MSI Devices

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MSI Devices
M. Mano C. Kime Logic and Computer Design
Fundamentals (Chapter 5)
Dr. Costas Kyriacou and Dr. Konstantinos Tatas
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MSI Devices

• Medium Scale Integration (MSI) devices are
  digital devices that are build using a few tens
  to hundreds of logic gates.
• MSI devices are used as discrete devices packed
  in a single Integrated Circuit (IC), or as
  building blocks for other, more complex devices
  such as memory devices or microprocessors.
• Some typical MSI devices are the following
• Encoders and Decoders
• Multiplexers and Demultiplexers
• Full Adders
• Latches and flip flops
• Registers and Counters

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Examples of MSI Devices
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Decoders

• A decoder is a combinational digital circuit with
  a number of inputs n and a number of outputs
  m, where m 2n
• Only one of the outputs is enabled at a time. The
  output enabled is the one specified by the binary
  number formed at the inputs of the decoder.
• On the circuit below, the inputs of the decoder
  are connected on three switches, forming the
  number 5 (101)2, thus only LED 5 will be ON

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2 to 4 Line Decoder
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3 to 8 Line Decoder
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3 to 8 Line Decoder (Implementation using two
2-to-4 decoders)
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3 to 8 Line Decoder (Implementation using two
2-to-4 decoders)
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4 to 16 Line Decoder (Implementation using four
2-to-4 decoders)
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ENCODERS

• A decoder in general is a combinational digital
  circuit with with a number of inputs m and a
  number of outputs n, where n log2m
• A binary encoder has precisely the opposite
  functionality of the binary decoder.
• A priority encoder is a special case of encoder
  used in computer interrupt mechanisms to specify
  which device requests service and prioritize
  interrupts that occur at the same time

I3 I2 I1 I0 O1 O0 V
0 0 0 0 X X 0
0 0 0 1 0 0 1
0 0 1 X 0 1 1
0 1 X X 1 0 1
1 X X X 1 1 1
 
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Multiplexers

• A multiplexer is a device that has a number of
  data inputs m, and number of control inputs n
  and one output, such that m2n. The output has
  always the same value as the data input specified
  by the binary number at the control inputs.
• The rotary switch (selector) shown in figure (a)
  below, is equivalent to a 4-to-1 multiplexer.
• The sliding switch shown in figure (b) below, is
  equivalent to an 8-to-1 multiplexer.

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Internal structure of a 2-to-1 multiplexer.

• The design of a 2-to-1 multiplexer is shown
  below.
• If S0 then the output Y has the same value as
  the input I0
• If S1 then the output Y has the same value as
  the input I1

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4-to-1 Multiplexer (MUX)
S1 S0 O
0 0 I0
0 1 I1
1 0 I2
1 1 I3
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1-bit Full Adder
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4-bit Full Adder (Ripple-Carry Adder)

• To obtain a 4-bit full adder we cascade four
  1-bit full adders, by connecting the Carry Out
  bit of bit column M to the Carry In of the bit
  column M1, as shown below. The Carry In of the
  Least Significant column is set to zero.

• Example Find the bit values of the outputs
  Cout,S3..S0 of the full adder shown below, if
  A3..A0 1011 and B3..B0 0111.

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Example

• Design a 4-bit adder/subtracter using Full-adders
  and gates.

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Magnitude Comparator
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Review questions

• How many input/output signals are present in a
• 5-to-32 decoder?
• 32-to-1 MUX?
• 32-bit Ripple-Carry Adder (RCA)?
• How many 2-to-1 MUXs are required to build a
  32-to-1 MUX?
• Design a logic unit with 2 data inputs (A, B),
  three select inputs (S2, S1, S0) and the
  following specifications

S2 S1 S0 O
0 0 0 A AND B
0 0 1 A OR B
0 1 0 A XOR B
0 1 1 A NAND B
1 0 0 A NOR B
1 0 1 A XNOR B
1 1 0 A?
1 1 1 B?
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Review questions 2

• Use two 4-to-1 MUXs to build a full adder
• Implement the following Boolean algebra equation
  using only a single 8-to-1 MUX F(A,B,C,D)
  S(0,3,5,6,8,9,14,15)