DIGITAL LOGIC

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DIGITAL LOGIC

• By Maria Ramila I. Jimenez
• XUCC

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• Computers are composed of huge collections of
  switches connected by wires.
• The wires are signal pathways.
• Vacuum Tubes
• Transistors
• ICs
• SSI
• MSI
• LSI
• VLSI

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• Packaging Chips
• DIP
• PGA
• PQFP
• Manufacturers build circuits mainly out of wires,
  switches, resistors, capacitors, diodes, and
  chips. When manufacturers produce the wires by
  etching metallic

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• foils that they laminated on thin, nonconductive
  circuit boards, the wires are called lines or
  traces. The boards are called PCBs or simply
  cards.
• mother board
• daughter board

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• Designers represent chips schematically by
  diagrams that show and label the pins. In
  addition to power and ground pins, chips have
  signal pins for inputs and outputs sometimes
  signal pins are used for both input and output.
• The lines, which represent wires or traces,
  conduct the signals. A given line

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• Relationship between the physical implementation
  of a circuit and the logic diagram of the circuit.

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• can only be at one logic level at a time. If one
  device outputs or asserts high on an output pin,
  then any devices whose input pins are connected
  to that pin are said to receive, be driven to, or
  be held at logic level 1. Engineers use the
  phrases pull a signal down, assert low, or
  deassert to describe sending a false signal, and
  raise a signal,

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• assert high, or simply assert to describe sending
  a true signal.
• Chip manufacturers use standard conventions to
  label the pins on their devices. If they label an
  input pin with a bar above it, then an input must
  assert false to activate the function. For
  example RES ( reset) is an input pin on many
  chips that resets the circuit. To reset

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• the circuit, then an input must assert false at
  RES. Moreover, there is an implicit assumption
  that the external device will assert true during
  normal operation of the circuit.
• It is usually the case that within a given
  computers circuitry, all devices use similar or
  compatible technology. This means that designers
  can connect them together without using special
  interface circuits, or buffers.

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• said another way, the designers can connect the
  output pins from one device directly to the input
  pins of other devices, and the devices can share
  the same power supply.
• In addition to the two logic states, certain
  gates called tristate logic gates can enter a
  high impedance state, which is logically
  equivalent to being disconnected.

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• Unless an external device asserts 1 at the enable
  input to the gate, the outputs of the gate remain
  in their high-impedance or disconnected state and
  are said to float.

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• Designers frequently connect the outputs of
  several components directly to a common set of
  wires, called a bus.
• Special devices, bus controllers, control the
  activity on a bus.
• Transistors are the basic physical component of
  virtually all computers
• Gates are the basic logical element.

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• Bus Connections

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• Combinatorial Logic circuits are loop-free
  circuits. Loop-free means that the output of a
  gate in the circuit is not an input to the gate
  or to any other device that makes an input to the
  gate. The essential property of a combinatorial
  circuit is that its output does not depend on the
  history of inputs it has no memory.

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• A complete binary decoder is a device that has n
  inputs, I0, I1,In-1, and 2n outputs, O0,
  O1,,O2n-1. The decoder generates the unary
  representation of the input line Ok is active
  (true) when the input is the binary encoding of
  the value k, and all other output lines are
  inactive (false). A computer can use a decoder
  for converting a register number into the

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• signal that selects the specified register.
  Memory systems use decoders to convert addresses
  into control signals for selecting the specified
  memory location.
• Encoders do exactly the opposite of decoders. A
  unary-to-binary encoder, for example, generates a
  binary value when an external device activates a
  single

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A unary-to-binary encoder
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• output. A computer can use a unary-to-binary
  encoder to convert an interrupt signal into the
  device number of the device requesting the
  interrupt. If more than one device can generate
  simultaneous interrupt signals, however, the
  designer can use a priority network, which is
  another type of encoder, to

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• ensure that only one signal reaches the
  unary-to-binary encoder at a time. A priority
  network has N inputs, I1 to IN and N outputs, O1
  to ON. It generates a single output, Oi, where Ij
  is 0 for j lt i, and Ii 1.

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