How Computers Work Lecture 8 Asynchronous State Machines and Metastability

Yüklə 514 b.
ölçüsü514 b.

How Computers Work Lecture 8 Asynchronous State Machines and Metastability

Maximum Frequency


What we will cover today: The Trouble with Asynchronous Inputs

A Synchronizer as Arbiter

What Happens Now?

Buridan’s Ass

The Game Show Arbiter

A Simpler Arbiter

With Both Inputs High

Static Metastability

Static Metastability: Inverted Pendulum Analogy

What does Heisenberg Say ?

What if we relax accuracy?

The Remarkable Fact

Dynamic Metastability (More Stupid Classroom Tricks)

Lecture Demonstration Circuit

Is It Possible to Build This?

Practical Metastability

A Simple Model of Static Metstability

Evolution of Vout

How Small must Vin(0) be to make time to saturation take longer than time t?

If Vin(0) is uniformly distributed:

How much time do we need to achieve a certain pmetastable?

Example: How Long for 1 failure / year?

How often will failures occur if we wait 100 ns?

Is this a Good Excuse For Cruising a Light?

What did we Learn Today?

  • If we violate setup or hold times, a flip-flop can give a random digital output.

  • If we violate setup or hold times, we can’t bound the propagation delay of a flip-flop.

  • Metastability usually causes strange outputs, but flip-flops are sold that have valid, stable, outputs while internal nodes are metastable. They can still change their minds when coming out of metastability.

  • In practice, we can choose a propagation time that will have a forever stable output “most” of the time.

  • If we wait long enough (typ. 10-100 ns) “most of the time” is almost all of the time.

  • We can easily detect when settling happens, but we can’t say how long it will take.

Dostları ilə paylaş:

Verilənlər bazası müəlliflik hüququ ilə müdafiə olunur © 2019
rəhbərliyinə müraciət

    Ana səhifə