Gate-Level Minimization Mano & Ciletti Chapter 3 By Suleyman TOSUN Ankara University
Outline Intro to Gate-Level Minimization The Map Method 2-3-4-5 variable map methods Product-of-Sums Method Don t care Conditions NAND and NOR Implementations
Gate-Level Minimization Finding an optimal gate-level implementation of Boolean functions. Difficult to perform manually. Can use computer-based logic synthesis tool Exp: espresso logic minimization software Karnough Map (K-map) can be used for manual design of digital circuits.
The Map Method The truth table representation of a function is unique. But, not the algebraic expression Several versions of an algebraic expression exist. Difficult to minimize algebraic functions manually. The map method is a simple proceure to minimize Boolean functions. Pictorial form of a truth table. Called Karnough Map or K-Map.
Two-Variable Map Four Minterms Two variables Four squares for four minterms Figure b shows the relationship between the squares and the variables x and y.
Two-Variable Map (Cont.) May only be useful to represent 16 Boolean functions. Exp: If m1=m2=m3=1 then m1+m2+m3=x y+xy +xy=x+y (OR function)
Three-Variable Map There are 8 minterms for 3 variables. So, there are 8 squares. Minterms are arranged not in a binary sequence, but in sequence similar to the Gray code.
Three-Variable Map (Cont.) The square for square m5 corresponds to row 1 and column 01 (101). Another look to m5 is m5=xy z. When variables are 0, they are primed (ex: x ). Otherwise not primed (x).
Three-Variable Map (Cont.) Two adjacent squares differs one variable (one primed other is not). So, they can be minimized Ex: m5+m7=xy z+xyz=xz(y +y)=xz So, try to cover as many adjacent squares as possible by the orders of two. 1,2,4,8 squares that has the logical value1.
Example 1 Simplfy the Boolean function F(x,y,z)=Σ(2,3,4,5) F(x,y,z)=x y+xy
Adjacent squares Some adjacent squares don t touch each other. m0 is adjacent to m2 and m4 is adjacent to m6. m0+m2=x y z +x yz =x z (y +y)=x z m4+m6=xy z +xyz =xz (y +y)=xz
Example 2 Simplfy the Boolean function F(x,y,z)=Σ(3,4,6,7) F(x,y,z)=yz+xz
Example 3 Simplfy the Boolean function F(x,y,z)=Σ(0,2,4,5,6) F(x,y,z)=z +xy
Example 4 F=A C+A B+AB C+BC Express F as a sum of minterms. F(A,B,C)=Σ(1,2,3,5,7) Find the minimal sum-of-products expression F=C+A B
Four-Variable Map 16 minterms (and squares) for 4 variables.
Example 5 Simplify F(w,x,y,z)=Σ(0,1,2,4,5,6,8,9,12,13,14) F(w,x,y,z)=y +w z +xz
Example 6 Simplfy F=A B C +B CD +A BCD +AB C F=B D +B C +A CD
Prime Implicant A product term obtained by combining the max. possible number of adjacent squares. If a minterm is covered by only one prime implicant, that prime implicant is said to be essential.
Example F(A,B,C,D)=Σ(0,2,3,5,7,8,9,10,11,13,15)
Example F=BD+B D +CD+AD=BD+B D +CD+AB =BD+B D +B C+AD=BD+B D +B C+AB
Five-Variable Map Not simple, is not usually used. 5 variables, 32 squares.
Example 7 Simplfy F(A,B,C,D,E)=Σ(0,2,4,6,9,13,21,23,25,29,31) F=A B E +BD E +ACE
Product-of-Sums Simplification Mark the empty squares by 0 s. Combine them (as we did for sum-ofproducts) We obtain F (complement of the function) Take the complement of F ((F ) ) to obtain F.
Example 8 Simplfy F(A,B,C,D)=Σ(0,1,2,5,8,9,10) F=B D +B C +A C D F =AB+CD+BD => F=(A +B )(C +D )(B +D)
Example 8 gate impl. Two different implementations of the same function.
Don t Care Conditions In practice some combinations are not specified as 1 s or 0 s. Four bit binary codes has six unused combinations. Functions having unspecified outputs are called incompletely specified functions. We don t care the unspecified minterms These minterms are called don t-care conditions. They can be used for minimization. They are indicated as X s in the map. They can be assumed as 1 s or 0 s to have best simplification.
Example 9 Simplify F(w,x,y,z)=Σ(1,3,7,11,15) having don t care conditions d(w,x,y,z)= Σ(0,2,5)
NAND and NOR Implementation Generally used for circuit design Easier to fabricate. Basic gates Other functions can be generated from them. Rules have been developed to convert functions to NAND and NOR only implementations.
NAND Circuits NAND gate is universal Any digital system can be implemented with it. AND, OR and NOT can be implemented with NANDs.
Two graphic symbols of NAND
Two-Level Implementation Have the function in sum-of-product form. Put bubbles (inverters) to have two different representations of NAND gate Either AND-invert or Invert-OR
Example: F=AB+CD
Example 10 Implement F(x,y,z)=Σ(1,2,3,4,5,7) with only NAND gates.
Multilevel NAND Circuits F=A(CD+B)+BC
NOR Implementation Dual of NAND operation
Example: F=(A+B)(C+D)E
Example: F=(AB +A B)(C+D )
Exclusive-Or (XOR) Function XOR x y xy xy 1 if only x is one or if only y is 1 XNOR ( x y) xy xy 1 if both inputs are 1 or both inputs are 0 Some identities of XOR x 0 x x 1 x' x x 0 x x'1 x y' x' y ( x y)'
XOR Implementations
Odd Funct on N variable XOR function is odd function defined as The logical sum of the 2 n /2 minterms whose binary numerical values have an odd number of 1 s. If n=3, 4 minterms have odd number of 1 s.
Logic Diagram of odd and even functions
Parity Generation A parity bit is an extra bit included with a binary message If number of 1 s is odd, parity bit is 1; else 0. The circuit that generates the parity bit in the transmitter is called parity generator. Parity bit can be generated using XOR function.
3-Bit Parity Generator
Parity Checker Bits are transmitted to the destination with parity. The circuit that checks the parity in the receiver is called a parity checker. Parity checker can be implmeneted with XOR gates.
3-Bit Parity Checker