[SOLVED] Can metastability occur on a latch ?

[mr_vasanth]

Unlike flipflop, the latch is a level sensitive logic.

In a level sensitive logic, how metastability can occur ?

 

[slutarius]

There is no Metastability, but there is possibility of glitch at the latch output in case Enable and Data pins switching at the same time.

 

[mr_vasanth]

There is no Metastability, but there is possibility of glitch at the latch output in case Enable and Data pins switching at the same time.

Even if the input changes close to the closing edge of the latch enable also metastability will not occur ?

 

[slutarius]

Even if the input changes close to the closing edge of the latch enable also metastability will not occur ?

As I mentioned, if data and enable change together, or say very closed to each other, we only can see a glitch at the output.
There is no data lost of signal wrong level after that.

 

[jt_eaton]

You can build a flip flop out of two latches with an inverter on one of the enables so yes a latch can go metastable. That happens inside the latches in a flip flop when the flop goes metastable.

Any time you try to change the state of a latch but remove the gate to soon then you run the risk that the latch will be in the process of switching and then not have enough input to guarantee to finish the transition. The feedback loop in the latch then forces to one state or the other but may take longer than expected to get there. That is metastability.

John Eaton

 

[mr_vasanth]

You can build a flip flop out of two latches with an inverter on one of the enables so yes a latch can go metastable. That happens inside the latches in a flip flop when the flop goes metastable.

Any time you try to change the state of a latch but remove the gate to soon then you run the risk that the latch will be in the process of switching and then not have enough input to guarantee to finish the transition. The feedback loop in the latch then forces to one state or the other but may take longer than expected to get there. That is metastability.

John Eaton

Hi John,

I completely agree with your response. But can a standalone latch in the design go metastable ? I am skeptical about that because a standalone latch also involves setup and hold check time.

 

[FvM]

But can a standalone latch in the design go metastable ?

Yes, if the latch enable is deasserted near data edge and setup or hold time are violated.

 

[mr_vasanth]

Yes, if the latch enable is deasserted near data edge and setup or hold time are violated.

Thanks FvM. I believe if data edge changes near latch enable assertion it will not go metastable as the latch can borrow time when it is open. Metastability is possible only during latch enable deassertion. Is my understanding correct?

 

[grvkpr18]

Latch will go metastable at the closing edge, not the opening edge. It can borrow time only for the opening edge. When data toggles near the closing edge, it can go metastable.