Computer Architecture and Organization

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1 A-1 Appendix A - Digital Logic Computer Architecture and Organization Miles Murdocca and Vincent Heuring Appendix A Digital Logic

2 A-2 Appendix A - Digital Logic Chapter Contents A.1 Introduction A.2 Combinational Logic A.3 Truth Tables A.4 Logic Gates A.5 Properties of Boolean Algebra A.6 The Sum-of-Products Form and Logic Diagrams A.7 The Product-of-Sums Form A.8 Positive vs. Negative Logic A.9 The Data Sheet A.10 Digital Components A.11 Sequential Logic A.12 Design of Finite State Machines A.13 Mealy vs. Moore Machines A.14 Registers A.15 Counters A.16 Reduction of Combinational Logic and Sequential Logic A.17 Reduction of Two-Level Expressions A.18 State Reduction

3 A-3 Appendix A - Digital Logic Some Definitions Combinational logic: a digital logic circuit in which logical decisions are made based only on combinations of the inputs. e.g. an adder. Sequential logic: a circuit in which decisions are made based on combinations of the current inputs as well as the past history of inputs. e.g. a memory unit. Finite state machine: a circuit which has an internal state, and whose outputs are functions of both current inputs and its internal state. e.g. a vending machine controller.

4 A-4 Appendix A - Digital Logic The Combinational Logic Unit Translates a set of inputs into a set of outputs according to one or more mapping functions. Inputs and outputs for a CLU normally have two distinct (binary) values: high and low, 1 and 0, 0 and 1, or 5 V. and 0 V. for example. The outputs of a CLU are strictly functions of the inputs, and the outputs are updated immediately after the inputs change. A set of inputs i 0 i n are presented to the CLU, which produces a set of outputs according to mapping functions f 0 f m.

5 A-5 Appendix A - Digital Logic Truth Tables Developed in 1854 by George Boole further developed by Claude Shannon (Bell Labs) Outputs are computed for all possible input combinations (how many input combinations are there? Consider a room with two light switches. How must they work? Don't show this to your electrician, or wire your house this way. This circuit definitely violates the electric code. The practical circuit never leaves the lines to the light "hot" when the light is turned off. Can you figure how?

6 A-6 Appendix A - Digital Logic Alternate Assignments of Outputs to Switch Settings Logically identical truth table to the original (see previous slide), if the switches are configured up-side down.

7 A-7 Appendix A - Digital Logic Truth Tables Showing All Possible Functions of Two Binary Variables The more frequently used functions have names: AND, XOR, OR, NOR, XOR, and NAND. (Always use upper case spelling.)

8 A-8 Appendix A - Digital Logic Logic Gates and Their Symbols Logic symbols for AND, OR, buffer, and NOT Boolean functions Note the use of the inversion bubble. (Be careful about the nose of the gate when drawing AND vs. OR.)

9 A-9 Appendix A - Digital Logic Logic symbols for NAND, NOR, XOR, and XNOR Boolean functions

10 A-10 Appendix A - Digital Logic Variations of Basic Logic Gate Symbols

11 A-11 Appendix A - Digital Logic The Inverter at the Transistor Level Power Terminals Transistor Symbol A Transistor Used as an Inverter Inverter Transfer Function

12 A-12 Appendix A - Digital Logic Allowable Voltages in Transistor- Transistor-Logic (TTL) Assignments of logical 0 and 1 to voltage ranges (left) at the output of a logic gates, (right) at the input to a logic gate.

13 A-13 Appendix A - Digital Logic Transistor-Level Circuits For 2-Input NAND and NOR Gates

14 A-14 Appendix A - Digital Logic CMOS Configurations Schematic symbols for (left) n-channel transistor and (right) p-channel transistor. CMOS configurations for (a) NOT, (b) NOR, and (c) NAND gates.

15 A-15 Appendix A - Digital Logic Tri-State Buffers Outputs can be 0, 1, or electrically disconnected.

16 A-16 Appendix A - Digital Logic The Basic Properties of Boolean Algebra Principle of duality: The dual of a Boolean function is made by replacing AND with OR and OR with AND, constant 1s by 0s, and 0s by 1s A, B, etc. are literals; 0 and 1 are constants.

17 A-17 Appendix A - Digital Logic DeMorgan s Theorem Discuss: Applying DeMorgan s theorem by pushing the bubbles, and bubble tricks.

18 A-18 Appendix A - Digital Logic NAND Gates Can Implement AND and OR Gates Inverted inputs to a NAND gate are implemented with NAND gates.

19 A-19 Appendix A - Digital Logic The Sum-of-Products (SOP) Form Truth Table for The Majority Function Transform the function into a two-level AND-OR equation Implement the function with an arrangement of logic gates from the set {AND, OR, NOT} M is true when A=0, B=1, and C=1, or when A=1, B=0, and C=1, and so on for the remaining cases. Represent logic equations by using the sum-of-products (SOP) form

20 A-20 Appendix A - Digital Logic The SOP Form of the Majority Gate The SOP form for the 3-input majority gate is: M = ABC + ABC + ABC + ABC = m3 + m5 +m6 +m7 = Σ (3, 5, 6, 7) Each of the 2 n terms are called minterms, running from 0 to 2 n - 1 Note the relationship between minterm number and boolean value. Discuss: common-sense interpretation of equation.

21 A-21 Appendix A - Digital Logic A 2-Level AND-OR Circuit Implements the Majority Function The encircled T intersections are electrically common (see next slide).

22 A-22 Appendix A - Digital Logic Notation Used at Circuit Intersections

23 A-23 Appendix A - Digital Logic A 2-Level OR-AND Circuit Implements the Majority Function

24 A-24 Appendix A - Digital Logic Positive vs. Negative Logic Positive logic: truth, or assertion is represented by logic 1, higher voltage; falsity, de- or unassertion, logic 0, is represented by lower voltage. Negative logic: truth, or assertion is represented by logic 0, lower voltage; falsity, de- or unassertion, logic 1, is represented by lower voltage

25 A-25 Appendix A - Digital Logic Positive and Negative Logic (Cont d.)

26 A-26 Appendix A - Digital Logic Bubble Matching Active low signals are signified by a prime or overbar or /. Active high: enable Active low: enable, enable, enable/ Ex: microwave oven control: Active high: Heat = DoorClosed Start Active low:? (hint: begin with AND gate as before.)

27 A-27 Appendix A - Digital Logic Bubble Matching (Cont d.)

28 A-28 Appendix A - Digital Logic The Data Sheet

29 A-29 Appendix A - Digital Logic Digital Components High level digital circuit designs are normally made using collections of logic gates referred to as components, rather than using individual logic gates. The majority function can be viewed as a component. Levels of integration (numbers of gates) in an integrated circuit (IC): Small scale integration (SSI): gates. Medium scale integration (MSI): 100 to 1000 gates. Large scale integration (LSI): ,000 logic gates. Very large scale integration (VLSI): 10,000-upward. These levels are approximate, but the distinctions are useful in comparing the relative complexity of circuits. Let us consider several useful MSI components:

30 A-30 Appendix A - Digital Logic The Multiplexer

31 A-31 Appendix A - Digital Logic Gate-Level Layout of Multiplexer

32 A-32 Appendix A - Digital Logic Implementing the Majority Function with an 8-1 Mux Principle: Use the mux select to pick out the selected minterms of the function.

33 A-33 Appendix A - Digital Logic Efficiency: Using a 4-1 Mux to Implement the Majority Function Principle: Use the A and B inputs to select a pair of minterms. The value applied to the MUX input is selected from {0, 1, C, C} to pick the desired behavior of the minterm pair.

34 A-34 Appendix A - Digital Logic The Demultiplexer (DEMUX)

35 A-35 Appendix A - Digital Logic The Demultiplexer is a Decoder with an Enable Input

36 A-36 Appendix A - Digital Logic A 2-to-4 Decoder

37 A-37 Appendix A - Digital Logic Using a 3-to-8 Decoder to Implement the Majority Function

38 A-38 Appendix A - Digital Logic The Priority Encoder An encoder translates a set of inputs into a binary encoding, Can be thought of as the converse of a decoder. A priority encoder imposes an order on the inputs. A i has a higher priority than A i+1

39 A-39 Appendix A - Digital Logic Programmable Logic Arrays (PLAs) A PLA is a customizable AND matrix followed by a customizable OR matrix:

40 A-40 Appendix A - Digital Logic Using a PLA to Implement the Majority Function

41 A-41 Appendix A - Digital Logic Using PLAs to Implement an Adder

42 A-42 Appendix A - Digital Logic A Multi-Bit Ripple- Carry Adder PLA Realization of a Full Adder

43 A-43 Appendix A - Digital Logic Sequential Logic The combinational logic circuits we have been studying so far have no memory. The outputs always follow the inputs. There is a need for circuits with memory, which behave differently depending upon their previous state. An example is a vending machine, which must remember how many coins and what kinds of coins have been inserted. The machine should behave according to not only the current coin inserted, but also upon how many coins and what kinds of coins have been inserted previously. These are referred to as finite state machines, because they can have at most a finite number of states.

44 A-44 Appendix A - Digital Logic Classical Model of a Finite State Machine An FSM is composed of a combinational logic unit and delay elements (called flipflops) in a feedback path, which maintains state information.

45 A-45 Appendix A - Digital Logic NOR Gate with Lumped Delay The delay between input and output (which is lumped at the output for the purpose of analysis) is at the basis of the functioning of an important memory element, the flip-flop.

46 A-46 Appendix A - Digital Logic S-R Flip-Flop The S-R flip-flop is an active high (positive logic) device.

47 A-47 Appendix A - Digital Logic NAND Implementation of S-R Flip-Flop A NOR implementation of an S-R flip-flop is converted into a NAND implementation.

48 A-48 Appendix A - Digital Logic A Hazard It is desirable to be able to turn off the flip-flop so it does not respond to such hazards.

49 A-49 Appendix A - Digital Logic A Clock Waveform: The Clock Paces the System In a positive logic system, the action happens when the clock is high, or positive. The low part of the clock cycle allows propagation between subcircuits, so that the signals settle at their correct values when the clock next goes high.

50 A-50 Appendix A - Digital Logic Scientific Prefixes For computer memory, 1K = 2 10 = For everything else, like clock speeds, 1K = 1000, and likewise for 1M, 1G, etc.

51 A-51 Appendix A - Digital Logic Clocked S-R Flip-Flop The clock signal, CLK, enables the S and R inputs to the flip-flop.

52 A-52 Appendix A - Digital Logic Clocked D Flip-Flop The clocked D flip-flop, sometimes called a latch, has a potential problem: If D changes while the clock is high, the output will also change. The Master-Slave flip-flop (next slide) addresses this problem.

53 A-53 Appendix A - Digital Logic Master-Slave Flip-Flop The rising edge of the clock loads new data into the master, while the slave continues to hold previous data. The falling edge of the clock loads the new master data into the slave.

54 A-54 Appendix A - Digital Logic Clocked J-K Flip-Flop The J-K flip-flop eliminates the disallowed S=R=1 problem of the S-R flip-flop, because Q enables J while Q disables K, and vice-versa. However, there is still a problem. If J goes momentarily to 1 and then back to 0 while the flip-flop is active and in the reset state, the flip-flop will catch the 1. This is referred to as 1 s catching. The J-K Master-Slave flip-flop (next slide) addresses this problem.

55 A-55 Appendix A - Digital Logic Master-Slave J-K Flip-Flop

56 A-56 Appendix A - Digital Logic Clocked T Flip-Flop The presence of a constant 1 at J and K means that the flip-flop will change its state from 0 to 1 or 1 to 0 each time it is clocked by the T (Toggle) input.

57 A-57 Appendix A - Digital Logic Negative Edge-Triggered D Flip-Flop When the clock is high, the two input latches output 0, so the Main latch remains in its previous state, regardless of changes in D. When the clock goes high-to-low, values in the two input latches will affect the state of the Main latch. While the clock is low, D cannot affect the Main latch.

58 A-58 Appendix A - Digital Logic Example: Modulo-4 Counter Counter has a clock input (CLK) and a RESET input. Counter has two output lines, which take on values of 00, 01, 10, and 11 on subsequent clock cycles.

59 A-59 Appendix A - Digital Logic State Transition Diagram for Mod-4 Counter

60 A-60 Appendix A - Digital Logic State Table for Mod-4 Counter

61 A-61 Appendix A - Digital Logic State Assignment for Mod-4 Counter

62 A-62 Appendix A - Digital Logic Truth Table for Mod-4 Counter

63 A-63 Appendix A - Digital Logic Logic Design for Mod-4 Counter

64 A-64 Appendix A - Digital Logic Example: A Sequence Detector Example: Design a machine that outputs a 1 when exactly two of the last three inputs are 1. e.g. input sequence of produces an output sequence of Assume input is a 1-bit serial line. Use D flip-flops and 8-to-1 multiplexers. Start by constructing a state transition diagram (next slide).

65 A-65 Appendix A - Digital Logic Sequence Detector State Transition Diagram Design a machine that outputs a 1 when exactly two of the last three inputs are 1.

66 A-66 Appendix A - Digital Logic Sequence Detector State Table

67 A-67 Appendix A - Digital Logic Sequence Detector State Assignment

68 A-68 Appendix A - Digital Logic Sequence Detector Logic Diagram

69 A-69 Appendix A - Digital Logic Example: A Vending Machine Controller Example: Design a finite state machine for a vending machine controller that accepts nickels (5 cents each), dimes (10 cents each), and quarters (25 cents each). When the value of the money inserted equals or exceeds twenty cents, the machine vends the item and returns change if any, and waits for next transaction. Implement with a PLA and D flip-flops.

70 A-70 Appendix A - Digital Logic Vending Machine State Transition Diagram

71 A-71 Appendix A - Digital Logic Vending Machine State Table and State Assignment

72 A-72 Appendix A - Digital Logic PLA Vending Machine Controller

73 A-73 Appendix A - Digital Logic Moore Counter Mealy Model: Outputs are functions of Inputs and Present State. Previous FSM designs were Mealy Machines, in which next state was computed from present state and inputs. Moore Model: Outputs are functions of Present State only.

74 A-74 Appendix A - Digital Logic Four-Bit Register Makes use of tri-state buffers so that multiple registers can gang their outputs to common output lines.

75 A-75 Appendix A - Digital Logic Left-Right Shift Register with Parallel Read and Write

76 A-76 Appendix A - Digital Logic Modulo-8 Counter Note the use of the T flip-flops, implemented as J-K s. They are used to toggle the input of the next flip-flop when its output is 1.

77 A-77 Appendix A - Digital Logic Reduction (Simplification) of Boolean Expressions It is often possible to simplify the canonical SOP (or POS) forms. A smaller Boolean equation generally translates to a lower gate count in the target circuit. We cover three methods: algebraic reduction, Karnaugh map reduction, and tabular (Quine-McCluskey) reduction.

78 A-78 Appendix A - Digital Logic Reduced Majority Function Circuit Compared with the AND-OR circuit for the unreduced majority function, the inverter for C has been eliminated, one AND gate has been eliminated, and one AND gate has only two inputs instead of three inputs. Can the function by reduced further? How do we go about it?

79 A-79 Appendix A - Digital Logic The Algebraic Method Consider the majority function, F. We apply the algebraic method to reduce F to its minimal two-level form:

80 A-80 Appendix A - Digital Logic The Algebraic Method This majority circuit is functionally equivalent to the previous majority circuit, but this one is in its minimal two-level form:

81 A-81 Appendix A - Digital Logic Karnaugh Maps: Venn Diagram Representation of Majority Function Each distinct region in the Universe represents a minterm. This diagram can be transformed into a Karnaugh Map.

82 A-82 Appendix A - Digital Logic K-Map for Majority Function Place a 1 in each cell that corresponds to that minterm. Cells on the outer edge of the map wrap around

83 A-83 Appendix A - Digital Logic Adjacency Groupings for Majority Function F = BC + AC + AB

84 A-84 Appendix A - Digital Logic Minimized AND-OR Majority Circuit F = BC + AC + AB The K-map approach yields the same minimal two-level form as the algebraic approach.

85 A-85 Appendix A - Digital Logic K-Map Groupings Minimal grouping is on the left, non-minimal (but logically equivalent) grouping is on the right. To obtain minimal grouping, create smallest groups first.

86 A-86 Appendix A - Digital Logic K-Map Corners are Logically Adjacent

87 A-87 Appendix A - Digital Logic K-Maps and Don t Cares There can be more than one minimal grouping, as a result of don t cares.

88 A-88 Appendix A - Digital Logic 3-Level Majority Circuit K-Kap Reduction results in a reduced two-level circuit (that is, AND followed by OR. Inverters are not included in the two-level count). Algebraic reduction can result in multi-level circuits with even fewer logic gates and fewer inputs to the logic gates.

89 A-89 Appendix A - Digital Logic Truth Table with Don t Cares A truth table representation of a single function with don t cares.

90 A-90 Appendix A - Digital Logic Tabular (Quine-McCluskey) Reduction Tabular reduction begins by grouping minterms for which F is nonzero according to the number of 1 s in each minterm. Don t cares are considered to be nonzero. The next step forms a consensus (the logical form of a cross product) between each pair of adjacent groups for all terms that differ in only one variable.

91 A-91 Appendix A - Digital Logic Table of Choice The prime implicants form a set that completely covers the function, although not necessarily minimally. A table of choice is used to obtain a minimal cover set.

92 A-92 Appendix A - Digital Logic Reduced Table of Choice In a reduced table of choice, the essential prime implicants and the minterms they cover are removed, producing the eligible set. F = ABC + ABC + BD + AD

93 A-93 Appendix A - Digital Logic Multiple Output Truth Table The power of tabular reduction comes into play for multiple functions, in which minterms can be shared among the functions.

94 A-94 Appendix A - Digital Logic Multiple Output Table of Choice F 0 (A,B,C) = ABC + BC F 1 (A,B,C) = AC + AC + BC F 2 (A,B,C) = B

95 A-95 Appendix A - Digital Logic Speed and Performance The speed of a digital system is governed by: the propagation delay through the logic gates, and the propagation delay across interconnections. We will look at characterizing the delay for a logic gate, and a method of reducing circuit depth using function decomposition.

96 A-96 Appendix A - Digital Logic Propagation Delay for a NOT Gate (Adapted from: Hamacher et. al. 2001)

97 A-97 Appendix A - Digital Logic MUX Decomposition

98 A-98 Appendix A - Digital Logic OR-Gate Decomposition Fanin affects circuit depth.

99 A-99 Appendix A - Digital Logic State Reduction Description of state machine M 0 to be reduced.

100 A-100 Appendix A - Digital Logic Distinguishing Tree A next state tree for M 0.

101 A-101 Appendix A - Digital Logic Reduced State Table A reduced state table for machine M 1.

102 A-102 Appendix A - Digital Logic Sequence Detector State Transition Diagram

103 A-103 Appendix A - Digital Logic Sequence Detector State Table

104 A-104 Appendix A - Digital Logic Sequence Detector Reduced State Table

105 A-105 Appendix A - Digital Logic Sequence Detector State Assignment

106 A-106 Appendix A - Digital Logic Sequence Detector K-Maps K-map reduction of next state and output functions for sequence detector.

107 A-107 Appendix A - Digital Logic Sequence Detector Circuit

Principles of Computer Architecture. Appendix A: Digital Logic

A-1 Appendix A - Digital Logic Principles of Computer Architecture Miles Murdocca and Vincent Heuring Appendix A: Digital Logic A-2 Appendix A - Digital Logic Chapter Contents A.1 Introduction A.2 Combinational