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Practical Aspects of Sequential Machine Design

  There are many practical issues which must be taken into account when designing state machines. These include:

•   Asynchronous inputs. These are inputs which can arrive at any time, and are not synchronised to the clock signal.
  • metastability, improper detection of signal
  • pulse catching, stetching and synchronisation
  • state selection
  • switch debouncing

• Output glitches. Due mainly to
  • hazards in output logic
  • race conditions
  May not be a problem, but if so, then
  • eliminate, or
  • filter out

•   State code assignment. Careful selection of states and state code can minimise circuitry and reduce problems associated with glitches and asynchronous inputs.
  • States having the same next state for a given input condition should have logically adjacent state code assignments.
  • States that are next states of a single state should have logically adjacent state code assignments.

• Reset and initialization. On power up, you cannot predict the state a flip flop will go into. A circuit, sometimes called a sanity circuit, is used to force the flip flops into a known or specified state. This is often done using PRESET or PRECLEAR (or other) features of the flip flops.

We will look further into some of these points in the following sections.

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