Saturday, February 13, 2016

Metastability and the Combinatorial Logic

In digital electronics any signal can hold four values that are  
  • 0 - LOW
  • 1 - HIGH
  • x - Unknown - Either 0 or 1.
  • z - High Impedance

Consider a case where we have D flip-flop and there is change in data signal (D) at positive edge of the clock (clk). As shown in shaded reason in timing diagram.

D Flip-Flop and Timing Diagram

Here we can not predict the output (Q) because 'D' is unstable at positive edge of 'clk'. So it is Neither 0 nor 1. This state is called metastable state. Generally it accrues when there are setup and hold violations in design. We should take care of it in advance otherwise it can make whole system metastable.

There is one more condition which can make system unstable that is due to combinational logic change. Let us take an example of 2 input AND gate. Truth table of 2 input AND gate is
AND Gate and Truth Table
AND Gate and Truth Table
From the above truth table the output of AND gate is ‘zero’ for input ‘01’ and ‘10’. When input change from ‘01’ to ‘10’, it can take following two path
AND Gate and Transitions between 01 to 10 and via-versa 
  1.     01 -> 00 -> 10
  2.     01 -> 11 -> 10
When it takes 1st path then output remains ‘0’, but when it take 2nd path then output goes from 0 to 1 then 0. This HIGH output accrue for very short duration. This is called Glitch and this may cause metastability in system. Similar behavior for OR gate also there is LOW output accrue for very short duration.
OR Gate and Transitions between 01 to 10 and via versa
OR Gate and Transitions between 01 to 10 and via-versa
Sometime we can see this behavior in netlist simulation. Designers use Gray Code Technique where small glitch can affect the design. In Gray code only one bit change at a time.

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