Modification used to prevent lock out in digital counter?

Question

Modification used to prevent lock out in digital counter?

Answer 1

Make sure all of the possible states of the counter (both used and unused) are accounted for.  For all unused states, make sure they move to one of the used states, either directly, or to another used state that eventually makes it back to a used state.

Example:  A 4-bit binary counter that counts from 0 - 9 (and back to 0).  The unused states of a 4-bit binary number are 10, 11, 12, 13, 14, and 15.  If the counter (state machine) for any reason ever got set to one of these numbers, it might be stuck there (locked out).  In each case you need a logic path in the next-state logic to force any unused state (10 - 15) back into the range of 0 - 9.

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