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.

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16 votes
6 answers
5k views

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. ...
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11 votes
2 answers
14k views

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 ...
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8 votes
2 answers
2k views

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 ...
  • 105
7 votes
3 answers
1k views

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 ...
5 votes
2 answers
1k views

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 ...
3 votes
1 answer
172 views

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 ...
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3 votes
1 answer
293 views

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 (...
  • 31
3 votes
1 answer
839 views

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 ...
3 votes
1 answer
402 views

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 ...
  • 44.8k
2 votes
2 answers
982 views

Metastability simulation

I am trying to observe the metastability by simulating (LTSpice) a chain of inverters and probe the signals in between. The oscillation never happens (I put more than 5 inverters to ensure enough ...
  • 832
2 votes
1 answer
364 views

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 ...
  • 171
2 votes
3 answers
157 views

Clock domain cross and metastablilty problem

I understand the problem of metastability and understand that we can't get a stable value in a bounded time so we need unbonded time but it is not practical, so we put another flip flop with no ...
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2 votes
2 answers
102 views

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 ...
  • 230
2 votes
2 answers
114 views

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 ...
  • 230
2 votes
2 answers
385 views

VHDL: Metastability check for hold time fails

I'm trying to model an SN74HC573 D-type latch in VHDL to get back into it. Here's what I got so far: ...
  • 244
2 votes
1 answer
71 views

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 ...
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2 votes
2 answers
474 views

Crossing independent domain clocks (slow to fast)

I have 2 time domain clocks (completely independent) and a bit stream (single bit) The first clocks is at 12.29 MHz . I want to asynchronously reclock it to a second time domain. Meta stability is ...
  • 339
1 vote
1 answer
1k views

Violating the minimum clock pulse width of a D-type flip flop

Any D-type flip flop has a specification for a minimum clock pulse width. For example, the 74LVC374 has a typical time of 1.5ns for Vcc=3V. But what can happen to the flip flop if a shorter pulse is ...
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1 vote
1 answer
105 views

Problem of metastability: simulation of dual port ram

i simulate Dual Port Ram Register in VHDL and have some doubts about the metastability problem. Dual Port Ram has 2 clock signal, one for Port A , the second for Port B. In my simulation I create two ...
1 vote
1 answer
91 views

Do any flip-flops use separate clock levels for "sample" and "propagate" events

Many kinds of sequential logic require that the output of one register be fed into the input of another register which is strobed by the same clock. In such logic, it's necessary to ensure that a ...
  • 44.8k
1 vote
1 answer
175 views

FPGA SPI slave doesn't work if driving it with the fast FPGA clock instead of with the SPI master clock (oversampling)

I have an slave SPI device implemented within an FPGA (Basys 3). I have had problems to route the SPI clock signal provided by a master to my slave device through one of the board PMOD pins (see this ...
  • 1,057
1 vote
3 answers
2k views

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

Intuitively, I understand that with an even number of stages, the output of the last would be the same logic level as the input of the first, so that the output eventually latches to a certain logic ...
1 vote
1 answer
145 views

Frequency-to-Voltage Converter: Unexpected Stability Regions

I am using an AD652 as a synchronous frequency-to-voltage converter in the layout shown below. I conducted a series of measurements of the output voltage (Vout) across a wide range of input ...
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1 vote
2 answers
95 views

FPGA Simulation Metastability

I have a quick question related to metastability during transfer of data across two clock signals. For a homework assignment, I designed a VHDL module to transfer data across two clock signals. To ...
1 vote
0 answers
156 views

What is the best design practice to view multiple clocks that are generated from a single PLL within an FPGA?

Assume we have two clocks of 100 mhz and 200 mhz both generated from a PLL within an FPGA. If they are seen as two independent clock domains, then everything should work fine in the design, but there ...
0 votes
2 answers
53 views

How to handle metastable input using a microcontroller? [closed]

I have a metastability problem and I want to know how to best handle it using the C language. The problem I have two GPIOs that define the state of my system. Because of the conditioning circuit, they ...
  • 591
0 votes
1 answer
908 views

Why double synchronizer alone is not enough for multi byte transfer between two clock domains?

When one bit information is transferred between two different clock domains, we use 2 Flip-flops or double synchronizers. But when we transfer multi bit signals across two different clock domains, why ...
0 votes
2 answers
332 views

Metastability in 3 or 2 flop synchronizer if input is valid for at least 2 clocks

In this image: Figure 1: metastability in 2 or 3 flop synchronizer If the metastability of first flop doesn't get resolved in 4th clock, is it possible that it may get resolved to '0' in 5th clock? ...
  • 1
0 votes
1 answer
1k views

How is asynchronous reset physically implemented in a flip-flop?

In Cliff Cumming's excellent paper on asynchronous vs synchronous resets, the following paragraph about the risk of metastability appears on page 19: Attention must be paid to the release of the ...
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0 votes
1 answer
5k views

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

I understand that stability for an LTI system is defined with respect to Bounded input bounded output condition. However I'm not clear on why non repeated poles on the imaginary axis makes the system ...
  • 354
0 votes
1 answer
563 views

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

I was reading sunburst paper on Clock Domain Crossing and got stuck with this doubt. Here in the 3rd flip flop, input is metastable state but at the rising edge of the clock output was set to high. ...
  • 2,992
0 votes
2 answers
521 views

Temperature and Voltage variation affects on Metastability

Are there any effects of temperature and voltage variations on Metastability of the flip-flop? If Yes, What are the effects?
0 votes
3 answers
135 views

What is the difference between "2 synchronize" and "metastability"?

As far as I know, 2 synchronize used to be used in in asynchronous FIFO for preventing metastability in multi clock domain. as the below, cross clock domain databus But I came across about ...
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0 votes
0 answers
89 views

Flipflop value a long time after startup

After power up in an ASIC, is it save to assume that all flipflops are in a stable state ('0' or '1')? I do not care in which stable state they are, but it is crucial that all metastability has ...
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