STA

Static timing analysis (STA) is a vast domain involving many sub-fields. It involves computing the limits of delay of elements in the circuit without actually simulating it. In this post, we have tried to list down all the posts that an STA engineer cannot do without. Please add your feedback in comments to make reading it a more meaningful experience.

  • Metastability - This post discusses the basics of metastability and how to avoid it.
  • Lockup latch - The basics of lockup latch, both from timing and DFT perspective have been discussed in this post.

  • Clock latency - Read this if you wish to get acquainted with the terminology related to clock latency

  • Data checks - Non-sequential setup and hold checks have been discussed, very useful for beginners

  • Synchronizers - Different types of synchronizers have been discussed in detail

  • On-chip variations - Describes on-chip variations and the methods undertaken to deal with these
  • Temperature inversion - Discusses the concept of temperature inversion and conductivity trends with temperature

  • Timing arcs - Discusses the basics of timing arcs, positive and negative unateness, cell arcs and net arcs etc.

  • Basics of latch timing - Definition of latch, setup time and hold timing of a latch, latch timing arcs are discussed

XOR/XNOR gate using 2:1 MUX

2-input XOR gate using a 2:1 multiplexer: As we know, a 2:1 multiplexer selects between two inputs depending upon the value of its select input. The function of a 2:1 multiplexer can be given as:

OUT = IN0 when SEL = 0 ELSE IN1

Also, a 2-input XOR gate produces a ‘1’ at the output if both the inputs have different value; and ‘0’ if the inputs are same. The truth table of an XOR gate is given as:

A
B
OUT
0
0
0
0
1
1
1
0
1
1
1
0
Truth table of XOR gate

In the truth table of XOR gate, if we fix a value, say B, then

OUT = A WHEN B = 0 ELSE A’


Both the above equations seem equivalent if we connect negative of IN0 to IN1 in a multiplexer. This is how a 2:1 multiplexer will implement an XOR gate. Figure 1 below shows the implement of a 2-input XOR gate using a 2:1 Multiplexer.

An XOR gate can be implemented from a mux simply by connecting the select to one of the inputs, and the inputs to A and Abar respectively.
Implementing a 2-input XOR gate using a 2:1 Multiplexer


I hope you’ve found this post useful. Let me know what you think in the comments. I’d love to hear from you all.


2-input XNOR gate using a 2:1 multiplexer: Similarly, the truth table of XNOR gate can be written as:

A
B
OUT
0
0
1
0
1
0
1
0
0
1
1
1
Truth table of XNOR gate

In the truth table, if we fix, say A, then

OUT = B WHEN A = 1, ELSE B’


Thus, XNOR gate is the complement of XOR gate. It can be implemented if we connect A to IN1 and Abar to IN0.

An XNOR gate can be implemented from a mux simply by connecting the select to one of the inputs, and the inputs to A and Abar respectively.
2-input XNOR gate using 2:1 multiplexer


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