S5-4.4 Logic Families

Work through these

• Digital IC characteristics: fan-out, noise margin, propagation delay, power dissipation

  The four numbers that specify a family, and the ones you check before connecting two families together. Interfacing safely is exactly a matter of making these line up.

• The TTL logic family and TTL series

  The older of the two families, and still the one textbooks explain first. The series numbering is worth knowing because it encodes speed and power.

• TTL output configurations: open collector, totem pole, tri-state logic

  Three output stages for three different needs. The tri-state one is how every bus in every computer works, so it is the one to be sure of.

• The MOS logic family

  The transistor technology that took over, introduced before the family built on it. Short, and it sets up the next three items.

• CMOS logic family and its series characteristics

  The family that won, and why: static power near zero. That is the same reason it dominated large scale integration, which is a thread running into other subjects.

• CMOS transmission gate

  A switch built from two transistors, and a genuinely useful building block. It shows up inside multiplexers and latches, so recognising it pays off later.

• CMOS open drain and high impedance outputs

  The equivalents of the open collector and tri-state outputs above, in this family. Comparing the pairs is quicker than learning them separately.

• Comparison of TTL and CMOS logic families

  The comparison the whole topic has been building towards, on speed, power and noise margin. State it as three trades rather than a winner.

• Lab: measure the characteristic parameters of TTL and CMOS gates

  The bench version, and the measurement that makes the specification real. Measuring your own noise margin is more convincing than reading one.