Standard & Canonical Forms

Transcription

1 Standard & Canonical Fors CHAPTER OBJECTIVES Learn Binary Logic and BOOLEAN AlgebraLearn How to ap a Boolean Expression into Logic Circuit Ipleentation Learn How To anipulate Boolean Expressions and Siplify TheLesson OjectivesLearn how to derive a Boolean expression of a function defined by its truth table. The derived expressions ay be in one of two possible standard fors: The Su of in-ters or the Product of ax-ters. 2. Learn how to ap these expressions into logic circuit ipleentations (2- Level Ipleentations). inters Consider a syste of 3 input signals (variables) x, y, & z. A ter which ANDs all input variables, either in the true or copleent for, is called a inter. Thus, the considered 3-input syste has 8 inters, naely: x y z, x y z, x y z, x y z, x y z, x y z, x y z & x y z Each inter equals 1 at exactly one particular input cobination and is equal to 0 at all other cobinations Thus, for exaple, x y z is always equal to 0 except for the input cobination xyz = 000, where it is equal to 1. Accordingly, the inter x y z is referred to as 0. In general, inters are designated i, where i corresponds the input cobination at which this inter is equal to 1.

2 For the 3-input syste under consideration, the nuber of possible input cobinations is 2 3, or 8. This eans that the syste has a total of 8 inters as follows: 0 = x y z =1 IFF xyz = 000, otherwise it equals 0 1 = 2 = x yz =1 IFF xyz = 001, otherwise it equals 0 x yz =1 IFF xyz = 010, otherwise it equals 0 3 = 4 = 5 = x yz =1 IFF xyz = 011, otherwise it equals 0 x y z =1 IFF xyz = 100, otherwise it equals 0 x yz =1 IFF xyz = 101, otherwise it equals 0 6 = xy z =1 IFF xyz = 110, otherwise it equals 0 7 = xyz =1 IFF xyz = 111, otherwise it equals 0 In general, For n-input variables, the nuber of inters = the total nuber of possible input cobinations = 2 n. A inter = 0 at all input cobinations except one where the inter = 1. axters Consider a circuit of 3 input signals (variables) x, y, & z. A ter which ORs all input variables, either in the true or copleent for, is called a axter. With 3-input variables, the syste under consideration has a total of 8 axters, naely: ( x + y + z),( x + y + z),( x + y + z),( x + y + z), ( x + y + z),( x + y + z), ( x + y + z)&( x + y + z ) Each axter equals 0 at exactly one of the 8 possible input cobinations and is equal to 1 at all other cobinations. For exaple, ( x + y + z) equals 1 at all input cobinations except for the cobination xyz = 000, where it is equal to 0. Accordingly, the axter ( x + y + z) is referred to as 0. In general, axters are designated i, where i corresponds to the input cobination at which this axter is equal to 0.

3 For the 3-input syste, the nuber of possible input cobinations is 2 3, or 8. This eans that the syste has a total of 8 axters as follows: 0 = ( x + y + z) =0 IFF xyz = 000, otherwise it equals 1 1 = ( x + y + z) = 0 IFF xyz = 001, otherwise it equals 1 2 = ( x + y + z) = 0 IFF xyz = 010, otherwise it equals 1 3 = ( x + y + z) = 0 IFF xyz = 011, otherwise it equals 1 4 = ( x + y + z) = 0 IFF xyz = 100, otherwise it equals 1 5 = ( x + y + z) = 0 IFF xyz = 101, otherwise it equals 1 6 = ( x + y + z) = 0 IFF xyz = 110, otherwise it equals 1 7 = ( x + y + z ) = 0 IFF xyz = 111, otherwise it equals 1 In general, For n-input variables, the nuber of axters = the total nuber of possible input cobinations = 2 n. A axter = 1 at all input cobinations except one where the axter = 0. Iprtant Result Using De-organ s theore, or truth tables, it can be easily shown that: i = i i =0, 1, 2,...,( 2 1) Expressing Functions as a Su of inters and Product of axters Exaple: Consider the function F defined by the shown truth table n x y z F Now let s rewrite the table, with few added coluns. A colun i indicating the input cobination Four coluns of inters 2, 4, 5 and 7 One last colun OR-ing the above inters ( )

4 i x y z F Fro this table, we can clearly see that F = This is logical since F = 1, only at input cobinations i= 2, 4, 5 and 7 Thus, by ORing inter 2 (which has a value of 1 only at input cobination i= 2) with inter 4 (which has a value of 1 only at input cobination i= 4) with inter 5 (which has a value of 1 only at input cobination i= 5) with inter 7 (which has a value of 1 only at input cobination i= 7) the resulting function will equal F. In general, Any function can be expressed by OR-ing all inters ( i ) corresponding to input cobinations (i) at which the function has a value of 1. The resulting expression is coonly referred to as the SU of inters and is typically expressed as F = (2, 4, 5, 7), where indicates OR-ing of the indicated inters. Thus, F = (2, 4, 5, 7) = ( ) Exaple: Consider the function F of the previous exaple. We will, first, derive the su of inters expression for the copleent function F`. The truth table of F` shows that F` equals 1 at i = 0, 1, 3 and 6, then, F` = , i.e F` = (0, 1, 3, 6), (1) F = (2, 4, 5, 7) (2) Obviously, the su of inters expression of F` contains all inters that do not appear in the su of inters expression of F. i x y z F F`

5 Using De-organ theore on equation (2), ( ) = F = = This for is designated as the Product of axters and is expressed using the sybol, which is used to designate product in regular algebra, but is used to designate AND-ing in Boolean algebra. Thus, F` = (2, 4, 5, 7) = (3) Fro equations (1) and (3) we get, F` = (0, 1, 3, 6) = (2, 4, 5, 7) In general, any function can be expressed both as a su of inters and as a product of axters. Consider the derivation of F back fro F` given in equation (3): F = F = =... = F = (2, 4, 5, 7) = (0, 1, 3, 6) F` = (2, 4, 5, 7) = (0, 1, 3, 6) Conclusions: Any function can be expressed both as a su of inters ( i ) and as a product of axters. The product of axters expression ( j ) expression of F contains all axters j ( j i) that do not appear in the su of inters expression of F. The su of inters expression of F` contains all inters that do not appear in the su of inters expression of F. This is true for all copleentary functions. Thus, each of the 2 n inters will appear either in the su of inters expression of F or the su of inters expression of F but not both. The product of axters expression of F` contains all axters that do not appear in the product of axters expression of F. This is true for all copleentary functions. Thus, each of the 2 n axters will appear either in the product of axters expression of F or the product of axters expression of F but not both.

6 Exaple: Given that F (a, b, c, d) = (0, 1, 2, 4, 5, 7), derive the product of axters expression of F and the 2 standard for expressions of F`. Since the syste has 4 input variables (a, b, c & d) The nuber of inters and axters = 2 4 = 16 F (a, b, c, d) = (0, 1, 2, 4, 5, 7) 1. List all axters in the Product of axters expression F = (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15). F = (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15). 2. Cross out axters corresponding to input cobinations of the inters appearing in the su of inters expression F = (3, 6, 8, 9, 10, 11, 12, 13, 14, 15). Siilarly, obtain both canonical for expressions for F` F` = (3, 6, 8, 9, 10, 11, 12, 13, 14, 15). F` = (0, 1, 2, 4, 5, 7)

7 Canonical Fors: The su of inters and the product of axters fors of Boolean expressions are known as the canonical fors القانونية) (الصيغ of a function. Standard Fors: A product ter is a ter with ANDed literals *. Thus, AB, A B, A CD are all product ters. A inter is a special case of a product ter where all input variables appear in the product ter either in the true or copleent for. A su ter is a ter with ORed literals *. Thus, (A+B), (A +B), (A +C+D) are all su ters. A axter is a special case of a su ter where all input variables, either in the true or copleent for, are ORed together. Boolean functions can generally be expressed in the for of a Su of Products (SOP) or in the for of a Product of Sus (POS). The su of inters for is a special case of the SOP for where all product ters are inters. The product of axters for is a special case of the POS for where all su ters are axters. The SOP and POS fors are Standard fors for representing Boolean functions. * A Boolean variable in the true or copleent fors

8 Two-Level Ipleentations of Standard Fors Su of Products Expression (SOP): Any SOP expression can be ipleented in 2-levels of gates. The first level consists of a nuber of AND gates which equals the nuber of product ters in the expression. Each AND gate ipleents one of the product ters in the expression. The second level consists of a SINGLE OR gate whose nuber of inputs equals the nuber of product ters in the expression. Exaple Ipleent the following SOP function F = X + Y` + X`Y Level 1 X Level 2 Y` F X` Y Two-Level Ipleentation (F = X + Y` + X`Y ) Level-1: AND-Gates ; Level-2: One OR-Gate

9 Product of Sus Expression (POS): Any POS expression can be ipleented in 2-levels of gates The first level consists of a nuber of OR gates which equals the nuber of su ters in the expression, each gate ipleents one of the su ters in the expression. The second level consists of a SINGLE AND gate whose nuber of inputs equals the nuber of su ters. Exaple Ipleent the following SOP function F = (X+ )(Y`+)(X`+Y+ ) Level 1 X Level 2 Y` F X` Y Two-Level Ipleentation {F = (X+ )(Y`+)(X`+Y+ )} Level-1: OR-Gates ; Level-2: One AND-Gate