13 votes
Accepted

Why do cascading D-Flip Flops prevent metastability?

Metastability cannot be 'cured', but if you wait long enough, the likelihood of it occurring can be made arbitrarily small. Once you've got it down to once in the age of the universe, it's probably ...
Neil_UK's user avatar
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10 votes

Why do cascading D-Flip Flops prevent metastability?

It reduces the probability of metastability affecting the circuit by allowing more time until the signal is actually used. With two flip-flops, it allows a whole extra clock cycle for the signal to ...
user253751's user avatar
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10 votes

How does 2-ff synchronizer ensure proper synchonization?

The simple answer is that they don't on their own. The synchroniser is there not to ensure the data gets across, but the ensure you don't end up with metastable signals feeding lots of other signals ...
Tom Carpenter's user avatar
8 votes

After metastability, does the value eventually settle to the correct value?

You have some misconceptions about metastability 1) You talk about AFTER the metastability period. The whole point is that, although you can scope the waveform after the event, and figure out when ...
Neil_UK's user avatar
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7 votes

Why do we need to synchronise asynchronous inputs in FSM?

As you already understand, your traffic light controller state machine is a synchronous machine. That is, it is driven by some clock in such a way that all combinatorial logic changes happen and ...
Trevor_G's user avatar
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7 votes
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Understanding metastability in Technion Paper

Why does it take the circuit so long to stabilize in the 3rd case compared to the first? The 3rd case is much closer to the balance point of this particular gate. The search time step determines how ...
jpa's user avatar
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6 votes
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Metastability simulation

Metastability is generally not oscillation, but the signal from a latch, not an inverter, hovering around 50% of rail for an extended period of time before settling to one or other state. Just a few ...
Neil_UK's user avatar
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6 votes
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Violating the minimum clock pulse width of a D-type flip flop

It depends on how short the pulse is. If it is extremely short, the transmission gate or tristate element which grants access to the master latch will not have time to even properly turn on, so the ...
jbord39's user avatar
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6 votes
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How to calculate the number of required flip-flop stages needed for clock-domain crossing?

The number of flip flops needed depends on three things: target MTBF requirement clock rate ‘crunchiness’ of the flip flops The latter point, ‘crunchiness’, is also called metastable hardness, and ...
hacktastical's user avatar
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5 votes
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Why double synchronizer alone is not enough for multi byte transfer between two clock domains?

Transferring a single bit is simple. It has only two states, and when a transition occurs, it can only be either in the previous state or the new state. Therefore the ONLY concern is metastability, ...
Dave Tweed's user avatar
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5 votes

How to handle metastable input using a microcontroller?

This is not what's known as a metastability problem. This is a state coding problem. You have designed your system so that on transitioning from one state to another, it flips two signals, which can ...
Neil_UK's user avatar
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5 votes

Understanding metastability in Technion Paper

They are trying to show you that meta stability is essentially a chaotic process, i.e. a butterfly flaps its wings here and a tornado happens 1000km away. They are showing you that as long as the ...
LorenzoDonati4Ukraine-OnStrike's user avatar
4 votes

Metastability simulation

It may happen that you are observing genuine numerical metastability in your ring oscillator simulation. As the metastable state is the state of unstable equilibrium, two inverters with the outputs of ...
V.V.T's user avatar
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4 votes

Why do cascading D-Flip Flops prevent metastability?

They don't prevent metastability from affecting the output, but they can greatly increase the mean time between incidents since the metastability would have to be of relatively long duration. ...
Spehro Pefhany's user avatar
4 votes
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Why is a reset with asynchronous assert safe?

The first circuit that you have shown is a case of RDC (Reset Domain Crossing) as the launching flop and capturing flops are in two different reset domains. As you said, if you assert the async reset ...
Mitu Raj's user avatar
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4 votes
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Metastability concern in bang-bang phase detector

My question is, how PD Logic handles such situations when B is metastable? It depends what you mean by 'handle'. It doesn't really handle, as in 'do something sensible', it just behaves, as in 'this ...
Neil_UK's user avatar
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3 votes

Why do we need to synchronise asynchronous inputs in FSM?

One problem with asynchronous designs, is the presence of race or hazards. Say for example in this circuit, let A = 1, B = 0 and C = 1 initially and so D is stable and settled at 1. Now let B ...
Mitu Raj's user avatar
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3 votes

After metastability, does the value eventually settle to the correct value?

If the flip flop is sampling an input signal -- e.g. a D flip-flop and the 'D' input is changing at the same time as the clock, then the correct slew is indeterminate -- there is no 'correct value' -- ...
jp314's user avatar
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3 votes

Metastability error propagation with flip flop

The first FF is not always metastable. Assuming that input edges are uniformly distributed with respect to its clock, the first FF has a certain probability of going metastable that is related to the ...
Dave Tweed's user avatar
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3 votes
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Why don't 2 flip-flop synchronizers have a reset?

Even if you don't reset 2FF-synchroniser, you can still make it work. When such a 2FF-synchroniser is initially power-on and clocked, it drives an unknown value at its output for 2 clock cycles at ...
Mitu Raj's user avatar
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3 votes

Synchronization of handshake channel with different clock domains

Metastability can occur when the setup time to a register is not met. By register, consider a D-type Flip-Flop (DFF). On such a bus, handshaking signals are intended to indicate when the associated ...
TonyM's user avatar
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2 votes

Why do cascading D-Flip Flops prevent metastability?

Because the first flip-flop, even if it is metastable, will have all the period of the clock to stabilize. By the time the second flip-flop samples the first flip-flop, its output could be already ...
Claudio Avi Chami's user avatar
2 votes
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Crossing independent domain clocks (slow to fast)

You can never expect 100% reliability, but you can expect 99.99999...% reliability. As you can see in the picture below (from here), the MTBF is proportional to the clock frequency and number of ...
crj11's user avatar
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2 votes

Metastability Deserialization and clock crossing domain

You say that TXCLK and INCLK both come from the ADC, so they shouldn't be asynchronous with respect to each other. They should be derived from a common internal source, which means they really belong ...
Dave Tweed's user avatar
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2 votes

How would a ring oscillator with even number of inverters behave?

It likely wouldn't do anything interesting. With an even number of inverters, there should be phase shift through the ring of 0 degrees, but there is no DC inversion at the end of the chain, meaning ...
Tom Carpenter's user avatar
2 votes
Accepted

What will the output of filp-flop if its input is metastable?

In the paper it is explained that metastability is inevitable in a system with asynchronous signals: In a multi-clock design, metastability cannot be avoided, but the detrimental effects of ...
Vicente Cunha's user avatar
2 votes

Why non repeated poles at imaginary axis makes LTI system marginally stable?

Complex conjugate poles on the \$j\omega\$ axis can also produce an unbounded output, just like a pole at \$s=0\$. It just depends on the input signal. If you excite a system with a single pole at \$s=...
Matt L.'s user avatar
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2 votes
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How is asynchronous reset physically implemented in a flip-flop?

Asynchronous reset is level-sensitive so the timing relative to the clock edge is of no importance. Think of the output cross-coupled gates of a master-slave flip-flop and add inputs to the gates to ...
Spehro Pefhany's user avatar
2 votes

Clock domain cross and metastablilty problem

Yes, if the metastable state occurs, it is equally likely to resolve to either 0 or 1. This really doesn't matter in the grand scheme of things, however. Let's say that the asynchronous input makes a ...
Dave Tweed's user avatar
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2 votes
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VHDL: Metastability check for hold time fails

I am answering because I can't comment. You have deasserted le before changing the input d in your testbench. In the DUT, ...
Vinay Madapura's user avatar

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