ECE 331 - Digital System Design

ECE 331 - Digital System Design

ECE 331 Digital System Design Electrical and Timing Characteristics of Standard Logic Gates (Lecture #2) Standard Logic Gates Device Logic Gate 74xx08 Quad 2-input AND gate 74xx32 Quad 2-input OR gate

74xx04 Hex Inverter (NOT gate) 74xx00 Quad 2-input NAND gate 74xx02 Quad 2-input NOR gate 74xx86 Quad 2-input XOR gate Note: xx refers to the logic family Spring 2011

ECE 331 - Digital Systems Design 2 Logic Families Transistor TTL CMOS Spring 2011 Logic Family xx Low Power

L High Speed H Schottky S Low Power Schottky LS Advanced Schottky AS Adv Low Power Schottky

ALS Fast F High Speed HC Advanced AC ECE 331 - Digital Systems Design 3

Comparison of Logic Families Spring 2011 ECE 331 - Digital Systems Design 4 Example: 74LS08 (see data sheet for 74LS08) Spring 2011 ECE 331 - Digital Systems Design 5 Example: 74HC08

(see data sheet for 74HC08) Spring 2011 ECE 331 - Digital Systems Design 6 Basic Electrical Characteristics Spring 2011 ECE 331 - Digital Systems Design 7 Logic Gates

Logic gates are the basic building blocks for (combinational and sequential) logic circuits. They are, however, abstractions. Spring 2011 ECE 331 - Digital Systems Design 8 Logic Gates In fact, logic gates are electrical circuits. Spring 2011

ECE 331 - Digital Systems Design 9 Logic Gates As such, the logic levels must be represented using an electrical characteristic. Most technologies use voltages to represent the logic levels. TTL

CMOS Some, but very few, technologies use currents to represent the logic levels. Spring 2011 ECE 331 - Digital Systems Design 10 Representing Logic Levels Ideally, a single voltage value is specified for each logic level.

VDD (power) Logic 1 GND (ground) Logic 0 Logic 1 = high voltage Logic 0 = low voltage Spring 2011

11 Representing Logic Levels In reality, a range of voltages is specified for each logic level. VDD Logic 1 V1,MIN Undefined Threshold voltages V0,MAX Logic 0 GND Spring 2011

ECE 331 - Digital Systems Design 12 Representing Logic Levels Furthermore, voltage ranges, for logic 1 and logic 0, are specified for both the input and the output of a logic gate. They are defined in terms of four parameters

VOH = output high voltage VIH = input high voltage VOL = output low voltage VIL = input low voltage These are specified in the data sheet for the corresponding logic gate. They differ from one logic family to another. Spring 2011 ECE 331 - Digital Systems Design

13 Representing Logic Levels Input Output VDD VDD Logic 1 VOH Logic 1 VIH

Undefined Undefined VIL Logic 0 GND VIH = min. volt. for Logic 1 VIL = max. volt. for Logic 0 Spring 2011 VOL Logic 0 GND VOH = min. volt. for Logic 1 VOL = max. volt. for Logic 0

ECE 331 - Digital Systems Design 14 Example: 74LS08 VIH, VIL VOH, VOL Spring 2011 ECE 331 - Digital Systems Design 15 Example: 74LS32 VIH, VIL VOH, VOL

Spring 2011 ECE 331 - Digital Systems Design 16 Example: 74HC32 VIH, VIL VOH, VOL Spring 2011 ECE 331 - Digital Systems Design 17

Example: 74LS04 VIH, VIL VOH, VOL Spring 2011 ECE 331 - Digital Systems Design 18 Basic Timing Characteristics Spring 2011 ECE 331 - Digital Systems Design 19

Time Delay (aka. Latency) A standard logic gate does not respond to a change on one of its inputs instantaneously. There is, instead, a finite delay between a change on the input and a change on the output. The propagation delay of a standard logic gate is defined for two cases: Spring 2011 tPLH = delay for output to change from low to high

tPHL = delay for output to change from high to low ECE 331 - Digital Systems Design 20 Time Delay low-to-high transition high-to-low transition tPHL Spring 2011

tPLH ECE 331 - Digital Systems Design 21 Time Delay The time delay (both tPLH and tPLH) for a logic gate is specified in its data sheet. The time delay is also known as the Spring 2011 gate delay

propagation delay of the logic gate latency ECE 331 - Digital Systems Design 22 Example: 74LS08 tPHL, tPLH Spring 2011

ECE 331 - Digital Systems Design 23 Example: 74LS32 tPHL, tPLH Spring 2011 ECE 331 - Digital Systems Design 24 Example: 74HC32 tPHL, tPLH Spring 2011

ECE 331 - Digital Systems Design 25 Example: 74LS04 tPHL, tPLH Spring 2011 ECE 331 - Digital Systems Design 26 Time Delay

The propagation delay of a logic circuit can be determined using the time delay of the individual logic gates. The critical path in the logic circuit must be identified. The critical path is the path with the greatest delay. The propagation delay of a logic circuit can be used to define Spring 2011 When the output of the logic circuit is valid.

The maximum speed of a combinational logic circuit. The maximum frequency of a sequential logic circuit. ECE 331 - Digital Systems Design 27 Questions? Spring 2011 ECE 331 - Digital Systems Design 28

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