Lec0.ppt
- 1. 1 Switching Theory & Logic Design Laboratory Debdeep Mukhopadhyay Associate Professor Dept of Computer Science and Engineering IIT Kharagpur
- 2. Chapter 0 2
- 3. Introduction 3 Logic design is one of the disciplines that has enabled digital revolution, which has dramatically altered our economics, communication, and life in general! Cars are growingly more computers!
- 5. What is Design? • Design is the process of coming up with a solution to a problem: – Need to understand the problem – Need to understand the constraints We desire an ‘efficient’ design! • Consider, the problem of building an university: – Create an xyz sq ft of floor area, where say 10,000 people can work efficiently. – Constraints: • Dimensions • Aesthetics • Departments, rooms • Other services • Lifts • Parking Space • Emergency Exits… • Money and Time 5
- 6. Divide and Conquer • A very efficient methodology which humans have developed because of their inability of handling too many details! • Divide the problem into sub-problems: – Associate sub-contractors (Split/Divide) • Each have their own design problems • Continues in a recursive fashion – Merge the solutions (Conquer) • The chief-architect is responsible for integrating • Toughest job! • Good communication and team work is a key! – It is difficult for the stair-case contractor to ask to change the dimensions of the hall for better flight. – Or, to widen the elevator shaft after developing the building. 6
- 7. Top-down and Bottom-up Approaches • Both strategies of information processing and knowledge ordering • A top down approach breaks up a bigger picture into sub- systems, which are often like black-boxes, initially. • A Bottom-up approach starts with systems and connects them together to form bigger systems. • Top down approach is about planning, and starts implementations after the complete picture is clear. – Delays testing • Bottom up approach on the other hand stresses on implementation and testing from the beginning. – Problem may be integration/linking of modules. – Good for reuse of design. • Real life design combines both. 7
- 8. Facets of Design 8 Design Creative Process (Visualize the solution) Engineering Process (Explore the trade-offs, make decisions) Optimization Process (Choosing best combination among components)
- 9. Digital/Logic Design • A digital designer uses components from digital electronics to solve problems in real life. • Transistors from CMOS (Complementary MOS) forms the core. • A digital designer abstracts it like a switch. • Circuit: An inter-connected collection of switches 9
- 10. Gate Path 0 Closed 1 Open Gate Drain Source Gate Source Drain Gate Path 0 Open 1 Closed pMOS nMOS • Transmits 1 well • Transmits 0 poorly • Transmits 0 well • Transmits 1 poorly Transistors as Switches: 1st Abstraction
- 11. CMOS Transmission Gate • Transmit signal from INPUT to OUTPUT when Gate is closed Gate (complementary of Gate) Source Drain Gate INPUT OUTPUT Gate pMOS nMOS OUTPUT 0 OFF OFF Z 1 ON ON INPUT Z : High-Impedance State, consider the terminal is “floating”
- 12. An Example • Problem: Make a two-way switch for a bulb. 12
- 13. An Example • Problem: Make a two-way switch for a bulb. 13
- 14. An Example • Problem: Make a two-way switch for a bulb. 14
- 15. An Example • Problem: Make a two-way switch for a bulb. 15
- 16. Constraints of a digital design • Logic designers job is to choose the right components and connect them to solve the problem meeting: – Size – Cost – Power – Security http://www.engadget.com/2014/08/23/elect rical-potential-data-theft/ Power consumption 0 1 1 1 Secret information 0 1
- 17. Cost, Size, Performance 17 Cost and Size are closely related • Complexity of a digital circuit depends on: • Number of components used • Cost of each component • Wires used to connect these components As technology becomes smaller, routing delays dominate!
- 18. Moore’s law for Intel Microprocessors Source: Evolution of Intel Microprocessors (from Jean Baer, Microprocessor Architecture, Cambridge University Press) In 1965, Gordon Moore noted that the number of transistors on a chip doubled every 18 to 24 months. He made a prediction that semiconductor technology will double its effectiveness every 18 months
- 19. Some figures • 1971: Intel 4004, 1.08 MHz, 2,300 transistors • 2003: Intel Pentium 4, 3.4 GHz, 1.7 billion transistors – Frequency increases roughly double per 2.5 years – Number of transistors roughly double every two years (Moore’s Law). • How will the trend continue in the future?
- 20. Power Dissipation in Intel Processors Source: Evolution of Intel Microprocessors (from Jean Baer, Microprocessor Architecture, Cambridge University Press) Dynamic Power: Depends on how fast the switches are turned on and off! Static Power: Device leakage, which has started To dominate!
- 21. Logic Levels: Another Abstraction 21 • The input and output signals are analog signals: continuous voltage levels (say any real number from 0V to 5V) • They are abstracted as logic ‘0’ and logic ‘1’: • Below VL : logic ‘0’ • Above VL: logic ‘1’ • Inputs and Outputs generally have different thresholds to improve the noise margin*. • Robust: as variations in analog signals are removed due to the thresholding! *voltage difference between output of one gate and input of next. Noise must exceed noise margin to make second gate produce wrong output.
- 22. 22 8 9 10 11 12 13 14 15 4 5 6 7 12 13 14 15 2 3 6 7 10 11 14 15 1 3 5 7 9 11 13 15 Think of a number between 1 and 15
- 23. 23 Binary Numbers • Number 7 appears on the four cards in the pattern ‘No, Yes, Yes, Yes’ • The number 7 in binary code is 0111 • This is the Digital Language! Yes = 1 No = 0
- 24. 24 Why binary? • Information is stored in computer via voltage levels. • Using decimal would require 10 distinct and reliable levels for each digit. • This is not feasible with reasonable reliability and financial constraints. • Everything in computer is stored using binary: numbers, text, programs, pictures, sounds, videos, ...
- 25. 25 Bit, Byte, and Word 0 0 1 1 0 1 1 0 0 0 1 1 1 0 1 1 0 0 1 1 1 1 0 0 1 1 1 1 0 0 0 1 1 0 0 0 0 0 1 0 0 cont’d A bit is a size that can store 1 digit of a binary number, 0 or 1. A byte is 8 bits, which can store eight 0’s or 1’s. A word is either 32 or 64 bits, depending on computers. Regular PC’s are 32-bit word in size, higher-end workstations are 64-bit. Word size is the size of the registers. What do these bits mean is a matter of interpretation! All information in a computer are represented in a uniform format of bit patterns.
- 26. Types of Digital Circuits • Combinational Circuits: Inputs directly influence the output. • Example 1: NOT gate 26 Vin Vcc
- 27. Example 2: 3 INPUT CMOS NAND GATE PULL UP NETWORK PULL DOWN NETWORK A B C Y=~(ABC) Types of Digital Circuits
- 28. • Sequential Circuits: These circuits not only depend on current input, but also to the past history of values on the same inputs. • An Example: Memory Element • When EN is high, then DOUT is assigned the value of DIN • Otherwise, DOUT is held indefinitely. • Applications: – Storing and Recalling data – Sequencing program instructions 28 Types of Digital Circuits DIN EN DOUT
- 29. 29 Summary: Abstraction Levels for Digital Designs n+ n+ S G D + DEVICE CIRCUIT GATE MODULE SYSTEM
- 30. Summary: Logic Design 30 • Set of abstractions and methodologies that help us configure large number of digital circuits to solve a given problem. • Need to learn methodologies for developing a systematic design flow to take a concept and transform into reality. • We also need to learn optimization techniques, and meet constraints better! • Necessity is the mother of invention! • Constraints give rise to innovations, and make the job worthy!