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Tutorial 2 : Digital Electronics 2
Question 1
A logic diagram of one typical stage for the Bidirectional Shift Register is given as below:
a) Draw a function table for the given Bidirectional Shift Register using mode
selection inputs S1 and S 0 .
b) Write four equivalent Register Transfer Language (RTL) for the register function
table given above.
c) For the given conditional RTL statement, draw the logic diagram associated with
it.
K1 : R0  R1  R 2, KI K 2 : R0 
 R1  R 2  1
Question 2
Given is the schematic for a 3-bit counter. Assuming that the initial stage/count = 000,
a) Analyze the schematic given and produce the state table (present & next state) for
the circuit. Show the necessary steps in getting the state table.
b) Give the full counting sequence for this counter (bi-directional if applicable).
Question 3
Modify the register of figure shown below so that it will operate according to the
following function table using mode selection inputs S1 and S 0 .
S1
0
0
1
1
S0
0
1
0
1
Register operation
No change
Clear register to 0
Shift down
Load parallel data
Question 4
The logic diagram for a 4-bit arithmetic circuit, using full adder and multiplexer is shown
in Figure 1.
MUX
Cin
Ai
Bi
0
1
Bi
0
1
2
3
X
FA
Gi
Y
Cout
S1
S0
S
Figure 1
Rajah 1
a)
b)
Determine the simplified Boolean function/expression for Inputs Xi and Yi .
Determine the arithmetic operation performed for each of the four
combinations of S and Cin : 00, 01, 10, 11. Specify it in a function table.
Question 5
a)
For each of the RTL (Register Transfer Level) micro-operations given, state
the results. Assume the initial values of the register contents:
R1 = 10011001
and
R2 = 11000011.
No.
RTL Code
i)
R2 
 R1  R2  1
ii)
R0 
 R1  R 2
iii)
R0 
 R1  R 2
iv)
R0 
 R1  R 2
b)
Using D flip-flops, design a register cell for an 8-bit register R0 that has the
following register transfer functions:
S1  S 0 : R0  0
S1  S 0 : R0  R0  R1
S1  S 0 : R0  R0  R1
S1  S 0 : R0  R0  R1
Question 6
(a) A logic circuit is used to implement transfers between four registers, R1, R2, R3 and
R4. The register transfers are as follows:
T1 : R1  R 3, R 4  R 3
T2 : R3  R 2
T3 : R 4  R1, R 2  R1
(i)
Let say the transfers between these registers need to be implemented by
using a bus-based transfer (single bus).
a. Sketch a logic circuit that converts the control signals T1, T2 and T3
as inputs to outputs that are the select inputs for the multiplexers (S0
and S1) and load signals for the registers (L1, L2, L3 and L4).
b. Sketch a detailed logic diagram of the hardware that implements the
register transfers.
Question 7
Give an explanation of serial mode in digital system. With the help of suitable diagram,
describe how serial transfer between two 4-bit registers can be done by utilizing shift
registers. (Hint : Use flip-flop to sketch the diagram)
Question 8
Serial addition is one the example of serial micro-operations. It might be slower but it
does require only small numbers of hardware.
(i)
By using a one single-bit Full Adder (FA), two four-bit shift registers
(Register X and Register Y), one D flip-flop and few gates, draw a
complete logic diagram of a serial adder. Also include other control
signals such as Reset, Shift and Clock. Note that the result of the addition
needs to be store in Register X.