# COMBINATIONAL and SEQUENTIAL LOGIC CIRCUITS Hardware implementation and software design

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1 PH-315 COMINATIONAL and SEUENTIAL LOGIC CIRCUITS Hardware implementation and software design A La Rosa I PURPOSE: To familiarize with combinational and sequential logic circuits Combinational circuits are logic circuits whose outputs respond immediately to the inputs; there is no memory In a sequential logic circuit the outputs depend on the inputs plus its history; ie it has memory Experimental Section-1 You will build an ADDER (using 7400-NAND and 7402-NOR gates), as an example of combinational logic circuit Experimental Section-2 Sequential logic circuits are introduced through the construction of a RS latch (using NAND gates), which will help us to attain an understanding about how memory is developed in logic circuits Stability in the RS latch is obtained by implementing a series of gate controls, all of which lead to the development of the JK flip flop Commercially available JK flip flops will be used to construct an hexadecimal and a decimal ring counter To gain hands on experience on the software design, you will be required to LabView design a 3-to-8 decoder using combinational logic circuits II THEORETICAL CONSIDERATIONS II1 How is information coded in electronic digital form? II1A Defining the digital levels using a transistor switch II1 Counting objects: Decimal and binary system II1C Digital electronics II1A Digital levels Consider the transistor switch circuit shown in Fig1 Notice, if V in < 21 Volts The E diode would be reversed biased, therefore there will be no flow of electrons from E to That is, the transistor would be OFF No I current, no collector current It implies V out = V CC = 5 volts (Digital level 1) If V in > 21 As V in increases, the transistor moves out from cutoff along the loading line Further increase of V in makes the transistor reach the saturation stage, I C = 5 ma For a transistor of = 100, a base current equal to I = 50 A will saturate the transistor Thus, by applying an input voltage equal to, for example V in = 3(07) + (10k )(50 A) = 26 V the transistor will be saturated So, we expect that for input voltages in the range 21V <V in < 26 V the transistor will work in the active region

2 Note: In these digital electronics applications the transistor is not used in the active region V in I 10k I C npn V CC (+5 V) R C =1k C E V CE V out I C 5 ma Saturation 50 A Cutoff +5 V V CE =V out I C = V CC /R C (1/R C )V out Fig1 Transistor switch For Vin< 21V the output level is 5V; for Vin>26 V the output levels is close to 0 V If V in = 26 V As indicated above, for an input voltage of 26 V the transistor will be saturated, and the collector current would be I C = 5 ma The corresponding voltage drop across R E is then 1k x 5 ma = 5 Volts, which makes V out = 0 Volts If V in > 26 V The transistor remains saturated and V out = 0 Volts (Digital level 0) V CE 5 V Cutoff Active (forbidden) Saturation V in Logical output = 1 2 Logical 1 output = V 26 V Fig2 Switch transistor response and corresponding definitions of digital output signal levels Digital 1 Digital 0 2

3 II1 Decimal and binary systems Fig3 How to systematically count the elements of this system? Using an arbitrary numerical system We will count them in sub-groups of sizes A,, and C A 2 groups of size A 3 groups of size 1 group of size C C which can be expressed in he following notation 2A 3 1C A Fig4 Grouping under an arbitrarily given numerical system Using the decimal system We will count them in sub-groups of 10 0, 10 1, 10 2, 10 3, 3

4 4 groups of 10 5 groups of 1 4 (10 1 ) 5 (10 0 ) Then, as we assume that the decimal system is being used, we just write: 4 5 Array of decimal digits Fig 5 Grouping under the decimal numerical system The position of a digit gives the increasing powers of 10 in the number inary system We will count them in sub-groups of 2 0, 2 1, 2 2, 2 3, group of group of group of group of group of group of (2 5 ) 0 (2 4 ) 1 (2 3 ) 1 (2 2 ) 0 (2 1 ) 1 (2 0 ) When the binary system is assumed implicitly being used, we just write: Array of binary digits Fig 6 Grouping under the binary numerical system The position of a digit gives the increasing powers of 2 in the number 4

5 II1C Digital electronics Using an array of transistor circuits V in V CC (+5 V) V out 5 V Interpreted as logic levels 1 V CC (+5 V) V in V out 0 V 0 V CC (+5 V) V in V out 5 V 1 Fig 7 III EXPERIMENTAL CONSIDERATIONS III1 Combinational Logic Circuits III1A Logic gates III1 Digital Arithmetic: Adder circuit III2 Sequential Logic Circuits III21 How memory is developed in logic circuits: SR LATCH III22 Adding control to the SR latch: GATED FLIP=FLOP III23 Reducing the gating time: EDGE TRIGGERED FLIP FLOPS III24 Eliminating the forbidden sates: JK FLIP FLOP III25 JK Flip-flop applications III3 LabView Design of a Decoder III4 Registers III5 Memory Circuits III1 COMINATIONAL LOGIC CIRCUITS Combinational circuits are logic circuits whose outputs respond immediately to the inputs; there is no memory 5

6 III1A Digital logic gates Combinational Digital gates are circuits that pass or block signals moving through a logic circuit NOT gate (Integrated circuit 7404 INVERTER ) A Input The small circle indicates inversion A Output Input Output A A Note: The overscore on the symbol A means NOT or logical complement AND gate A Inputs AND Output Inputs Output A =A = A NAND gate (Integrated circuit 7400 NAND ) A Inputs Output = A Inputs Output A =A

7 OR gate (Integrated circuit 7432 OR) A Inputs OR Output Inputs Output A =A = A NOR gate (Integrated circuit 7402 NOR) A Inputs Output = A + Inputs Output A =A EXCLUSIVE OR gate A Inputs XOR Output Inputs Output A =A = A

9 A (A ) A Experimental implementation of A (A ) A A (A ) A A A A + + A Inverter NOR gate NAND gate NOR gate Fig 9 XOR design with NAND and NOR gates Hence the following implementation constitutes a half adder circuit A 1 1 A 1 1 A 1 1 C 1 A A 1 1 S 1 Fig 10 Half adder circuit FULL ADDER Task: uild the circuit below and verify that it works as a full adder (it adds two digits plus a previous carrier) In particular, explain in detail how the OR gate makes the trick for the full-adder to work 9

10 A 2 2 C 1 Half adder C 2 A Half adder C 2 S 2 Fig 11 Full adder circuit III2 SEUENTIAL LOGIC CIRCUITS III21 How memory is developed in logic circuits: SR LATCH III22 Adding control to the SR latch: GATED FLIP FLOP III23 Reducing the gating time: EDGE TRIGGERED FLIP FLOPS III24 Eliminating the forbidden sates: JK FLIP FLOP III25 JK Flip-flop applications Logic circuits, like the adder circuit, are called combinational logic circuits Their characteristics are: The output responds immediately to the inputs There is no memory In contrast, in a sequential logic circuit The output not only depend on the inputs, but also on the inputs history That is, a sequential logic circuit has a memory III21 How memory is developed in logic circuits: S-R LATCH Task: Implement the circuit shown in Fig 10 and verify the table of truth S R P I N P U T S O U T P U T S Fig 10 Latch circuit displaying electronic memory properties S R P Unambiguous output Remembers the previous state Unambiguous output Remembers the previous state P Notice, except when S=R=0, the output satisfies Since we want the latter relation to hold, we will forbid the S=R=0 input state Hence, the above result is equivalently expressed as follows: 10

11 S R Fig 11 S-R latch with complementary outputs I N P U T S O U T P U T S S R Forbidden Sets Memory Sets memory III22 Adding control to the SR latch: GATED FLIP FLOP The SR latch requires a few refinements For example, it responds to its input signals immediately and at all times Problems can occur when logic signals that are supposed to arrive at the same time actually arrive at slightly different times due to separate delays Such timing problems can create short unwanted pulses called glitches The gated flip flop shown below corrects this problem n+1 n S R Fig 12 Gated latch Notice: The circuit responds to input logic signals only when the clock input is in state 1 When is in state 0, the outputs of the NAN gates on the left become equal to 1 and, thus, the outputs and remains in memory state The table of truth for the circuit in Fig12 can be obtained directly from the table of truth of the circuit in Fig 11 by simply interchanging the levels 1 and 0 S R FF While is high I N P U T S O U T P U T S S R Forbidden Sets Memory Sets memory 11

12 Alternatively the table of truth ca be expresses in such a way as to list the output state after a clock gating pulse : 010 I N P U T S O U T P U T S S R n+1 n Forbidden Sets Sets n n III23 Reducing the gating time: EDGE TRIGGERED FLIP FLOPS To even further protect the flip flops from glitches, the gating time (the time during which the input signals affect the output signals) can be reduced by making the circuit sensitive only when the clock signal makes transitions from either high to low or vice versa This is known as edge triggering S FF S FF R R Leading edge triggering Trailing edge triggering Fig 13 Symbols for edge triggered flip flops Triggering at the edges limits the time during which the inputs are active III24 Eliminating the forbidden sates: JK FLIP FLOP A problem with the S-R latches is the forbidden state at the inputs The circuit below shows an alternative to correct such shortcoming 12

13 J S J S FF K R K R Fig 14 Version of a J-K flip flop (No need to implement this circuit in this lab session) The corresponding table if truth is, I N P U T S O U T P U T S J K FF J K n+1 n n n TOGGLE Sets Sets n n Memory Fig 15 J-K flip flop and its standard table of truth When the inputs J and K are equal to 1, the outputs and will change to its complementary value after each clock pulse The toggle feature reveals the advantage of edge triggering for the JF flip flop: if the gating time were extended in time, the output state would oscillate back and forth and the eventual final output (when the gating is off) would be undetermined The JK flip-flop is a very versatile device, and is probably the most commonly used form of flipflop in digital electronic and control circuits D- FLIP FLOP 13

14 D FF I N P U T O U T P U T S D n+1 n Fig 16 D flip flop Notice it has the effect of transferring the input to the output at the active clock edge T- FLIP FLOP T FF T n+1 n+1 I N P U T O U T P U T S 1 n n 0 n n Fig 17 The T flip flop toggles with the clock pulse when T=1 and does not toggle when T=0 Commercial JK FLIP FLOP Use a commercially available JK flip flop chip (IC DUAL JK EDGE-TRIG F/F 16 DIP) and familiarize with the its functioning The data sheet is available on the website of this course The JK flip flop is considered a universal flip flop The flip flop is SET when it store a binary 1 (=1) This is obtained by applying momentarily a LOW at the PR input The flip flop is CLEARED (also known as RESET) when it store a binary 0 ( = 0) This is obtained by applying momentarily a LOW at the CLR input Clear first the flip flop and then check the different mode of operations: SET MODE: Place J=1 and K=0 and verify it causes the flip flop to set (=1) when the clock transits from high to low RESET MODE: Place J=0 and K=1 and verify it causes the flip flop to clear (or reset; ie =1) when the clock transits from high to low HOLD MODE: Place J=0 and K=0 and verify it the out does not change upon the arrival of clock pulses TOGGLE MODE: Place J=1 and K=1 and verify changes back and forth to the high and low levels upon the arrival of clock pulses III23 JK FLIP APPLICATIONS 14

15 Hexadecimal Ring Counter TASK: Construct a hexadecimal ring counter exploiting the toggle mode of the JK flip flop Implement into the counter the capability to be reset (or clear) at any arbitrary time Also, make a diagram displaying the digital signals of the clock and the four -outputs as a function of time All J=1 J CLK K PR CLR 0 J K PR CLR 1 J K PR CLR 2 J K PR CLR 3 Fig 18 Asynchronic counter Hints: It may occur that when connecting the -outputs to the monitoring LEDs, the latter may affect the functioning of the counter (the -output not being able to drive the clocks) As potential solutions, you may: Opt to display the output of the counter by monitoring the outputs instead (thus relieving the -outputs to do its job driving the clock of the next flip-flop) Opt to keep using the same design of Fig 18, but inserting a resistor (try 1k, or 10k) between the -output and the corresponding LED Decade Ring Counter It often more convenient to have counters based on 10 rather than 16 The ring counter you built above can be converted to a decade counter by providing a RESET or CLEAR every time the system reaches 10 Since = an NAND gate with inputs could make the trick Such gate will output 1 when the input varies from 0=0000 to 9=1001, but will transition to zero at 1010 Such output can be feedback to the CLEAR input of the JK flip flops TASK: Implement a decade ring counter Implement the CLEAR feature described above using the 2-input NAND gates III3 LAVIEW DESIGN: 3 to 8 Decoder The figure below shows a LabVIEW design of a 2-to-4 decoder (see figure below) That is, for a binary input 00 only the O LED lights up; for the binary input 01 only the 1 LED lights up; etc 15

16 Fig 19 LabView design of a 2 to 4 decoder TASK: Use LabVIEW software to build a 3-to-8 decoder using combinational logic circuits Helpful references: Getting started with LabView III4 REGISTERS A register is a series of flip flops arranged for organized storage or processing of binary information Information is represented in a computer by groups of 0 s and 1 s called words A 8-bit word is called a byte Large computers work with words of 32 or more bits A register in a computer with 8-bit words would require 8 flip flops to store or process simultaneously the 8 bits of information Words of information are moved around in a computer on a bus The bus consists of a number of conducting paths connecting all potential source-registers with all potential destination-registers 16

17 D D D D Register LOAD Fig 20 Parallel input and parallel output Loading a register of 4 D-type flip flops from a bus At the trailing edge of the LOAD signal, the information on the bus is stored in the register us Shift register Sometimes digital information must be sent over one channel In this case, bits are sent in serial form When digital information must be received in serial form, a shift register mat be used to accept the serial information and convert it to parallel form Input D D D D Register Fig 21 Shift register The input at the D flip flop is shifted to the output at the action of a clock pulse III5 MEMORY CIRCUITS Read-Only Memories The decoder alluded in section III3 above are an example of what has come to be called a read-only memory, or ROM A ROM associates a specific output binary number with each input binary number according to its fixed internal logic The fixed relationship between input and output distinguishes the ROM from other memory circuits An important application of ROMs is to provide look-up tables for mathematical functions, such as trigonometric, exponential, square root, and logarithmic functions 17

18 In certain applications, most notably in microprocessors circuits, it proves useful to be able to enter the information in a ROM after the fabrication of the device In such a programmable ROM, or PROM, the desired memory bits are stored by electrically altering the circuit connections Similarly, erasable PROM are available in which information is stored as charge on stray capacitance at the gate electrodes of a MOSFET ROM without actually destroying the gate electrodes These bit patterns can be erased by irradiation with ultraviolet light to discharge the gate capacitors or other electrical signals[ref 3] Shift Register Memories In many applications it proves useful to store digital information temporarily for recall at later time This is a memory into which information can be rapidly written and changed, as well as read out Shift registers are convenient and effective memory circuits for this purpose Random-Access Memories The access time in a shift-register memory depends upon the word address and upon the word storage capacity of the memory since information is only available sequentially at the shift register outputs In a random-access memory (RAM) the access time is independent of the location of information in the memory; addressing logic permits immediate access to any information stored in the memory A RAM is organized into words lines and bit lines, and information is stored at each intersection by the state of a flip flop memory cell References 1 J R Cogdell, "Foundations of Electronics," Prentice Hall (1999) 2 The JK flip flop 3 J rophy, "asic Electronics for Scientists," 5th Ed McGraw-Hill (1990) See chapter 9 18

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