Combinational Circuits

1. Combinational Circuits Dr. Bernard Chen Ph.D. University of Central Arkansas Fall 2009

2. Outline • Boolean Algebra • Decoder • Encoder

3. Boolean function and logic diagram • Boolean algebra: Deals with binary variables and logic operations operating on those variables. • Logic diagram: Composed of graphic symbols for logic gates. A simple circuit sketch that represents inputs and outputs of Boolean functions.

4. Gates • Refer to the hardware to implement Boolean operators. • The most basic gates are

5. Boolean function and truth table

6. Basic Identities of Boolean Algebra(Existence of 1 and 0 element) • x + 0 = x • x · 0 = 0 • x + 1 = 1 • x · 1 = 1

7. Basic Identities of Boolean Algebra (Existence of complement) (5) x + x = x (6) x · x = x (7) x + x’ = x (8) x · x’ = 0

8. Basic Identities of Boolean Algebra (Commutativity): (9) x + y = y + x (10) xy = yx

9. Basic Identities of Boolean Algebra (Associativity): (11) x + ( y + z ) = ( x + y ) + z (12) x (yz) = (xy) z

10. Basic Identities of Boolean Algebra (Distributivity): (13) x ( y + z ) = xy + xz (14) x + yz = ( x + y )( x + z)

11. Basic Identities of Boolean Algebra (DeMorgan’s Theorem) (15) ( x + y )’ = x’ y’ (16) ( xy )’ = x’ + y’

12. Basic Identities of Boolean Algebra (Involution) (17) (x’)’ = x

13. The other type of question Show that; 1- ab + ab' = a 2- (a + b)(a + b') = a 1- ab + ab' = a(b+b') = a.1=a 2- (a + b)(a + b') = a.a +a.b' +a.b+b.b' = a + a.b' +a.b + 0 = a + a.(b' +b) + 0 = a + a.1 + 0 = a + a = a

14. More Examples • Show that; (a) ab + ab'c = ab + ac (b) (a + b)(a + b' + c) = a + bc (a) ab + ab'c = a(b + b'c) = a((b+b').(b+c))=a(b+c)=ab+ac (b) (a + b)(a + b' + c) = (a.a + a.b' + a.c + ab +b.b' +bc) = …

15. DeMorgan's Theorem (a) (a + b)' = a'b' (b) (ab)' = a' + b' Generalized DeMorgan's Theorem (a) (a + b + … z)' = a'b' … z' (b) (a.b … z)' = a' + b' + … z‘

16. DeMorgan's Theorem Show that: (a + b.c)' = a'.b' + a'.c'

17. More DeMorgan's example • Show that: (a(b + z(x + a')))' =a' + b' (z' + x') • (a(b + z(x + a')))' = a' + (b + z(x + a'))' • = a' + b' (z(x + a'))' • = a' + b' (z' + (x + a')') • = a' + b' (z' + x'(a')') • = a' + b' (z' + x'a) • =a‘+b' z' + b'x'a • =(a‘+ b'x'a) + b' z' • =(a‘+ b'x‘)(a +a‘) + b' z' • = a‘+ b'x‘+ b' z‘ • = a' + b' (z' + x')

18. More Examples (a(b + c) + a'b)'=b'(a' + c') ab + a'c + bc = ab + a'c (a + b)(a' + c)(b + c) = (a + b)(a' + c)

19. Outline • Boolean Algebra • Decoder • Encoder

20. Decoder • Accepts a value and decodes it • Output corresponds to value of n inputs • Consists of: • Inputs (n) • Outputs (2n , numbered from 0  2n - 1) • Selectors / Enable (active high or active low)

21. The truth table of 2-to-4 Decoder

22. 2-to-4 Decoder

23. 2-to-4 Decoder

24. The truth table of 3-to-8 Decoder

25. 3-to-8 Decoder

26. 3-to-8 Decoder with Enable

27. Decoder Expansion • Decoder expansion • Combine two or more small decoders with enable inputs to form a larger decoder • 3-to-8-line decoder constructed from two 2-to-4-line decoders • The MSB is connected to the enable inputs • if A2=0, upper is enabled; if A2=1, lower is enabled.

28. Decoder Expansion

29. Combining two 2-4 decoders to form one 3-8 decoder using enable switch The highest bit is used for the enables

30. How about 4-16 decoder • Use how many 3-8 decoder? • Use how many 2-4 decoder?