Product of Sum

1. Definition and Basic Concept

Product of Sum: Definition and Basic Concept

The Product of Sum (POS) is a canonical form in Boolean algebra that represents a logical function as a conjunction (AND) of disjunctive (OR) terms. It is the dual of the Sum of Products (SOP) form and is particularly useful in digital logic design, optimization, and circuit synthesis.

Mathematical Formulation

A POS expression is written as:

$$ F(X_1, X_2, \dots, X_n) = \prod_{i=1}^k \left( \sum_{j=1}^m X_j \right) $$

where:

Maxterms and Canonical POS

In canonical POS, each OR term is a maxterm, which evaluates to 0 for exactly one combination of inputs. For an \( n \)-variable function, there are \( 2^n \) possible maxterms. A maxterm \( M_i \) is constructed such that:

$$ M_i = \sum_{j=1}^n \ell_j $$

where \( \ell_j \) is the literal \( X_j \) (if the corresponding bit in the binary representation of \( i \) is 0) or \( \overline{X_j} \) (if the bit is 1).

Example: 2-Variable POS

Consider a Boolean function \( F(A, B) \) with the following truth table:

A B F(A, B)
0 0 0
0 1 1
1 0 1
1 1 0

The canonical POS is derived by identifying the rows where \( F = 0 \) and writing the corresponding maxterms:

$$ F(A, B) = (A + B) \cdot (\overline{A} + \overline{B}) $$

Practical Applications

POS forms are widely used in:

Comparison with SOP

While SOP is a disjunction of minterms (AND terms), POS is a conjunction of maxterms (OR terms). The choice between SOP and POS depends on:

For example, the function \( F = \overline{A}B + A\overline{B} \) in SOP is equivalent to \( F = (A + B)(\overline{A} + \overline{B}) \) in POS.

1.2 Comparison with Sum of Products (SOP)

The Product of Sum (POS) and Sum of Products (SOP) are two canonical forms of Boolean expressions, each with distinct structural and computational properties. While both can represent any Boolean function, their differences impact circuit design, optimization, and performance.

Structural Differences

POS expresses a Boolean function as a conjunction (AND) of disjunctive (OR) terms, whereas SOP is a disjunction (OR) of conjunctive (AND) terms. For a function f with variables A, B, C, the forms are:

$$ \text{POS: } f = (A + B + C)(\overline{A} + B + \overline{C}) $$
$$ \text{SOP: } f = \overline{A}BC + A\overline{B}C + AB\overline{C} $$

POS naturally represents functions where the output is 0 for specific input combinations, while SOP captures cases where the output is 1.

Karnaugh Map Interpretation

In K-map optimization, POS groups 0 cells to form maxterms, while SOP groups 1 cells for minterms. For example, a K-map for the function f(A,B):

0 1 1 0 A=0 A=1 B=0 B=1

SOP yields f = A\overline{B} + \overline{A}B, while POS gives f = (A + B)(\overline{A} + \overline{B}).

Practical Implications

Conversion Between Forms

De Morgan’s theorems enable conversion between POS and SOP:

$$ \text{POS to SOP: } \overline{(A+B)} = \overline{A} \cdot \overline{B} $$
$$ \text{SOP to POS: } \overline{A \cdot B} = \overline{A} + \overline{B} $$

This duality is exploited in logic synthesis tools to optimize for area, speed, or power.

1.3 Standard and Canonical Forms

Definition and Mathematical Representation

The Product of Sum (POS) expression exists in two principal forms: standard and canonical. The canonical POS, also known as the maxterm expansion, is a complete representation where each sum term (maxterm) contains all variables in either true or complemented form. For a Boolean function of n variables, the canonical POS consists of up to 2n maxterms.

$$ F(X,Y,Z) = (X + Y + Z)(X + \overline{Y} + Z)(\overline{X} + Y + \overline{Z}) $$

Standard POS Form

In contrast to canonical form, the standard POS does not require every sum term to include all variables. This often leads to more compact expressions. For example:

$$ F(A,B,C) = (A + B)(\overline{A} + C) $$

Here, the first term lacks C, and the second term lacks B. Standard POS forms are derived through algebraic simplification or Karnaugh map reduction of canonical forms.

Conversion Between Forms

To convert a truth table to canonical POS:

  1. Identify all input combinations where the output is 0.
  2. For each such combination, create a maxterm by OR-ing the variables (complemented if their value is 1, uncomplemented if 0).
  3. AND all maxterms together.

For example, given a truth table with F(0,1,1) = 0, the corresponding maxterm is X + \overline{Y} + \overline{Z}.

Practical Applications

Canonical POS finds use in:

Optimization Considerations

While canonical forms provide unambiguous representations, they are rarely used in practical implementations due to their verbosity. Modern synthesis tools:

$$ \text{Optimized POS} = \prod_{i=1}^{k} \left( \sum_{j=1}^{n_i} l_{ij} \right) $$

Where k ≤ 2n and ni ≤ n, showing the reduction from canonical form.

2. Truth Table to POS Conversion

2.1 Truth Table to POS Conversion

Fundamentals of Product of Sum (POS) Representation

The Product of Sum (POS) form is a canonical representation of a Boolean function where the output is expressed as a conjunction (AND) of disjunctive (OR) terms. Unlike the Sum of Products (SOP) form, which focuses on minterms (input combinations where the output is 1), POS operates on maxterms (input combinations where the output is 0). This makes POS particularly useful in scenarios where the function's off-set (zeros) is smaller or more logically intuitive than its on-set (ones).

Step-by-Step Conversion from Truth Table to POS

To convert a truth table to POS form, follow these steps:

  1. Identify Maxterms: For each row in the truth table where the output is 0, note the input combination. Each such combination corresponds to a maxterm.
  2. Express Maxterms as Sums: For each maxterm, create a sum (OR) term where each variable appears in its complemented form if it is 1 in the input combination, and in its uncomplemented form if it is 0.
  3. Combine Sum Terms with AND: The final POS expression is the product (AND) of all the sum terms derived from the maxterms.

Example Conversion

Consider a 3-input Boolean function with the following truth table:

A B C Output (F)
0 0 0 1
0 0 1 0
0 1 0 1
0 1 1 0
1 0 0 1
1 0 1 0
1 1 0 1
1 1 1 1

The output F is 0 for input combinations (A,B,C) = (0,0,1), (0,1,1), and (1,0,1). These correspond to the maxterms:

$$ M_1 = A + B + \overline{C} $$ $$ M_2 = A + \overline{B} + \overline{C} $$ $$ M_3 = \overline{A} + B + \overline{C} $$

The POS expression for F is then:

$$ F = (A + B + \overline{C})(A + \overline{B} + \overline{C})(\overline{A} + B + \overline{C}) $$

Practical Applications and Optimization

POS forms are particularly advantageous in PLA (Programmable Logic Array) implementations and certain types of FPGA architectures, where OR-AND structures align naturally with available logic resources. Additionally, POS can sometimes yield more compact representations than SOP for functions with sparse on-sets, reducing the number of required logic gates.

Tools like Karnaugh Maps or the Quine-McCluskey algorithm can further optimize POS expressions by identifying and eliminating redundant terms, though the initial conversion from a truth table remains a straightforward mechanical process.

2.2 Karnaugh Map Simplification for POS

Fundamentals of POS in K-Maps

Karnaugh maps (K-maps) provide a graphical method for simplifying Boolean expressions into their minimal Product of Sum (POS) form. Unlike Sum of Products (SOP), which groups minterms (1s), POS groups maxterms (0s) to represent the complement of the function. The resulting expression is a product (AND) of sums (ORs).

$$ F = \prod (M_0, M_1, \dots) = (A + B)(\overline{A} + C) $$

Step-by-Step POS Simplification

  1. Plot Maxterms (0s): Identify and mark all 0s in the K-map corresponding to the truth table’s output.
  2. Group Adjacent 0s: Form the largest possible rectangular groups of 0s in powers of 2 (1, 2, 4, 8). Wrap-around adjacency is allowed.
  3. Derive Sum Terms: For each group, determine the variables that remain constant. A variable is inverted if it appears as 1 in the group and non-inverted if 0.
  4. Combine Sum Terms: AND all the derived sum terms to form the minimal POS expression.

Example: 3-Variable K-Map

Consider the truth table for \( F(A,B,C) \) with maxterms \( M(0, 2, 4, 6) \):

$$ F = (A + B + C)(A + \overline{B} + C)(\overline{A} + B + C)(\overline{A} + \overline{B} + C) $$

After grouping adjacent 0s in the K-map, the simplified POS becomes:

$$ F = (A + C)(\overline{A} + C) $$

Practical Considerations

Applications in Circuit Design

POS simplification is preferred in scenarios where:

Comparison with SOP

Criteria POS SOP
Grouping 0s (maxterms) 1s (minterms)
Output AND of ORs OR of ANDs
Gate-Level Efficiency Optimal for NOR-based logic Optimal for NAND-based logic
3-Variable K-Map for POS Simplification A 3-variable Karnaugh map (K-map) for Product of Sum (POS) simplification, showing maxterms (0s) and grouped regions with dashed borders. 3-Variable K-Map for POS Simplification A'B' A'B AB AB' C' C C C' 0 0 1 1 0 1 1 0 1 1 0 0 0 0 1 1 (A + B) (A + C') (B' + C') (A + B) (A + C') (B' + C')
Diagram Description: The section involves grouping adjacent 0s in a Karnaugh map, which is a highly visual and spatial process.

2.3 Handling Don't Care Conditions

In digital logic design, don't care conditions refer to input combinations for which the output of a function is unspecified. These conditions arise when certain input states are either impossible or irrelevant to the system's operation. Exploiting don't cares effectively allows for significant simplification of Boolean expressions, particularly in Product of Sum (POS) implementations.

Mathematical Representation of Don't Cares

Don't care terms are typically denoted by X or d in truth tables and Karnaugh maps. For a Boolean function f(A,B,C) with don't cares, the specification may appear as:

$$ f(A,B,C) = \sum m(0,2,5) + \sum d(3,7) $$

Here, m(0,2,5) represents the minterms where the output is 1, while d(3,7) indicates the don't care conditions. In POS form, this translates to maximizing the flexibility in choosing between 0 and 1 for the don't care terms to form the largest possible prime implicants.

Karnaugh Map Optimization with Don't Cares

When constructing a POS expression from a K-map containing don't cares, these X values can be treated as either 0 or 1 to maximize the grouping of adjacent 0s (for POS). Consider the following K-map representation:

AB\CD 00 01 00 01 11 10 0 X 1 0 X 0 1 0

For POS minimization, we group the 0s while strategically including don't cares (X) to form larger clusters. In this example, treating the X at (00,01) as 0 and the X at (01,00) as 1 allows the formation of a prime implicant covering cells (00,00), (00,01), and (01,01).

Algorithmic Approach to Don't Care Handling

The Quine-McCluskey algorithm extends naturally to handle don't care conditions when deriving minimal POS expressions:

  1. Initialization: Separate the specified maxterms (0s) and don't care terms.
  2. Prime Implicant Generation: Treat don't cares as either 0 or 1 to maximize term merging.
  3. Coverage Table Construction: Include only the specified maxterms (not don't cares) in the coverage table.
  4. Essential Prime Implicant Selection: Choose the smallest set of prime implicants that cover all specified maxterms.

Practical Implications in Circuit Design

In physical implementations, properly handled don't care conditions lead to circuits with fewer gates and reduced propagation delays. For example, a BCD-to-7-segment decoder typically utilizes don't cares for input combinations above 9 (1010-1111), allowing up to 30% reduction in gate count compared to a fully specified implementation.

$$ \text{POS}_{\text{optimized}} = (A + B + \overline{C})(\overline{A} + C + D) $$

This optimized POS form, achieved through strategic don't care utilization, demonstrates both the mathematical elegance and practical efficiency gains in digital system design.

K-map with Don't Cares for POS Optimization A 4x2 Karnaugh map showing 0, 1, and X (don't care) values with dashed rectangles indicating prime implicant groupings for POS optimization. 00 01 11 10 AB 00 01 11 10 CD 0 1 0 X X 1 0 0 0 0 0 X 0 1 X 0
Diagram Description: The Karnaugh map visualization is critical to demonstrate how don't care conditions (X) are strategically grouped with 0s to form prime implicants in POS optimization.

3. Logic Gates for POS Implementation

3.1 Logic Gates for POS Implementation

The Product of Sum (POS) form is a canonical representation of Boolean functions where the output is expressed as a conjunction (AND) of disjunctive (OR) terms. Implementing POS efficiently requires careful selection and arrangement of logic gates.

Fundamental Gates for POS Construction

POS expressions are constructed using:

For example, the POS expression:

$$ F = (A + B + \overline{C}) \cdot (\overline{A} + C) \cdot (B + \overline{D}) $$

requires three OR gates (one for each maxterm), one AND gate (for the product), and inverters for $$\overline{A}$$, $$\overline{C}$$, and $$\overline{D}$$.

Universal NAND-NOR Implementation

While POS is naturally implemented using OR-AND networks, in practice, NAND or NOR gates are often preferred due to their universality and fabrication advantages in CMOS technology.

The transformation from standard POS to NOR-only logic follows:

$$ (A + B) \cdot (C + D) = \overline{\overline{A + B} + \overline{C + D}} $$

which requires two levels of NOR operations.

Practical Considerations in VLSI Design

In Very Large Scale Integration (VLSI) circuits:

Modern synthesis tools automatically convert between POS and other forms based on the target technology library's characteristics.

Comparison with SOP Implementation

When compared to Sum of Products (SOP) implementations:

The choice between POS and SOP often comes down to which form yields a more efficient implementation for the specific Boolean function and target technology.

POS Implementation with Logic Gates A schematic diagram showing two parallel implementations of a Product of Sum (POS) expression: left side with OR-AND gates and right side with NOR-only implementation using De Morgan's transformation. POS Implementation with Logic Gates OR-AND Implementation A B C D OR OR AND F NOR-NOR Implementation A B C D NOR NOR NOR F
Diagram Description: The section explains gate-level implementations and transformations (like NOR-NOR conversion), which are highly visual spatial arrangements of logic gates.

3.2 Practical Use Cases in Digital Circuits

The Product of Sum (POS) form is a canonical representation of Boolean functions that finds extensive application in digital circuit design. Unlike the Sum of Products (SOP) form, which is an OR of AND terms, POS is an AND of OR terms, expressed as:

$$ F = \prod_{i} \left( \sum_{j} (x_j) \right) $$

This structure is particularly advantageous in scenarios where the output is naturally expressed as a conjunction of conditions. For instance, in error detection circuits, POS forms simplify the logic by directly encoding failure conditions.

Karnaugh Map Simplification for POS

When minimizing a Boolean function using Karnaugh maps for POS, the zeros of the function are grouped instead of the ones. The resulting simplified expression is derived by complementing the variables and applying De Morgan’s theorem. Consider a 3-variable function F(A, B, C) with the following truth table:

A B C F
0 0 0 1
0 0 1 0
0 1 0 1
0 1 1 0
1 0 0 1
1 0 1 1
1 1 0 0
1 1 1 1

The POS form is obtained by grouping the zeros (F=0) and writing the complemented terms:

$$ F = (A + \overline{B} + C)(\overline{A} + B + \overline{C}) $$

Implementation in Programmable Logic Arrays (PLAs)

PLAs often utilize POS forms to optimize the number of AND-OR gates required. For example, a 4-input function implemented in a PLA using POS may reduce gate count by 30% compared to an SOP implementation, as demonstrated in a 2021 study on FPGA resource optimization.

Case Study: Majority Voting Circuit

A 3-bit majority voter outputs 1 when two or more inputs are high. The POS representation:

$$ F = (A + B)(A + C)(B + C) $$

This requires three 2-input OR gates and one 3-input AND gate, whereas the SOP equivalent would need four 3-input AND gates and one 4-input OR gate. The POS implementation reduces transistor count in CMOS designs by eliminating series-connected PMOS networks.

Timing Analysis and POS Optimization

In ASIC design, POS forms can reduce critical path delay. A 2020 IEEE paper showed that for a 16-bit comparator, the POS-based design had a 12% lower propagation delay than SOP due to reduced fan-in in the OR plane. The trade-off between POS and SOP depends on:

POS vs SOP Delay Comparison Number of Inputs Delay (ps)

Power Consumption Considerations

POS implementations typically exhibit lower static power consumption in technologies where OR gates have fewer leakage paths. For a 45nm CMOS process, measurements show:

$$ P_{static}^{POS} = 0.82 \times P_{static}^{SOP} $$

This advantage diminishes below 28nm due to the dominance of gate tunneling currents, making the choice between POS and SOP technology-dependent.

Karnaugh Map for POS Simplification A 3-variable Karnaugh Map showing zero groupings for Product of Sum (POS) simplification. Variables A, B, and C are labeled on the axes with complemented terms marked. B'C' B'C BC BC' A' A 0 0 1 0 1 0 1 1 A' + B' B' + C Karnaugh Map for POS Simplification Group 1 (A' + B') Group 2 (B' + C)
Diagram Description: The Karnaugh Map simplification process for POS is inherently spatial and requires visualization of zero groupings and variable complementation.

3.3 Advantages and Limitations

Advantages of Product of Sum (POS) Form

The Product of Sum representation offers several key benefits in digital logic design and Boolean algebra:

$$ F = (A+B+C)(\overline{A}+B+\overline{C})(A+\overline{B}+C) $$

Limitations and Practical Considerations

Despite its advantages, POS formulation presents several challenges:

Performance Tradeoffs

The choice between SOP and POS affects multiple performance metrics:

$$ \text{Delay}_{POS} = t_{OR} + t_{AND} $$ $$ \text{Delay}_{SOP} = t_{AND} + t_{OR} $$

While the nominal delay appears symmetric, practical implementations differ due to:

Case Study: POS in Error Correction

In Hamming code implementations, POS form provides a 12-18% reduction in gate count compared to SOP when implementing parity check equations. The inherent OR-AND structure aligns perfectly with the syndrome calculation:

$$ S_0 = (D_1 \oplus D_3 \oplus D_5 \oplus D_7) $$ $$ S_1 = (D_2 \oplus D_3 \oplus D_6 \oplus D_7) $$ $$ \text{Error} = (S_0 + S_1)(\overline{S_0} + S_1)(S_0 + \overline{S_1}) $$

4. Recommended Textbooks

4.1 Recommended Textbooks

4.2 Online Resources and Tutorials

4.3 Research Papers and Advanced Topics