K-maps and Decoders Prof. Sin-Min Lee Department of Computer Science

Bit-Serial and Ripple-Carry Adders Half-adder (HA): Truth table and block diagram Full-adder (FA): Truth table and block diagram

Half-Adder Implementations c Three implementations of a half-adder.

Full-Adder Implementations

Truth table to K-Map ABP B A minterms are represented by a 1 in the corresponding location in the K map. The expression is: A.B + A.B + A.B

K-Maps Adjacent 1 ’ s can be “ paired off ” Any variable which is both a 1 and a zero in this pairing can be eliminated Pairs may be adjacent horizontally or vertically B A a pair another pair B is eliminated, leaving A as the term A is eliminated, leaving B as the term The expression becomes A + B

Two Variable K-Map A B CP A.B.C + A.B.C + A.B.C BC A One square filled in for each minterm. Notice the code sequence: – a Gray code.

Grouping the Pairs BC A equates to B.C as A is eliminated. Here, we can “wrap around” and this pair equates to A.C as B is eliminated. Our truth table simplifies to A.C + B.C as before.

Groups of 4 BC A Groups of 4 in a block can be used to eliminate two variables: The solution is B because it is a 1 over the whole block (vertical pairs) = BC + BC = B(C + C) = B.

Karnaugh Maps Three Variable K-Map –Extreme ends of same row considered adjacent A BC

Karnaugh Maps Three Variable K-Map example A BC X =

The Block of 4, again A BC X = C

Two Variable K-Map A B CP A.B.C + A.B.C + A.B.C AB C There is more than one way to label the axes of the K-Map, some views lead to groupings which are easier to see.

Karnaugh Maps Four Variable K-Map –Four corners adjacent AB CD

Karnaugh Maps Four Variable K-Map example AB CD F =

Product-of-Sums We have populated the maps with 1’s using sum-of-products extracted from the truth table. We can equally well work with the 0’s AB C ABCP AB C P = (A + B).(A + C) P = A.B + A.C equivalent

Inverted K Maps In some cases a better simplification can be obtained if the inverse of the output is considered –i.e. group the zeros instead of the ones –particularly when the number and patterns of zeros is simpler than the ones

Karnaugh Maps Example: Z5 of the Seven Segment Display X1X2X3X4Z X1010X 1011X1011X 1100X1101X1110X1111X1100X1101X1110X1111X X 1 X 2 X3 X Z5 = Better to group 1’s or 0’s?

and sum-of-product

Example: Majority Function Three inputs: A, B, C One output: M Output takes truth value of majority inputs. I.e. –M is 1 iff two of A,B,C is 1 –M is 0 iff two of A, B, C is 0 Notice writing large truth tables is cumbersome

Alternative Representation Collect the combinations of variable that give 1 for output. Write the function as a SUM of these terms In terms, write variable name for value 1, and a bar over the name for 0. EG: M = ABC+ABC+ABC+ABC

Rationale for New Notation Consider ABC: The product is for AND Consider ABC+ABC: The sum is for OR So we are writing the function as a sum of products I.e. AND-ing OR-terms – Called conjunctive normal form. Consider ABC: This is 1 iff A=0, B=1 and C=1 A function of N variables can be given as sum of 2**N n-variable products

Creating Circuits for Boolean Functions M=ABC+ABC+ABC+ABC 1,2,3 are NOT gates feeding lines A,B,C 4,5,6,7 are AND gates corresponding to the four product terms 8 is an OR term corresponding to the sum A,B,C have been inserted to avoid clutter – they could be connected directly out of NOT gate

Implementing Boolean Functions Write the truth table Provide inverters for complementing inputs Draw an AND gate for each term with 1in output column Wire the AND gates to appropriate inputs Feed the outputs of all AND gates into an OR gate

Using A Single Gate Type It is desirable to use only one type of gate generate the whole circuit. Can use NAND or NOR gate. In order to do so, enough to show that –NOT, AND, OR NAND can be generated by NOR gates –NOT, AND, OR, NOR ca be generated by NAND gates. We say that NAND, NOR are complete for Boolean circuits

Completeness of NAND

Completeness of NOR

Circuit Equivalence Sometimes need to minimize number of elements on a board: –get minimum number of gates –Two input gates instead of four input gates Need to find an equivalent circuit for the given circuit Equivalent= having same input – output behavior = computing same Boolean function Use Boolean Algebra

Example: Using AB+AC =A(B+C)

Some Laws of Boolean Algebra

Consequences of De Morgan ’ s Law

Using De Morgan ’ s Laws to covert sum of products to NAND

De Morgan again A NAND gate: Y = A.B = A + B is the same as an OR gate with two NOT gates Similarly a NOR gate is the same as an AND gate with two inverters Y = A + B = A.B not the individual terms change the sign not the lot

Dual gates not the individual inputs change the gate not the output

Truth Tables and Boolean Notation NAND Gate Representation –It is possible to implement any boolean expression using only NAND gates XX NOT AND A B A.B OR A A+B B

Truth Tables and Boolean Notation NAND Gate representation –Implement the following circuit using only NAND gates x3 x2 x4 De Morgan can also be represented visually:

Exercise Implement NOT, AND and OR using NOR gates Example AND gate dual circuit:

Solution Similar pattern to using NAND gates (not surprising) NOT AND OR XX A B A.B A A+B B X X A B A.B A+B A A.B B

Truth Tables and Boolean Notation NOR Gate representation –It is also possible to implement any boolean expression using only NOR gates –Implement the following circuit using only NOR gates X4 X3 X2

Solution Two NOR gates in sequence acting as NOT ’ s can be eliminated: X4 X3 X2