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Chapter 3.pptx
- 1. Chapter Three Digital LogicGates (Lec 1) Prepared by: Esubalew Mulat Target Group: 2nd Year CS Students Adapted from Addisalem Hailegnaw, DBU . Debre Berhan University, March 2015 E.C.
- 2. Topics • Introduction toLogic Gates • Types of Logic Gates • Introduction to Digital Logic Family • Realization of Logic Gates
- 3. Digital Logic Gates •Digital Logic Gates or in other words Logic gates or Gates for short are electronic circuits that execute logical operations. • Logic gates are the building blocks of digital circuits. • Generally, logic gates operate on one or more inputs to provide a single logical output.
- 4. Digital Logic Gates… •Electronic gates require a power supply. • Gate INPUTS are driven by voltages having two nominal values. • The OUTPUT of a gate provides only two nominal values of voltage. • There is always a time delay between an input being applied and the output responding.
- 5. Types of DigitalLogic Gates
- 6. Inverter • The Inverterhas a single input. • The inverter performs the Boolean NOT operation. When the input is LOW, the output will be HIGH; when the input is HIGH, the output will be LOW. Symbol • The NOT operation (complement) is shown with an overbar. • The Boolean expression for the aforementioned inverter is X = 𝐴
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- 8. Inverter… Example Give the waveformof a signal A as in the figure below, determine the wave form for the output X. Answer:
- 9. AND Gate • TheAND gate produces a HIGH output when all inputs are HIGH. Otherwise, the output is LOW. • The AND gate has two or more inputs. Symbol • The AND operation is usually shown with a dot between the variables but it may be implied with no dot. • Thus, the AND operation is written as X = A.B or X = AB
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- 12. AND Gate… • TheAND operation is used in computer programming as a selective mask. If you want to retain certain bits of a binary number but reset the other bits to 0, you could set a mask with an AND. • Example: If the binary number 10100011 is ANDed with the mask 00001111, what is the result? Answer: 00000011
- 13. OR Gate • TheOR gate has two or more inputs. • The OR gate produces a HIGH output if any input is HIGH; if all inputs are LOW, the output is LOW. Symbol • The OR operation is shown with a plus sign (+) between the variables. • Thus, the OR operation is written as X = A + B.
- 14. Truth Table forOR Gate Truth Table Example Waveform
- 15. OR Gate… • TheOR operation can be used in computer programming to set certain bits of a binary number to 1. • Example: ASCII letters have a 1 in the bit 5 position for lower case letters and a 0 in this position for capitals. (Bit positions are numbered from right to left starting with 0.) What will be the result if you OR an ASCII upercase letter with the 8-bit mask 00100000? • Solution: The resulting letter will be lower case.
- 16. NAND Gate • TheNAND gate produces a LOW output when all inputs are HIGH; otherwise, the output is HIGH. • The NAND operation is shown with a dot between the variables and an overbar covering them. Thus, the NAND operation is written as X = 𝐴𝐵 (Alternatively, X = 𝐴. 𝐵) Symbol
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- 19. NAND Gate… • TheNAND gate is particularly useful because it is a universal gate (all other basic gates can be constructed from NAND gates). • Question: How would you connect a 2-input NAND gate to form a basic inverter? • Answer: • Exercise: How do you construct an AND gate and an OR gate from a NAND gate?
- 20. NOR Gate • TheNOR gate accepts two or more inputs. • The NOR gate produces a LOW output if any input is HIGH; if all inputs are HIGH, then the output is LOW. • Symbol • The NOR operation is shown with a plus sign (+) between the variables and an overbar covering them. Thus, the NOR operation is written as X = 𝐴 + 𝐵.
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- 22. NOR Gate… Exemplary Waveform . Likea NAND Gate, a NOR gate is Universal Gate
- 23. NOR Gate… Example: Whenis the LED turned ON for the circuit shown? Solution: The LED will be on when any of the four inputs are HIGH.
- 24. XOR Gate • TheXOR Gate takes two or more inputs. • The XOR gate produces a HIGH output only when both inputs are at opposite logic levels. • Symbol • The XOR operation is written as X = 𝐴𝐵 + 𝐴𝐵. Alternatively, it can be written with a circled plus sign between the variables as
- 25. XOR Gate… • ExemplaryWaveform • Question: If the A and B waveforms are both inverted for the above waveforms, how is the output affected? • Answer: There is no change in the output
- 26. XNOR Gate • TheXNOR Gate takes two or more inputs. • The XNOR gate produces a HIGH output only when both inputs are at the same logic level.
- 27. XNOR Gate… • ExemplaryWaveform • Question: If the A waveform is inverted but B remains the same, how is the output affected? • Answer: The output will be inverted
- 28. Digital Circuits andLogic Families
- 29. Digital Circuit Examples Binaryto Octal Decoder
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- 33. Digital Circuit Examples… CPUand Others
- 34. Logic gates arethere whenever more complex digital systems are designed!!!
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- 36. Digital Logic Gates •Digital ICs can be classified by the specific circuit technology to which they belong. • The circuit technology is referred to as a digital logic family. • Each logic family has its own basic electronic circuit upon which more complex digital circuits and components are developed. • The basic circuit in each technology is a NAND, NOR, or Inverter gate.
- 37. Digital Logic Gates… •RTL : (Resistor-Transistor Logic) • DTL : (Diode-Transistor Logic) • TTL : (Transistor-Transistor Logic) • ECL : (Emitter-Coupled Logic) • MOS : (Metal-Oxide Semiconductor) • CMOS : (Complementary MOS)
- 38. Characteristics of DigitalIC 1. Propagation delay (speed of operation). 2. Power dissipation. 3. Fan in. 4. Fan out. 5. Noise immunity. 6. Operating temperature. 7. Power supply requirement. 8. Current and voltage parameters.
- 39. Bipolar Transistor Characteristics •Can be ‘npn’ or ‘pnp’ • Germanium or Silicon • Bipolar IC transistors are made with silicon and are usually ‘npn’ type.
- 40. Bipolar Transistor Characteristics… Example:Inverter with a BJT
- 41. Bipolar Transistor Characteristics… •hFE is transistor parameter called dc current gain.
- 42. Resistor Transistor Logic(RTL) • The basic circuit of RTL digital logic family is the NOR gate. • Low level = 0.2 V and high level from 1 to 3.6V
- 43. Resistor Transistor Logic(RTL) • The RTL logic circuit has many drawbacks. • The resistors increase the input resistance and reduce the switching speed of the circuit. • This degrades the rise and fall times of any input pulse. (Read More on It!)
- 44. Diode Transistor Logic(DTL) • Diode is adapted from transistor. • Behaves like base-emitter junction of a transistor.
- 45. Diode Transistor Logic(DTL)… • The basic circuit of DTL digital logic family is the NAND gate. • Low level = 0.2 V and high level from 4 to 5 V
- 46. Diode Transistor Logic(DTL)… • If any input of the gate is low to 0.2 V, the corresponding diode is forward biased and conducts current through VCC and RD. (RD=5k resistor and RC is 2K resistor) • The voltage at point P is equal to the input voltage 0.2 V plus one diode drop of 0.7 V, for a total of 0.9 V. • This is not sufficient to drive the transistor Q1 into conducting. • In order for the transistor to conduct the voltage at P it must overcome a potential of one VBE (0.6 V) drop in Q1 and two diode voltage drops (0.6 V each) • Hence, the transistor Q1 remains at cut-off condition and its collector to emitter behaves like an open circuit.
- 47. Diode Transistor Logic(DTL)… • If all the inputs of the gate are high to 5 V, all the diodes are reverse biased and the current will flow through RD, D1, D2, and the base of the transistor. • The transistor is now driven to saturation region. The voltage at P is equal to one VBE drop plus two diode drops across D1 and D2, or 3 × 0.7V = 2.1 V (VBE drop is 0.7 V while conducting and forward biased diode drop becomes 0.7 V). • This conforms that all the input diodes are reverse biased and off. With the transistor at saturated condition, the output drops to VCE(sat) = 0.2 V, which is low level for the gate.
- 48. Transistor - TransistorLogic (TTL) • Replaces diode with a transistor • Open collector TTL • L = 0.2 V and H = 2.4 to 5 V • The multiple emitters in Q1 behave like the input diodes in DTL, because they form a p-n junction.
- 49. Transistor - TransistorLogic (TTL)… Debre Berhan University, February 2013 E.C. Example • NAND
- 50. Emitter Coupled Logic(ECL) • Non-saturated digital logic family. • Propagation Delay : 1~ 2 ns (The fastest) • Was mostly used in systems requiring very high speed. E.g. Mainframe computers. • Very bad power dissipation and noise immunity !!! • Noise margin : 0.3 V • High fan-out possible • Obsolete !!
- 51. Metal Oxide Semiconductor(MOS) • MOS (Metal-Oxide Semiconductor) like other FETs is a unipolar transistor. • Consume low power • Consume small area
- 52. MOS Symbol • Enhancementtype symbolized with broken line connection between source and drain.
- 53. MOS… • We canview MOS transistors as electrically controlled switches • Voltage at gate controls path from source to drain
- 54. MOS… Example: (Inverter, NANDand NOR gates realization with NMOS) Note: If NOMS is on, it drives a logic to zero. While PMOS drives a logic to high.
- 55. CMOS • CMOS devicestake advantage on MOS devices because of the fact that both n-channel and p-channel can be fabricated on the same chip. • CMOS has important advantages
- 56. CMOS • CMOS devicestake advantage on MOS devices because of the fact that both n-channel and p-channel can be fabricated on the same chip. • CMOS has important advantages extremely low power use (a reason of its popularity) higher noise immunity high fan out capability and wide range of supply voltage Simpler interface with other logic circuits
- 57. CMOS Logic Circuits CMOSInverter
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- 59. CMOS Logic Circuits CMOSNOR Gate
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