Using Combinational Circuits for Control Purposes
Mohammed Al-Husainy2009, Journal of Computer Science
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Problem statement: Combinational circuits are used in computers for generating binary control decisions and for providing digital components for data processing. Approach: The use of combinational circuits and logic gates to control other circuits was discussed. Different systems that use logic gates, multiplexers, decoders and encoders to control different circuits were presented. This study presented a design and implementation of some combinational circuits such as a decoder, an encoder, a multiplexer, a bus system and read/write memory operations. Results: When we connected some types of combinational circuits to the inputs/outputs of digital circuit, these combinational circuits can help us to manage and flow a different types of control signals through a large digital circuit. Conclusion: Many combinational circuits had a good function which can be used for controlling different parts of any digital system and they produce a suitable way to transfer a control signals between different digital components of any large digital system.
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Journal of Computer Science 5 (7): 507-510, 2009
ISSN 1549-3636
© 2009 Science Publications
Using Combinational Circuits for Control Purposes
Maher A. Nabulsi and Mohammed A.F. Al-Husainy
Department of Computer Science, Faculty of Sciences and Information Technology,
Al-Zaytoonah University of Jordan, Amman, Jordan
Abstract: Problem statement: Combinational circuits are used in computers for generating binary
control decisions and for providing digital components for data processing. Approach: The use of
combinational circuits and logic gates to control other circuits was discussed. Different systems that
use logic gates, multiplexers, decoders and encoders to control different circuits were presented. This
study presented a design and implementation of some combinational circuits such as a decoder, an
encoder, a multiplexer, a bus system and read/write memory operations. Results: When we
connected some types of combinational circuits to the inputs/outputs of digital circuit, these
combinational circuits can help us to manage and flow a different types of control signals through a
large digital circuit. Conclusion: Many combinational circuits had a good function which can be used
for controlling different parts of any digital system and they produce a suitable way to transfer a
control signals between different digital components of any large digital system.
Key words: Decoder, encoder, multiplexer, register, control unit
specific digital functions commonly needed in the
design of digital systems.
Some combinational circuits, such as decoders,
encoders and multiplexers, can be used to control other
devices such as decoder circuits[1,2], three-state
buffers[3,4], register circuits[3], bus circuits, read / write
memory operations[5] and others. Decoder and other
combinational circuits can be used in mobile system,
wireless networks and can be applied for other related
communication systems[6,7]. Decoder logic is essential
for control units and memories[8].
INTRODUCTION
Logic circuits for digital systems may be
combinational or sequential. A combinational circuit
consists of logic gates whose outputs at any time are
determined from the present combination of inputs. A
combinational circuit performs an operation that can
be specified logically by a set of Boolean functions.
Sequential circuits such as registers, counters and
memories employ storage elements in addition to logic
gates. Their outputs are a function of the inputs and
the state of the storage elements. The state of storage
elements, in turn, is a function of previous inputs. As a
consequence, the outputs of a sequential circuit
depend not only on present values of inputs, but also
on past inputs and the circuit behavior must be
specified by a time sequence of inputs and internal
states.
A combinational circuit consists of input variables,
logic gates and output variables. The logic gates accept
signals from the inputs and generate signals to the
outputs. This process transforms binary information
from the given input data to a required output data.
There are several combinational circuits that are
employed extensively in the design of digital systems.
These circuits are available in integrated circuits and
are classified as standard components. They perform
MATERIALS AND METHODS
Here, we use a simple control unit[3,8-12] to generate
control signals, which can be used to control different
devices illustrated in our examples. The control unit
contains an Instruction Register (IR) to store the
instruction code which is read from memory, (4×16)
decoder to decode a 4-bit operation code, having only
one output (control signal) active at any given time
(either D0 or D1 or …or D15) and some control logic
gates.
We can use a multiplexer to control a register
circuit. For example, we use two multiplexers to control
a 2-bit register constructed from two D flip-flops as
Corresponding Author: Maher A. Nabulsi, Department of Computer Science, Faculty of Sciences and Information Technology,
Al-Zaytoonah University of Jordan, Amman, Jordan
507
J. Computer Sci., 5 (7): 507-510, 2009
seen in Fig. 1. This register circuit is controlled by a
common selection line of multiplexers (LD):
•
•
If LD = 0 (when D0 is not active), then A0 and A1
are transferred to D flip-flops and we have a nochange state (no loading of new information)
If LD = 1 (when D0 is active with operation code
0000), then I0 and I1 are transferred to D flip-flops
and we have a loading state of new information.
This situation is shown in Table 1
S0 and S1 will be equal (00) out of the (4×2) encoder,
RA will be transferred to the bus lines, the LD input of
RB will be active out of the (2×4) decoder, RB will
receive information from the bus lines and the first
transfer is performed (D1: RB ← RA). All other
transfers will be performed in the same manner as
shown in Table 2.
A decoder, an encoder and a multiplexer can be
used to control some transfers through a bus system.
Suppose, for example, it is required to perform the
following four transfers through the bus system
constructed from multiplexers:
D1: RB ← RA
D2: RC ← RB
D3: RD ← RC
D4: RA ← RD
Where, RA, RB, RC, RD are 4 bit registers and D1, D2,
D3, D4 are control signals. Figure 2 shows the bus system
and the control of the required transfers. In this example,
the operation code for the first transfer is (0001), for
the second is (0010), for the third is (0011), for the
fourth is (0100) and the other operation codes are for
other operations in the system. If the operation code is
(0001), then D1 will be active out of the (4×16) decoder,
Fig. 1: Two multiplexers with one common selection
line to control a 2 bit register
Table 1: Data inputs selected for D flip-flops based on LD control
input
D
selection
Op code
active input: LD Output: D Operation
Any Op code except 0000 D0
0
A
Unchanged
state
0000
1
I
loading
D0
Fig. 2: Using a decoder, an encoder and a multiplexer to control some transfers through a bus system
508
J. Computer Sci., 5 (7): 507-510, 2009
Table 2: Four transfers through the bus system based on some control inputs
S0
Op. code
D active
S1
0
0
0
1
D1
0
0
0
0
1
0
D2
0
1
0
0
1
1
D3
1
0
1
1
0
1
0
0
D4
Table 3: Read and write operations on memory
Op. code
D active
Enabled decoder
0101
D5
Decoder 1
Decoder 2
0110
D6
Selected Reg.
Bus ← RA
Bus ← RB
Bus ← RC
Bus ← RD
LD active
LD of RB
LD of RC
LD of RD
LD of RA
Operation
RB ← RA
RC ← RB
RD ← RC
RA ← RD
Table 4: The control of some inputs of a register (INR and CLR)
The active control
Op code D active
AC test
input of PC
0111
D7
AC (15) = 0
INR
1000
D8
AC (15) = 1
INR
1001
D9
AC = 0
INR
AC = FFFF
CLR
1010
D10
Operation
Write
Read
Fig. 3: Two decoders for read and write operations on
memory
Fig. 4: The implementation of control and execution of
some instructions
Data items to be stored (write) in memory or which
are retrieved (read) from memory to be used in
calculation are sent along the data bus. The address in
which that data item is to be stored, or from which it is
to be retrieved, is sent along the address bus. All
activity within the microcomputer, including the
storage and retrieval of data items, is initiated by the
microprocessor but managed by the control bus. It is the
responsibility of the control bus, for example, to switch
the direction of travel on the data bus depending on
whether a data item is to be stored or retrieved from
memory.
For controlling purposes, we can use two decoders
for write to memory and read from memory operations
as shown in Fig. 3. In this example, we have a memory
unit of 64 locations, each location of 8 bits, constructed
from flip-flops with a 6-bit address bus. Here, if the
operation code is (0101), then D5 will be active out of
the (4×16) decoder. D5 is connected to the enable input
of Decoder1 for the write to memory operation and D6
is connected to the enable input of Decoder2 for the
read operation through three-state buffers. This
situation is shown in Table 3.
Also, we can use some logic gates such as AND,
NAND, OR, NOR, NOT and others to control and
execute some instructions. In this example, we have a
16-bit processor register (accumulator: AC) and a
program counter (PC) to hold and calculate the address
of instructions. Here, we show how to control the
following instructions: SPA (skip next instruction if AC
is positive), SNA (skip next instruction if AC is
negative), SZA (skip next instruction if AC is zero) and
COA (clear PC if AC is all 1's). We specify these
instructions with the control signals (D7, D8, D9, D10)
and micro-operations as following:
SPA D7: if (AC (15) = 0), then PC ← PC + 1
SNA D8: if (AC (15) = 1), then PC ← PC + 1
SZA D9: if (AC = 0), then PC ← PC + 1
COA D10: if (AC = FFFF), then PC ← 0
The implementation of these instructions is seen in
Fig. 4. The conditions necessary to set the required
control input of PC are shown in Table 4.
509
J. Computer Sci., 5 (7): 507-510, 2009
RESULTS
2.
Wakerly, J.F., 2006. Digital Design-Principles and
Practices. 4th Edn., Pearson Prentice Hall, USA.,
ISBN: 0132128381, pp: 859.
3. Mano,
M.M.,
1993.
Computer
System
Architecture. 3rd Edn., Prentice Hall, USA., ISBN:
0-13-175738-5, pp: 525.
4. Nabulsi, M.A. and A.M. Abdalla, 2008. The
relationship between exclusive-or and the unique
existential quantifier. J. Comput. Sci., 4: 741-743.
5. Mano, M.M., 2002. Digital Design. 3rd Edn.,
Prentice Hall, USA., ISBN: 0-13-062121-8.
6. Chaiwat Keawsai, Keattisak Sripimanwat and
Attasit Lasakul, 2004. Modified register exchange
method of viterbi decoder for 3GPP mobile system.
ECTI,
Pataya,
Thailand.
http://www.kmitl.ac.th/dslabs/download/paper/viter
bidecoder_ecti2004.pdf
7. Zhang, L., Y.J. Cheng and X. Zhou, 2009. Rate
avalanche: Effects on the performance of multi-rate
802.11 wireless networks. Simulat. Model. Pract.
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487-503.
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10.1016/j.simpat.2008.09.003
8. Rob Williams, 2001. Computer Systems
Architecture.
Addison-Wesley,
ISBN:
0201648598.
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Architecture Designing for Performance. 7th Ed.
Pearson Prentice Hall, USA., ISBN: 0131856448,
pp: 778
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design and optimization of control units for VLSI
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11. Tang, K.S., K.F. Man, G. Chen and S. Kwong,
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DOI: 10.1007/BF02836678
Some combinational circuits such as a decoder, an
encoder and a multiplexer were used to control other
devices:
•
•
•
•
Using multiplexers to control the load operation of
a register
Using a decoder, an encoder and a multiplexer to
control some transfers through the bus system
Using two decoders to control the read and write
operations on memory
Using some logic gates to control the INR and
CLR inputs of a register
DISCUSSION
This study tries to focus on the usefulness of some
common combinational circuits (i.e., a decoder, an
encoder and a multiplexer) in controlling many
different components and operations (register, memory,
bus, and execution of instructions) of a digital
computer. Through Fig.1 to Fig. 4, a novel suggestion
of hardware connections between a set of decoders,
encoders and multiplexers with different components in
a digital computer is presented. These connections
helped the system generate many control signals to
perform different operations. This demonstration sheds
light on the importance of these combinational circuits
in control. We recommend employing these
combinational circuits in controlling other components
of computer systems (like the Arithmetic Logic Unit
(ALU), input/output units, etc.) and communication
systems.
CONCLUSION
In this study, a small architecture of decoder,
encoder and multiplexer for register, bus system and
memory operations was presented. We conclude that
the use of these combinational circuits can help the
designer of the digital computers to generate different
types of digital signals for controlling many
components in the digital computer.
REFERENCES
1.
Stephen Brown and Zvonko Vranesic, 2005.
Fundamentals of Digital Logic with VHDL Design.
2nd Edn., McGraw-Hill Higher Education, USA.,
ISBN: 0072499389, pp: 939.
510
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