Unit 3 Digital Circuits (Logic)

Transcription

1 Unit 3 Digital Circuits (Logic) 1

2 2 A Brief History COMPUTERS AND SWITCHING TECHNOLOGY

3 3 Mechanical Computers Primarily gearbased Difference Engine and Analytic Engine designed and partially implemented by Charles Babbage Used mechanical levers, gears, and ball bearings, etc. Difference Engine (1823) prototype and not fully programmable Analytic Engine (1834) Never completed To be programmed with punch cards Designed to perform 4 basic arithmetic ops. (add, sub, mul, div) Charles Babbage and his Difference Engine

4 ENIAC was first, fully electronic computer Electronic Computers Used thousands of vacuum tubes as fundamental switching (on/off) technology Weighed 30 tons, required 15,000 square feet, and maximum size number was 10 decimal digits (i.e. ±9,999,999,999) Still required some patch panels (wire plugs) to configure it

5 5 Vacuum Tube Technology Digital, electronic computers use some sort of voltage controlled switch (on/off) Looks like a light bulb Usually 3 nodes 1 node serves as the switch value allowing current to flow between the other 2 nodes (on) or preventing current flow between the other 2 nodes (off) Example: if the switch input voltage is 5V, then current is allowed to flow between the other nodes A Switch Input (Hi or Lo Voltage) Current can flow based on voltage of input switch B Vacuum Tube

6 6 Vacuum Tube Disadvantages Relatively large Especially when you need 19,000 to make 1 computer Unreliable Can burn out just like a light bulb Dissipate a lot of heat (power)

7 7 Transistor Another switching device Invented by Bell Labs in 1948 Uses semiconductor materials (silicon) Much smaller, faster, more reliable (doesn't burn out), and dissipated less power Individual Transistors (About the size of your fingertip) Source Gate 5V Drain High voltage at gate allows current to flow from source to drain Source Gate 0V Silicon Drain Silicon Low voltage at gate prevents current from flowing from source to drain Transistor is 'on' Transistor is 'off'

8 8 Moore's Law & Transistors Moore's Law = Number of transistors able to be fabricated on a chip will double every years Transistors are the fundamental building block of computer HW Switching devices: Can conduct [on = 1] or notconduct [off = 0] based on an input voltage

9 9 How Does a Transistor Work Transistor inner workings

10 10 NMOS Transistor Physics Transistor is started by implanting two ntype silicon areas, separated by ptype ntype silicon (extra negative charges) Source Input Drain Input ptype silicon ("extra" positive charges)

11 11 NMOS Transistor Physics A thin, insulator layer (silicon dioxide or just "oxide") is placed over the silicon between source and drain Source Input Drain Output Insulator Layer (oxide) ntype silicon (extra negative charges) ptype silicon ("extra" positive charges)

12 12 NMOS Transistor Physics A thin, insulator layer (silicon dioxide or just "oxide") is placed over the silicon between source and drain Conductive polysilicon material is layered over the oxide to form the gate input Source Input Gate Input Drain Output conductive polysilicon Insulator Layer (oxide) ntype silicon (extra negative charges) ptype silicon ("extra" positive charges)

13 13 NMOS Transistor Physics Positive voltage (charge) at the gate input repels the extra positive charges in the p type silicon Result is a negativecharge channel between the source input and drain Source Input negativelycharge channel Gate Input positive charge "repelled" ptype Drain Output ntype

14 14 NMOS Transistor Physics Electrons can flow through the negative channel from the source input to the drain output The transistor is "on" Gate Input Source Input Drain Output ntype ptype Negative channel between source and drain = Current flow

15 15 NMOS Transistor Physics If a low voltage (negative charge) is placed on the gate, no channel will develop and no current will flow The transistor is "off" Gate Input Source Input Drain Output ntype ptype No negative channel between source and drain = No current flow

16 16 View of a Transistor Crosssection of transistors on an IC Moore's Law is founded on our ability to keep shrinking transistor sizes Gate/channel width shrinks Gate oxide shrinks Transistor feature size is referred to as the implementation "technology node" Electron Microscope View of Transistor CrossSection

17 DIGITAL LOGIC GATES 17

18 18 Transistors and Logic Transistors act as switches (on or off) A S1 S2 B Logic operations (AND / OR) formed by connecting them in specific patterns Series Connection Parallel Connection A Series Connection S1 AND S2 must be on for A to be connected to B S1 S2 Parallel Connection S1 OR S2 must be on for A to be connected to B B

19 19 Digital Logic Forms the basic processing circuits for digital signals (i.e. 1's and 0's) Digital Logic still abstracts many of the physical issues (voltage, current, parasitics, etc.) dealt with in the study of integrated circuits An alarm should sound if the key is in the ignition AND your seatbelt is NOT fastened If the voltage threshold sensor rises above 3 volts create a conductive channel to excite the ignition sensor Computer Architecture (Functional Blocks) Digital Logic Integrated Circuits (Transistors) OR

20 20 Gates Each logical operation (AND, OR, NOT) can be implemented in circuit form using the corresponding logic gate AND Gate OR Gate NOT Gate

21 21 AND Gates An AND gate outputs a '1' (true) if ALL inputs are '1' (true) Gates can have several inputs Behavior can be shown in a truth table (listing all possible input combinations and the corresponding output) X Y Z F X Y F X Y F=X Y F X Y Z F=X Y Z F input AND 3input AND

22 22 OR Gates An OR gate outputs a '1' (true) if ANY input is '1' (true) Gates can also have several inputs X Y Z F X Y F=XY F X Y F X Y Z F=XYZ F input OR 3input OR

23 23 NOT Gate A NOT gate outputs a '1' (true) if the input is '0' (false) Also called an "Inverter" X F X F F = X

24 24 NAND and NOR Gates Inverted versions of the AND and OR gate X Y NAND Z X Y NOR Z Z X Y Z X Y X Y Z X Y Z X Y Z X Y Z AND NAND OR NOR True if NOT ALL inputs are true True if NOT ANY input is true

25 25 XOR and XNOR Gates Exclusive OR gate. Outputs a '1' if either input is a '1', but not both. X Y Z X Y Z XOR XNOR Z X Y Z X Y X Y Z X Y Z True if an odd # of inputs are true = True if inputs are different True if an even # of inputs are true = True if inputs are same

26 26 Logic Example A B C 0 D F 1

27 27 Logic Example A B C 0 D F 0

28 28 Delay Example A B C 0 D Levels of Logic F 1 0 Change in D, C, or A must propagate through 4 levels of gates

29 29 Logical Operations Summary All digital circuits can be described using AND, OR, and NOT Note: You'll learn in future courses that digital circuits can be described with any of the following sets: {AND, NOT}, {OR, NOT}, {NAND only}, or {NOR only} Normal convention: 1 = true / 0 = false A logic circuit takes some digital inputs and transforms each possible input combination to a desired output values Triviaoftheday: The Apollo Guidance Computer that controlled the lunar spacecraft in 1969 was built out of 8,400 3input NOR gates. Inputs I 0 I 1 I 2 Logic Circuit O 0 O 1 Outputs

30 30 Sequential Devices (Registers) AND, OR, NOT, NAND, and other gates are known as combinational logic Outputs only depend on what the inputs are right now, not one second ago This implies they have no "memory" (can't remember a value) Sequential logic devices provide the ability to retain or "remember" a value by itself (even after the input is changed or removed) Outputs can depend on the current inputs, and previous states of the circuit (stored values.) Usually have a controlling signal that indicates when the device should update the value it is remembering vs. when it should simply remember that value This controlling signal is usually the "clock" signal

31 31 Registers Registers are the most common sequential device Registers sample the data input (D) on the edge of a clock pulse (CP) and stores that value at the output (Q) Analogy: Taking a picture with your digital camera when you press a button (clock pulse) the camera samples the scene (input) and remembers/saves it as a snapshot (output) Data Input (could be many bits) Clock pulse D CP Q Block Diagram of a Register Data Output (could be many bits) t = 0 ns t = 1 ns t = 5 ns t = 7 ns t = 10 ns The clock pulse (positive edge) here Clock pulse d(t) Some input value changing over time d(1) d(2) d(3) d(4) d(5) d(6) d(7) d(8) d(9) d(10) d(11) d(12) q(t) unk d(1) d(5) d(7) d(10) causes q(t) to sample and hold the current d(t) value

32 32 FlipFlops Flipflops are the building blocks of registers 1 Flipflop PER bit of input/output There are many kinds of flipflops but the most common is the D (Data) Flipflop (a.k.a. DFF) D Flipflop triggers on the clock edge and captures the Dvalue at that instant and causes Q to remember it until the next edge Positive Edge: instant the clock transition from low to high (0 to 1) PositiveEdge Triggered DFF Clock pulse d(t) Clock Signal D DFF CLK Q q(t) d(t) q(t)

33 33 Registers and Flipflops A register is simply a group of D flipflops that all trigger on a single clock pulse D0 D1 4bit Register D Q DFF D Q Q0 Q1 DFF CLK Q t1 D2 D Q Q2 Steady level of 0 or 1 0 Q t 1 Q t DFF Positive Edge D t D3 D Q Q3 CP DFF

34 34 Pulses and Clocks Registers need an edge to trigger We can generate pulses at specific times (creating an irregular pattern) when we know the data we want has arrived Other registers in our hardware should trigger at a regular interval For that we use a clock signal Alternating high/low voltage pulse train Controls the ordering and timing of operations performed in the processor 1 cycle is usually measured from rising/positive edge to rising/positive edge Clock frequency (F) = # of cycles per second Clock Period (T) = 1 / Freq. 1 (5V) 0 (0V) Clock Pulses Clock Signal 1 cycle Op. 1 Op. 2 Op GHz = 2.8*10 9 cycles per second = ns/cycle Processor

35 35 Combinational vs. Sequential Sequential logic (i.e. registers) is used to store values Each register is analogous to a variable in your software program (a variable stores a number until you need it) Combinational logic is used to process bits (i.e. perform operations on values Analogous to operators (,,*) in your software program