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# ECE 3110: Introduction to Digital Systems Chapter 6 Combinational Logic Design Practices

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1. ECE 3110: Introduction to Digital SystemsChapter 6Combinational Logic Design Practices Documentation Standards

2. Chapter Outline • Documentation Standards for digital systems. • Digital Circuit Timing and Propagation delay • Combinational Logic Design Structures :- Decoders- Encoders- Three-State Buffers- Multiplexers- Demultiplexers - EXCLUSIVE OR Gates and Parity Circuits- Comparators- Adders/ Subtractors- Arithmetic Logic Units ( ALUs)

3. Documentation Standards • Documentation of a digital system should provide the necessary information for building, testing ,operating , and maintaining the system. • Generally, documentation include: 1) A Specification describes what the circuit is supposed to do. 2) A block diagram showing the inputs, outputs, the main building blocks ( modules) of the system and how they are connected. 2) A schematic diagram showing all the components, their types, and all interconnections. 3) A timing diagram showing the logic signals as a function of time. 4) A structured logic device description showing the operation of the structures . For example, the function table of the multiplexer, or the program listing of a PLD ( Programmable Logic Device). 5) A Circuit description explaining of the operation of the logic circuit.

4. Block Diagram • A block diagram should show all inputs and outputs , the building blocks and their function names , and the data flow paths ( the logic signals).- The internal details of each block should not be shown.- Related logic signals are combined together and drawn with a double or heavy line, known as a bus • Example: Min/Max Circuit MIN/MAX X Comparator Y X>Y min(X,Y) Mux A bus is a collection of 2 or more related signal lines. Mux Z Mux max(X,Y)

5. Block Diagram

6. Basic rules • A block diagram must show the most important system elements and how they work together. • Each block diagram should not be more than 1 page. • Large systems may require additional block diagrams of individual subsystems. (a top-level diagram shows the entire system. • Important control signals and buses should have names. • Flow of control and data should be clearly indicated. • Should not include internal details of each block (generally, NO gate symbols, NO pins).

7. Schematic Diagram • Details of component inputs, outputs, and interconnections • Reference designators • Pin numbers • Gate symbols • Signal names and active levels • Bubble-to-Bubble Logic Design • Layouts Logic diagram: An informal drawing without this level of details.

8. Example schematic

9. Gate symbols

10. DeMorgan equivalent symbols Which symbol to use? Answer depends on signal names and active levels.

11. Signal Names • Signal name: a descriptive alphanumeric label for each input/output signal. • In real system, well-chosen names convey information to readers • A signal name indicates • An action that is controlled (ENABLE, REQUEST, GO, PAUSE) • A condition that it detects such as READY_L, ERROR,… • Data that it carries, such as INBUS[31..0], ADDR[15…0]

12. Active Levels • Each signal name should have an active-level associated with it. A signal is active-high if it performs the named action or denotes the named condition when it’s HIGH or 1. A signal is active-low if it performs the named action or denotes the named condition when it’s LOW or 0. • The signal is asserted when it is in its active level and negated ( or deasserted ) when its not in its active level. • Different naming conventions for active levels available.

13. Naming conventions • Active low signal has a suffix of _Las part of the variable name. • Example :- ERROR is active high means there is an error when the signal is HIGH ( logic 1).- READY_L is active low means the data is ready when the signal is LOW ( logic 0).

14. Example LOW when error occurs Logic Circuit ERROR ERROR1_L HIGH when error occurs Logic Circuit ERROR ERROR_L

15. Signal name, logic expression, logic equation • A signal name is just a name, an alphanumeric label. • A logic expression combines signal names using switching algebra operators (AND,OR,NOT). • A logic equation is an assignment of a logic expression to a signal name. Describe one signal’s function in terms of other signals. • For example: READY_L, READY are signal names. READY’ is a logic expression. • Equation: DATA=ASEL.A+ASEL’.B Logic expression

16. Active Levels for Pins • When we draw the outline of a logic symbol, we think of the given logic function as occurring inside that outline. • In logic gates and logic structures the inversion bubble indicates the active level of the signal • Examples:- 2-to- 4 Decoder- EN_L is active low- A and B are active high- Y0_L, Y1_L, Y2_L,Y3_L are active low EN_L EN Y0 Y0_L Y1 Y1_L A A Y2 Y2_L B B Y3 Y3_L

17. More ways of obtaining AND/OR functions (Generalized AND/OR function). • Four ways of AND function: output is asserted if both of its inputs are asserted. • AND gate (74x08): active-high input/output • NAND gate (74x00): active-high input, active-low output • NOR gate (74x02): active-low input, active-high output • OR gate (74x32): active-low input/output • Four ways of OR function: output is asserted if either of its inputs are asserted. • OR gate (74x32): active-high input/output • NOR gate (74x02): active-high input, active -low output • NAND gate(74x00): active-low inputs, active-high output • AND gate (74x08): active-low inputs/output

18. Next… • Bubble-to-Bubble logic design • Schematics • Drawing Layouts • Reading Wakerly CH-6.1