Logic Gates

Overview

When sourcing logic IC’s, note that the standard prefix used by many manufactures is “74”.

Logic gate inputs are normally labelled as a single letter, starting with A (e.g. a three input AND gate would have inputs A, B and C). The output is normally labelled Y, unless you are using a flip-flop or latch, and the output is labelled Q.

D Flip-Flops

Important Parameters

Parameter Symbol Description
Propagation Delay \(t_P\)

Triggering

Edge-triggered D flip-flops can be either positive or negative edge triggered. Edge-triggered flip-flops are shown by a triangle at the clock input, and negative edge-triggered ones have an additional bubble. However, positive-edge triggered is much more common, and standard practise is to make a negative edge triggered flip-flop by adding your own inverting gate on the clock signal.

Note that adding a inverting gate to the clock signal increasing the propagation delay for that clock input, and will have a significant impact on the operation in high-speed designs.