Digital Circuits

1
Digital Circuits

• ECGR2181
• Chapter 3 Notes
• Supplement A

Reading Chapter 3
2
Additional Terms

• Sinking/Sourcing Current (sect. 3.5.2, p. 106)
  current entering/leaving the output of a device.
• Fanout (sect. 3.5.4) how many gate inputs can a
  particular device drive and still maintain
  digital logic characteristics.
• Unused Inputs (sect. 3.5.6) always connect
  unused inputs to either power supply rail (Vcc or
  Gnd.)
• Static conditions may appear to be stable,
• Dynamic conditions could be unstable
• Makes circuit behavior unpredictable.
• ESD Electro-Static Discharge. (sect. 3.5.7)
  ESD involves the discharge of static electricity
  and is the deadly enemy of electronic circuits,
  especially, CMOS devices. ESD damage can be
  avoided with the use of ESD straps and rubber
  mats.
• Transition Time (sect. 3.6.1) time required for
  signal to transit the abnormal region. The time
  to transit the abnormal region may be different
  for traversing the region in different directions.

3
More Terms

• Propagation Delay (sect. 3.6.2) time required
  for a change on the input to produce a change on
  the output.
• Current Spikes (sect. 3.6.4) typically seen on
  the power rails. Produced when many outputs
  change at the same time. Switching Power
  Supplies often produce these effects. t/s note
  check frequency to help find source
• Decoupling Capacitors (sect. 3.6.4) distributes
  the filtering on the board and aids in the
  reduction of noise on the power rails.
• Ground Bounce (sect. 3.6.6) read the text.
  describe how it looks on an oscope
• Three-state Outputs (sect. 3.7.3) devices whose
  outputs are one of three states, high, low and
  high impedance. Used to drive an output from
  multiple, mutually exclusive sources (or
  devices.)
• Different types of Logic Families
• TTL Transitor-Transitor Logic
• CMOS Complementary Metal Oxide Semiconductor
• ECL Emitter-Coupled Logic
• Take extreme care when interfacing TTL and CMOS
  logic devices