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Digital Logic

Digital Logic. Abstract.

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Digital Logic

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  1. Digital Logic

  2. Abstract The basics of Boolean algebra will be introduced, starting from simple combinational logic functions and basic sequential logic circuits such as latches and flip-flops. Based on these fundamentals, more complex logic structures such as decoders, adders, registers, and memories will constructed and their performance analyzed. FET Transistor basics will be reviewed and circuits introduced that demonstrate implementations of the basic logic elements in CMOS technology. The logic functions used as the architectural basis for programmable devices used in spaceborne electronics will be examined and analyzed. Some common design problems found in flight circuits will be introduced.

  3. Logic Outline • Introduction • Basic Logic • Hardware • Combinational Logic • Sequential Logic

  4. Basic Digital Logic

  5. Logic Values • For real binary hardware • Either a ‘1’ or a ‘0’ • “Most of the time” • Other types of hardware can be have more than two values • Simulation can use a multi-value logic system • VHDL Std_Logic uses a nine-valued logic

  6. Binary Hardware Logic Values • Logic ‘1’ • Logic ‘0’ • Undefined value

  7. TTL Logic Levels • Logic ‘1’ threshold - VIH •  2.0 V • Logic ‘1’ threshold - VIL •  0.8 V • Undefined • 0.8 V < v < 2.0 V

  8. Logic ‘1’ 2.0V Undefined 0.8V Logic ‘0’ TTL Logic LevelsExamples of Undefined Values • Floating signals may take on illegal values • What happens during a signal transition?

  9. MIL-STD-1553 • Three physical values • > +VTH • < - VTH • -VTH < v < +VTH • All values have meaning in MIL-STD-1553

  10. Intel Flash Memory • Intel® StrataFlash™ memory components store two bits per cell for high density and low cost. • Smaller margins for radiation. Source: http://www.intel.com/design/flcomp/prodbref/298044.htm

  11. Physical Logic Values • Magnetic field (core memory, plated wire) • Voltage (static CMOS logic) • Charge (DRAM) • Position (contacts open/closed) • Sound (tones for a modem) • Light (fiber optics - MIL-STD-1773) • Etc.

  12. Logic Values and Basic Functions

  13. Logic Values • Levels • '0' or '1' • Pulses • Presence or absence of pulses • Pulse widths • Number of pulses • Zero crossings

  14. A Multi-valued Logic System TYPE std_ulogic IS ( ‘U’, -- Uninitialized ‘X’, -- Forcing Unknown ‘0’, -- Forcing 0 ‘1’, -- Forcing 1 ‘Z’, -- High Impedance ‘W’, -- Weak Unknown ‘L’, -- Weak ‘0’ ‘H’, -- Weak ‘1’ ‘-’ -- Don’t Care );

  15. Switching Algebra • Two-valued variable from {0,1} • Two binary operations • And ( • ) • Or ( + ) • One unary operation • Not (  )

  16. A Y 0 1 1 0 Unary Operation - Not

  17. A B Y 0 0 0 0 1 0 1 0 0 1 1 1 Binary Operation - And

  18. A B Y 0 0 0 0 1 1 1 0 1 1 1 1 Binary Operation - [Inclusive] Or

  19. Binary Functions • How many functions are there of two binary variables? • Basic combinatorics • 4 positions or rows: 00, 01, 10, 11 • For each position you can select one of two values {0, 1} • Order counts since it is a function • Number of functions = 2 x 2 x 2 x 2 = 24 = 16

  20. Binary Functions - cont’d • How many functions are there of n binary variables? • 2n positions or rows • for n = 2: 00, 01, 10, 11 • for n = 3: 000, 001, 010, 011, 100, 101, 110, 111 • For each position or row you can select one of two values {0, 1} • Number of functions = 2^2n • for n = 2: 22 = 4; 2 x 2 x 2 x 2 = 24 = 16 • for n = 3: 23 = 8; 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 = 28 = 64 • for n = 4: 24 = 16; 216 = 65536

  21. A Y AND 2 B A B Y AND 3 C A B Y AND 4 C D Fan-In • Number of inputs to a logic gate • Examples for And function • n = 2: Y = A • B • n = 3: Y = A • B • C • n = 4: Y = A • B • C • D

  22. A A Y AND 2 B B A Y A Y Y OR 2 INV B C A Y AND 2 B D Logic Functions • Any logic function can be implemented with And, Or, and Not. • One standard form is the sum of products • Example: Y = (A • B + C • D) Inputs And gates Or gates Inverters Outputs

  23. Universal Logic Element

  24. Universal Logic GateIntroduction • Some other logic functions • NOR ::= Negative OR Y = ( A + B )´ • NAND ::= Negative AND Y = ( A • B )´ • Multiplexor Y = A • S + B • S´ • Look up table (LUT) Small memory

  25. A Y A Y = A A Y Y NOR 2 B INV A Y A A A A NOR 2 B Y = Y Y AND 2 Y B A B B Y B NOR 2 B A A A A A Y Y Y = Y Y NOR 2 B B OR 2 NOR 2 B B B Universal Logic GateNOR Function • NOR ::= Negative OR Y = ( A + B )´ NOT AND NOR 2 OR

  26. A NOT Y Y A = Y A A Y B NAND 2 INV A A A AND A A Y Y Y = Y Y AND 2 B B B NAND 2 NAND 2 B B A Y A A A A B OR NAND 2 Y = Y Y Y B OR 2 A B NAND 2 B Y B B NAND 2 Universal Logic GateNAND Function • NAND ::= Negative AND Y = ( A • B )´

  27. A A B B A V C C V C C V C C Y Y S 0 Y S 1 Y S 1 S 0 S 1 S 0 D 0 D 0 D 0 D 1 G N D D 1 D 1 Y Y Y Y Y Y M X 4 M X 4 D 2 D 2 M X 4 D 2 D 3 D 3 D 3 Y Y Y G N D G N D G N D Universal Logic GateMultiplexor Function • Multiplexor Y = A • S + B • S´ OR AND NOT

  28. Universal Logic Gate Look up table (LUT) • Look up table (LUT) Small memory NOT OR AND A X Y A B Y A B Y 0 0 1 0 0 0 0 0 0 0 1 1 0 1 1 0 1 0 1 0 0 1 0 1 1 0 0 1 1 0 1 1 1 1 1 1

  29. Logic Assignments and Duality Logic Gate - Positive Logic A B Y 0 0 0 0 1 0 1 0 0 1 1 1 0 = 0V 1 = 5V Logic Gate and Voltage A B Y 0V 0V 0V 0V 5V 0V 5V 0V 0V 5V 5V 5V Logic Gate - Negative Logic A B Y 1 1 1 1 0 1 0 1 1 0 0 0 1 = 0V 0 = 5V Logic Gate - Negative Logic A B Y 0 0 0 0 1 1 1 0 1 1 1 1 1 = 0V 0 = 5V Reorder Rows

  30. Adders

  31. Truth Table x y C S 0 0 0 0 0 1 0 1 1 0 0 1 1 1 1 0 A X Y S XOR 2 B Y A Y C XAND 2 B Half-Adder Logic Equations C = x • y S = x  y Schematic

  32. Truth Table x y z C S 0 0 0 0 0 0 0 1 0 1 0 1 0 0 1 0 1 1 1 0 1 0 0 0 1 1 0 1 1 0 1 1 0 1 0 1 1 1 1 1 Full-Adder Logic Equations C = x’yz + xy’z + xyz’ + xyz = z • (x’y+xy’) + xy • (z+z’) = z • (x  y) + x • y = MAJ (x,y,z) S = x’y’z + x’yz’ + xy’z’ + xyz = x’yz’ + xy’z’ + x’y’z + xyz = z’(x’y + xy’) + z(x’y’ + xy) = z’(x  y) + z(x  y)’ = (x  y)  z = x  y  z

  33. Adding Two Numbers • Many Types of Adders: Some examples: • Bit Serial Adder • Add time = n x f • Cascade Cascade Stages • Ripple carry adder • Add time = n x tPD • Carry Look Ahead Adder • Generate carries in parallel • e.g., 4-bit AM2902. Can have “look ahead” of the “look ahead” units.

  34. A S B CIN COUT A COUT FULL ADDER B S CIN Full Adder Schematic

  35. CIN CIN CIN CIN A B B A A A B B FULL ADDER BIT 2 FULL ADDER BIT 0 FULL ADDER BIT 3 FULL ADDER BIT 1 S S S S COUT COUT COUT COUT 4-bit Ripple Carry Adder

  36. Cin A B C-out 0 0 0 “kill” 0 1 C-in “propagate” 1 0 C-in “propagate” 1 1 1 “generate” A0 S G B1 P C1 = G0 + C0 • P0 A S G B P C2 = G1 + G0 • P1 + C0 • P0 • P1 A S G B P C3 = G2 + G1 • P2 + G0 • P1 • P2 + C0 • P0 • P1 • P2 A S G G B P P C4 = ... Carry Lookahead Adder (CLA)

  37. Sample CLA Timing Analysis • Generate g and p: 1 gate delay • Generate C: 2 gate delays • Generate sum: 3 gate delays • Total: 6 gate delays • Total is independent of word length

  38. Negative Numbers and Subtraction • Several different codes for negative numbers • 2's complement • 1's complement • signed magnitude • others • For 2's complement, subtraction is implemented with the same hardware by negating the subtrahend. This is done by inverting each bit and adding one. The one can be added by setting the carry-in to the first stage equal to 1, saving an operation.

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