Electrical Engineering Stack Exchange
Electrical Engineering Stack Exchange

Questions tagged [metastability]

Questions about the behaviour of FlipFlops when the normal input setup or hold requrements are not respected, usually when launch and capture clocks are not synchronous.

When the D-input to a flop is not stable and in the relevant logic 0 or logic 1 voltage range at the time of the clock edge, the feedback paths within the flop can balance each other, and the output of the flop can (in the extreme case) remain at mid-rail until the subsequent clock edge.

This has a consequent effect on the following logic stages, so needs careful circuit design to manage the effects.

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Why do cascading D-Flip Flops prevent metastability?

I understand what metastability is but don't understand how linking together flip flops reduces this? If the output of the first flipflop is metastable, this gets used as input for the second one. But I don't see how the 2nd flip flop will be able…
Wouter A
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How does 2-ff synchronizer ensure proper synchonization?

Using 2-ff synchronizers has been a standard for a signal to cross clock boundaries. And there are lots of paper/figures illustrating the mechanism, such as this one: It seems bclk can only sample the pulse of adat once (at the second rising edge of…
fiedel
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Why do we need to synchronise asynchronous inputs in FSM?

I have been newly learning digital electronics. I know that there are gates which perform logic functions, I learned about RS-Latch, D-Latch and Master-Slave-D-Flip-Flop. Now that I can perform logic functions and store states, I can create a…
Eren
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After metastability, does the value eventually settle to the correct value?

I'm confused about metastability. I know that metastability is the condition wherein the output of a flip flop becomes unpredictable (either high or low) for some "duration of time"... Okay, from this definition, it states that metastability will…
ReubenMijares
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Understanding metastability in Technion Paper

I am trying to understand metastability as introduced in the Technion IEEE paper (link). But I am struggling a little with some of the concepts, and wanted to ask about that. This is the flip-flop circuit they are using: Question: They mention that…
Mohammed Arshaan
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Metastability error propagation with flip flop

I do have a confusion regarding the metastability resolution using flip flops , I know that I should add synchronizer of two or three d-flip flop to guarantee a safe transmission at clock domain crossing boundaries, but my confusion is that the…
Hachani Ahmed
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Why don't 2 flip-flop synchronizers have a reset?

This is similar to this question, asking if a reset is needed in a 2 flip-flop synchronizer. The answer to that question was: "no, not necessarily". So, my question is: Why do almost all of the 2 flip-flop (or N flip-flop) synchronizer…
Harry
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D-type LATCH output when minimum clock/enable pulse width is violated and input/output are the same

Can the output of a D-type latch go metastable when: There is a minimum CLK/ENable pulse width violation The input and the output of the latch have the same value?
Viktorinox
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How do I model a simple metastable flip-flop in ngspice?

Problem I'm trying to simulate the simplest possible model for a flip-flop: two inverters connected in a circle. I'm using ngspice 31 on Arch Linux. I based my model on the CMOS SOI Inverter example…
tg85
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Metastability Deserialization and clock crossing domain

I have a question on metastability and clock crossing domain. I need to deserialize a bitstream out of an ADC. TXCLK, TXOUT1, INCLK are the outputs of the ADC. So the idea was to register the DATA TXOUT1 on a shift register clocked by TXCLK then…
the dude
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Output of a D flipflop upon power up?

I guess the output state of a D-flipflop is unknown upon power up. But what are the chances that it is neither 0 nor 1 but an intermediate state such as VDD/2? The D-flipflop in this question has an output driver inverter.
Niranjan Kulkarni
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Do Schmitt-trigger specs give requirements to avoid metastability?

Although Schmitt triggers are not usually regarded as latches with setup/hold constraints, a Schmitt trigger is functionally a sort of latch which is forced one way when the input is above a certain threshold, forced the other way when the input is…
supercat
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Why is a reset with asynchronous assert safe?

As far as I understand, a reset with asynchronous assert, synchronous de-assert is considered absolutely safe. I understand that this prevents metastability at the output of a flip-flop using that reset because recovery and removal requirements are…
Harry
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How to calculate the number of required flip-flop stages needed for clock-domain crossing?

In a given scenario where I have two clock domains driven by a 200MHz and a 30 MHz external independent clocks, what would be the best way to calculate the number of flip-flop stages needed for proper clock-domain crossing? At the moment, I'm…
nanoeng
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4-bit synchronous counter IC: Do I need to pre-synchronize the count enables? (Metastability?)

Using a 74LVC163 synchronous 4-bit binary counter and would like to predicate the counting on an asynchronous signal. If I apply this signal directly to one of the enables (e.g. CEP), am I assured that the counter will either count or not count…
Aaron
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