SEMESTER ONE EXAMINATIONS 2002

Transcription

1 SEMESTER ONE EXAMINATIONS 2002 EE101 Digital Electronics Solutions Question 1. An assembly line has 3 failsafe sensors and 1 emergency shutdown switch. The Line should keep moving unless any of the following conditions arise: o If the emergency switch is pressed. o If sensor 1 and sensor 2 are activated at the same time. o If sensor 2 and sensor 3 are activated at the same time. o If all three sensors are activated at the same time. (a) Derive the truth table for this system. A = Switch B=Sensor1 C=Sensor2 D=Sensor3 Pressed or sensor activated = 1 F=Shutdown=1 A B C D F [4 marks for correct table] Page 1 of 1.

2 (b) Design, using Karnaugh Map techniques, a minimum AND-OR gate network for this system. Draw the resulting digital circuit diagram. Calculate Karnaugh Map: Resulting expression is : F = A + + CD [4 marks for map] [4 marks for expression & diagram] (c) Design, a digital circuit that will implement the minimal AND-OR gate network found in (b) using both (i) NAND gates only and (ii) NOR gates only. Assume that each logic gate can have any number of inputs and that inverted inputs are available. Use DeMorgan s theorem to convert to NAND/NOR gates. The student must use the theorem and cannot simply substitute the gates as this does not result in the minimal solution. (i) NAND F = A + + CD = A CD A.. CD Page 2 of 2.

3 (ii) NOR F A CD F A CD F A ( B C) ( C D) F F A ( B C) ( C D) (d) If the time delay experienced by a NAND gate is 8ns and the time delay experienced in a NOR gate is 10ns. Which implementation of (c) is faster By how long For the NAND gates the total delay is 2 x 8ns = 16ns For the NOR gates the total delay is 3 x 10ns = 30ns NAND gate implementation is faster by 14ns (e) Prove the rule of Boolean algebra: (A+B)(A+C) = A + ( A B)( A C) AA AC AB A AC AB A(1 C) AB A AB A(1 B) A [4 marks] Page 3 of 3.

4 Question 2. (a) Convert the following numbers to binary. Show each step clearly. (i) (ii) can be converted directly as as it is in hex form. Conversion to base 10 and then division by 2 will also give full marks do in two stages. 255/2 = 127 remainder 1 127/2 = 63 remainder 1 63/2 = 31 remainder 1 31/2 = 15 remainder 1 15/2 = 7 remainder 1 7/2 = 3 remainder 1 3/2 = 1 remainder 1 1 LHS = * 2 = (no carry) * 2 = carry of * 2 = carry of 1.5 * 2 = carry of 1.0 RHS =.0111 ANS (b) Perform the following using 9's complement Binary Coded Decimal (D) subtraction. (i) (ii) (get 9 s complement = 667) (1 st invalid) = (9 s complement of 555 = 444) (last invalid) 0110 Page 4 of 4.

5 (no end-around carry => negative) get nines complement of 562 = -437 (c) Explain the operation and outline the problems associated with asynchronous counters. Draw a graph of the outputs to help explain your answer. [4 marks] If the output of one counter is connected to the clock of the next counter then all 2 N states can be used in the binary counter. N flip-flops => 2 N states, base 2 N This counter is called an asynchronous or ripple counter as the clock pulse effectively ripples through the counter. The more correct waveform is shown here: There is a cumulative delay which is a major disadvantage. This cumulative delay of an asynchronous counter limits the rate at which a counter can be clocked and can also create decoding problems. False outputs that result are called glitches. [6 marks] Page 5 of 5.

6 Question 3. (a) Explain the operation of a right/left shift register. Draw the logic diagram to help explain its operation. [8 marks for diagram] Shift registers can be used to transfer data from right to left, shift left by connecting the output of a flip-flop back to the input of the flip-flop on its left. A shift from left to right, shift right, can be carried out during normal operation of the shift register A shift right and shift left register can be combined by suitable gating and control signals. In the operation above R / L 0 => shift right In the operation above R / L 1 => shift left Explain the operation of the logic gates simply an and gate with a value of 0 has an output of zero and an AND gate with one input of 1.A will have the output A. In effect the R/L line turns either the Right or Left AND gate on or off. The OR gate simply combines the output of the two AND gates to bring forward the correct output. [5 marks for description] (b) Design a two s complement adder/subtractor. Draw the logic diagram and explain how the design works in add mode and in subtract mode. Page 6 of 6.

7 This circuitry can generate the 2 s complement when we wish to do a subtraction When ADD / SUB = 0 => the circuit adds When ADD / SUB = 1 => the circuit subtracts XOR gates act as True/complement in that they invert B when the line is set to 1 and a 1 is added to the LSB, so that the two s complement will be generated. [8 marks for correct diagram] [4 marks for the description] Question 4. (a) Explain the operation of a shift register with feedback. Use diagrams to explain the operation. If the serial output of a shift register is connected back to the input [Q to J], [Q\ to K] then the sequence of numbers stored in the register will circulate. E.g. Q3 Q2 Q1 Q0 CLK pulse pulse pulse pulse 4 Note that the last row is the same as the first row. [7 marks for diagram] [3 marks for description] (b) Explain the operation of an n-bit ring counter with self correcting feedback. An N-bit ring counter has N states each of which contains all 0 s except for a single 1 An ring counter is wasteful of flip-flops as we only get N states with N flip-flops. Page 7 of 7.

8 We can use self correcting feedback to obtain desired sequences. In this case when occurs the AND gate will feed back a 1 (as the inputs to the AND gate are connected to the Q\ values) [Diagram 10 marks] [Description 5 marks] Question 5. (a) Explain the operation of a master-slave JK flip-flop. Draw the logic diagram for the flip-flop and describe in detail the two-stage approach of its operation, in particular explain why a master-slave configuration is preferable to a standard JK flip-flop. Two phase operation [8 marks for diagram] When the clock pulse is high the JK inputs are applied to the master flip-flop and the slave input is not affected [clk=0]. When the clock pulse falls to zero the data from the master is applied to the slave inputs and the outputs Q and Q\ then get their new values. But since CLK=0 the master flip-flop is unaffected by them and the flp-flop operates correctly. Problem with standard JK flip-flops is that if the data inputs change while the clock pulse is high then we get strange effects on the outputs. We need to insure that the Q and Q\ values do not assume their new values until after the trailing edge of the clock pulse ie. = 0. [7 marks for description] (b) Explain using diagrams the modifications required to convert (a) into a JK flipflop with asynchronous set and reset inputs. Why are these inputs a useful addition to the standard master-slave JK flip-flop Separate set and reset inputs can be added to the JK Flip-flop. These set and reset inputs [S\, R\] over-ride the JK inputs and are asynchronous [i.e. override the clock]. When S\=R\=1 we have normal flip-flop operation. Page 8 of 8.

9 [6 marks for addition to diagram] [4 marks for brief description] Page 9 of 9.