Circuit Components

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Circuit Components

• By Maria Ramila I. Jimenez
• XUCC

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Circuit Components

• Computer designers use multiplexors to connect
  several input devices to a common output device.
  The multiplexor therefore acts as a switch to
  establish a connection between one selected input
  and the common output. If a multiplexor can
  switch 2k inputs to one output, it will have k
  control inputs, called select-inputs

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Circuit Components

• that determine which input line the multiplexor
  will select, where k is generally a small number.

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• Demultiplexors are devices for taking a single
  input signal and distributing it to one of a
  fixed number of places. In essence, a
  demultiplexor performs the opposite function of a
  multiplexor. The selected output is the one whose
  binary value is presented to the select inputs.
  Designers frequently gang demultiplexors

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• so they will distribute a selected set of inputs
  to a common set of outputs, and they frequently
  use them for data bus switching. Note that a
  demultiplexor is essentially a decoder whose
  outputs are gated by an input bit.

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• Shifters shift their inputs to the left or right,
  perhaps circularly. An n-bit q-shifter, for
  example, shifts its n-bit input q bits left or
  right. A designer can implement an arbitrary
  k-bit shift operation as a sequence of m-bit
  shift operations, where m varies over the powers
  of 2. For example, an 11-bit shift

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• would become an 8-bit shift followed by a 2-bit
  shift followed by a 1-bit shift. A barrel shifter
  implements this idea.

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(a)
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• A half-adder is a circuit that accepts two binary
  inputs, A and B, and computes their sum, S, and
  their carry-out Co.

(b)
(c)
Half-adder. (a) Logic Circuit. (b) Symbolic
Diagram. (c) Truth Table
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• A full adder is a circuit that adds two binary
  inputs plus a carry-in and produces the binary
  sum and a carry-out.

(a) Logic Circuit
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(c)
(b) Symbolic diagram. (c) Truth table
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• An n-bit binary adder is a combinational device
  that adds two n-bit binary numbers. An n-bit
  binary adder receives 2n 1 inputs and generate
  n 1 outputs.

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(a)
4-bit binary adder. (a) Logic circuit. (b)
Symbolic diagram. Because the carry from each
stage ripples to the stage on the left, this type
of adder is called a ripple-carry adder.
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• There are many types of adders. Binary adders
  operate on unsigned binary operands to produce
  unsigned binary results. Some adders operate on
  specific numeric representations to produce
  results of the same type. For high-performance
  processors, designers often use special circuits
  to speed up the

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• Adders operation. The use of a carry look-ahead
  circuit is an example. An adder that uses a carry
  look-ahead circuit is faster than one that
  propagates the carries, because the carry
  look-ahead circuits can produce the carries
  faster that the sum generators can produce and
  propagate them.

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• ALUs
• Logic units are circuits that perform logical
  operations on their inputs. Often a single
  circuit performs all types of logical operations
  as well as addition and subtraction. For such a
  logic circuit, an external input device raises
  the enable input and selects an operation by
  asserting a value on its control inputs.

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• Because an adder has most of the gates necessary
  to perform all basic logic functions, most adders
  are packaged as ALUs.
• Sequential Logic
• All of the circuits in the previous slides have
  one property in common They are combinatorial or
  loop-free. Once a circuit has a feedback path,
  its output may depend on prior inputs. When this
  is the case, the

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• circuit has memory, is called sequential, and is
  decidedly more complex to analyze.
• Flip-flops
• The flip-flop is the simplest memory circuit in a
  computers processor. It stores a single logic
  value true or false. A flip-flop can only hold
  1 bit. The simplest type of flip-flop, called an
  SR flip-flop.

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• There are a variety of input labels for
  flip-flops, but for the one shown here, the R and
  S labels are common.

Logic diagram of an SR flip-flop.
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• In the absence of inputs on R and S (i.e., both R
  and S are 0, or low), the circuit will remain
  stable in one of two configurations, either reset
  (where the output Q 0 and the output Q 1) or
  set (where Q 1 and Q 0).
• A 1 input on R (and 0 input on S) will cause the
  circuit to go to the reset state.

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• Once the circuit is reset, it is no longer
  necessary to hold the R input high.
• A high input on S ( and a low input on R) will
  cause the circuit to flip to the set state. Once
  the flip-flop is set, it is no longer necessary
  to hold the S input high.
• Except during the transient condition where the
  flip-flop is actually changing

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• states, the two outputs Q and Q always have
  opposite values.
• If both inputs R and S are high at the same time,
  the subsequent behavior of the flip-flop is
  unpredictable.
• The time it takes for a flip-flop to switch
  states is its switching time.
• An SR flip-flop does not use clock signals to
  govern its switching times it

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• switches the moment an R or S input arrives. It
  is therefore asynchronous. In contrast,
  flip-flops that use clocks to govern their
  switching times are synchronous.
• Another common type of flip-flop is the JK
  master-slave, which is a synchronous flip-flop. A
  master-slave flip-flop really consists of two
  flip-flops, a

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• master and a slave.

A clocked JK master-slave flip-flop. (a) Logic
diagram. (b) Symbolic diagram.
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• The JK master-slave flip-flop does not change
  state when the clock input (C) is low regardless
  of what the J and K inputs do. However, when the
  clock is high, the master watches the inputs
  and changes state to the opposite of the current
  state of the slave if either (1) both J and K go
  high, or (2) J goes high

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• when the flip-flop is clear or K goes high when
  the flip-flop is set.
• The synchronous D flip-flop is another common
  flip-flop. It requires only one input, labeled D.
  A D flip-flop that is clocked when D is high
  becomes set one that is clocked when D is low
  becomes reset.