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Monday, January 22, 2018

Unateness of Complex Circuit: Timing Arc


STA & SI:: Chapter 1: Introduction
1.1a 1.1b 1.1c 1.2a 1.2b
INTRODUCTION Timing Arc Unate: Timing Arc Unateness of Complex Circuit: Timing Arc LIB File syntax for Logic Gates: Timing Sense LIB File syntax for Complex Circuit: Timing Sense

Unateness of Complex Circuit: Timing Arc:


In last article (Unate: Timing Arc), we have discussed about the unateness property of Timing arc with respect to Logic gates. In the Timing Library, "Timing Arc information" is stored with the syntax "timing_sense".

In this article, we are trying to extend timing arc concepts from simple "Logic gate" to complex combinational circuit. For that first we need to understand how we can calculate or figure out the overall unateness of a complex circuit or say a system. To understand this, We start with few standard logic functions like AOI (AND-OR-Inverter) which is not that complex but help us to understand the concept of unateness in system.

To understand the Timing Arc concept for combinational circuit, we should know how Timing Arc of a system calculated. Let's take an example to understand this.



In the above circuit, you can see that there are 2 type of Timing Arc (Net Timing Arc and Cell Timing Arc).
  • Net Timing Arc is always Positive Unate.
  • Cell Timing Arc, we have already discussed in previous Article. (Unateness of Logic Gates )
    • NAND Gate - Negative Unateness
    • NOT Gate - Negative Unateness
    • NOR Gate - Negative Unateness

Note: To know more about the Unateness of Inverter, please read Article

So, If I want to understand the behaviors of signal at Y with respect to A (remember only A, Not with respect to other pins like B & C), then we can conclude as
  • Rising Input at A - Falling Output at Y or No Change.
  • Falling Input at A - Rising Output at Y or No Change.


If you want to cross check this, there are several ways but I am going to explain (or say cover here) 3 ways.
  1. Truth table Method
  2. Circuit Method
  3. Function Method

1) Truth Table Method

Below is the Truth table ("Table_1") of AND-OR-INVERTER circuit of "Figure_1". I have highlighted all cases when A changes from 0 to 1 (keeping all other inputs constant at a time), you can see that output either "Not Changing" or changing from 1 to 0. You can try reverse case also (A changes from 1 to 0).


So in summary, I can say that it's a Negative Unate at Y with respect to A. I am not describing much about this method because we already studied this in previous Article (Unate: Timing Arc)

In a similar way, if you will try with respect to B, you will find similar result (Negative Unate at Y with respect to B).

2) Circuit Method

I am sure, you might be thinking about the shortest way to figure out the Unateness between 2 input-output combination. Because using the Truth table is not Feasible every time and it's Time consuming also. Lets try to understand the circuit method.

Below table (Table_2) help you to understand the Unateness from a Overall system point of view. It's very simple. I have explained with respect to 2 System (Output of 1st system become input of Second system). Using this table, we will try to understand the overall unateness of any complex circuit.


Now, if above table (Table_2) is clear - let's try to understand how this table help us in AOI case (AND-OR-INVERTER) (Figure_1).
  • A to u1   - System 1   - Negative Unate (AND gate)
  • u1 to u2   - System 2   - Positive Unate (Wire/Net)
  • u2 to u3   - System 3   - Negative Unate (NOT gate)
  • u3 to u4   - System 4   - Positive Unate (Wire/Net)
  • u4 to Y   - System 5   - Negative Unate (NOR Gate)

So now, if you will see across different Systems (from 1 to 5), you can see that overall unateness is Negative Unate.

3) Function Method

How will you identify the unateness in case you dnt have circuit, you only have equation (Boolean Equation) of circuit or design? Again, If I will ask you that draw a circuit or create a truth table, then I am sure, you will try to skip it. But there is a solution of that. :)

For that, below definitions can help you to determine unateness of any variable of Function.
  • f is “positive unate” function in a dependent variable "x" if x’ does not appear in the sum-of-products representation.
  • f is “negative unate” in a dependent variable "x" if x does not appear in the sum-of-products representation.
  • f is “non unate” (sometime known as biunate in switching theory) in a dependent variable "x" if you can not write a sum-of-products representation without appearing x and x' both together. Means both be the part of SOP.

For example 1: F(w, x, z)= wx + w’z’
In the above function, if you try to implement above definition, you can easily figure out that
  • Positive unate with respect to x
  • Negative Unate with respect to z
  • Non-unate with respect to w

For example 2: F(w, x, z)= wx + w’z’
Now, question is what will be with respect to y.
If you will see the equation, it's very much clear that even if you give rising edge or falling edge at "y", output is not going to change. That's means it's neither Negative unate nor Positive unate nor Non-Unate. I am sure, now you may be confuse that what's this? :) Right now, I am leaving this as a open topic of discussion for later on. You can comment about these type of variables.

Now, if above function definition is clear - let's try to understand how these can be implemented in AOI case (AND-OR-INVERTER) (Figure_1).

Figure_1 can be written in the form of equations as: Y = ((A.B) + C)'

Let's open in simplified SOP form.

Y = ((A.B) + C)'
  = ((A.B)').(C)'
  = (A'+B').C'
  = A'C' + B'C'

Now, you can easily say -
  • Y is Negative unate with respect to A
  • Y is Negative Unate with respect to B
  • Y is Negative Unate with respect to C

In Summary: We can use any method as per our convenience to see the unateness of a system or circuit. Most of the time, this is already part of Lib file, but to understand the tool behavior, we should have these understanding.

In next article, we will discuss Unateness of OAI (OR-AND-Inverter), MUX and few other complex circuit.

1 comment:

  1. For example 2 Y is redundant variable

    ReplyDelete

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