes to an indeterminate state. The pulses emitted from the a and b outputs are quadratureencoded, meaning that when the incremental encoder is moving at a constant velocity, the duty cycle of each pulse is 50% i. Synchronizers and data flipflops are different jerome cox blendics inc. Digital logic metastability and flip flop mtbf calculation. In flipflops, metastability means indecision of whether the output should be.
In synchronizers and arbiters, general purpose applications require mean time between failures on the order of one per year or tens of years. Betz, a comprehensive approach to modeling, characterizing and optimizing for metastability in fpgas, acm international symposium on field programmable gate arrays, 2010, pp. A study of metastability in cmos latches lehigh preserve. Originally, synchronizers were required when reading an asynchronous input that is, an input not synchronized with the clock so that it might change exactly when sampled. Metastabilitycontaining component consisting of a state machine and associated registers a datapath. An extended metastability simulation method for synchronizer. Metastability challenges for 65nm and beyond proceedings. Usb design house metastability 11 an asynchronous input driving two synchronizers through combinational logic. A tutorial metastability events are common in digital circuits, and synchronizers are a necessity to. Metastability denotes the phenomenon when an isolated system spends an extended time in a configuration other than the systems state of least energy. It explains how metastability mtbf is calculated, and highlights how various device and design parameters affect the result. Metastability related failures are likely to increase as process variability increases. However, if the input is around the vs2 point when the latch samples, theres a possibility the latch will. Stories of malfunction and bad synchronizers are plenty.
In fact, metastability failures are much more critical for betterthanworstcasedesigns. Synchroniser failure and metastability estimation when ff input changes close to clock edge, the ff may enter the metastable state. A ball resting in a hollow on a slope is a simple example of metastability. When the clock skewslew is too much rise and fall time are more than the tolerable values.
Index termssfq, metastability, synchronizers, mean time between failure. The need for synchronization and the timing nondeterminism introduced while metastability is resolved are two important aspects that must be considered. When the setup and hold times of a flipflop are not met, the flipflop could be put into the metastable state. Power aware cdc verification of dynamic frequency and voltage scaling dvfs artifacts. Clock and synchronization tie50206 logic synthesis arto perttula tampere university of technology. Typically, evaluation of these parameters has been done by empirical rules of thumb or simple circuit simulations to ensure. Questa verification solution datasheet pdf, 1mb questa clockdomain crossing datasheet datasheet pdf, 510kb how to avoid metastability on reset signal networks, aka reset check is the new cdc blog post. Metastability can arise whenever a signal is sampled close to a transition, leading to indecision as to its correct value. In a 2ff synchronizer, the first flipflop samples the asynchronous input signal into the destination clock domain and waits for a full destination clock cycle to permit any metastability on the stage1 output signal to decay, then the stage1 signal is sampled by the same clock into a second stage flipflop, with the intended goal. For more information about how metastability mtbf is calculated metastability reports metastability reports provide summaries of the metastability analysis results. Synchronization of control signals with 2ff synchronizers. A timing violation does occur, and the output moves to the wrong state.
Typically, evaluation of these parameters has been done by empirical rules of thumb or simple circuit simulations to ensure that the synchronizer. A simple synchronizer only one synchronizer per input. Metastability problems can be avoided by adding special structures known as synchronizers in the destination domain. Transition at input of synchronizer will propagate to output after 2 or 3 active edges of rx clock in silicon the tradeoff is that metastability causes the delays through synchronizers to be unpredictable. Figure 1 asynchronous clocks and synchronization failure. In digital logic circuits, a digital signal is required to be within certain voltage or current limits to represent a 0 or 1 logic level for correct circuit operation.
Metastability can be predicted by using the equation of mtbf. During a metastable state of finite lifetime all state. If the ball is only slightly pushed, it will settle back into its hollow, but a stronger push may start the ball rolling down the slope. Statistical modeling of metastability in adcbased serial io. Synchronizers play a key role in multiclock domain systems on chip. The synchronizers allow sufficient time for the oscillations to settle down and ensure that a stable output is obtained in the destination domain. Introduction with cmos technology facing increased challenges due.
Flipflops are the start and end point of critical path all flipflops within one clock domain have the same clock signal same frequency use the longest path delay to calculate the frequency. A comprehensive approach to modeling, characterizing and. Tutorial 7 april 2008 25 metastability filters zhalf levels due to metastability need to be removed low or high threshold inverters measure divergence zfilters define the time to reach a stable state vdd2 0 vdd2 vt vdd4 vdd2 0. In a multiclock design, metastability cannot be avoided but the detrimental effects of metastability can be neutralized. However, if the input is around the vs2 point when the latch samples, theres a possibility the latch will end up in the metastable vs2, vs2 state. A full tutorial on the corresponding topics is outside the scope of this paper. The typical flipflops in figure 2 comprise master and slave latches and decoupling inverters. Topic 6 slide 6 the way to do it one synchronizer per input carefully locate the synchronization points in a system.
Jones oracle labs redwood shores, california, usa ian. After introducing the metastability problem, this thesis provides a theory for latcb behavior. Understanding metastability in fpgas july 2009, ver. Clock a clock b component state machine ack req data req ack datapath op1 op2 op3 register 3 register 2 register 1 figure 1. Eecs150 digital design lecture 21 metastability, finite. But still a problem the synchronizer output may become metastable when setup and hold time are not met. No timing violation occurred, and the output moves to the appropriate state high or low. Using these data the designer can determine the influence of metastable states in an application and take any necessary countermeasures. Metastability denotes the phenomenon when a system spends an extended time in a configuration other than the systems state of least energy. Synchronizers must be designed to reduce the chances system failure due to metastability. Clock signal is connected only to flipflops and not to basic gates. Analysis of existing synchronizer synchronizers are classified as follows, 1 two flop synchronizer. Onchip measurement of deep metastability in synchronizers.
Modeling and characterization of metastability in single flux. Understanding metastability and the correct design of synchronizers is sometimes an art. A tutorial metastability events are common in digital circuits, and synchronizers are a necessity to protect us from their fatal effects. Managing metastability with the quartus ii software. Originally, synchronizers were requiredwhen reading an asynchronous input that is, an input not synchronized with. Noise will be amplified and push the flipflop one way or other. Jan 29, 2016 metastability denotes the phenomenon when a system spends an extended time in a configuration other than the systems state of least energy.