Mod-6 Counter: Self-Correction by Explicit Next-State Assignment
Mission-critical state machines cannot rely on undefined behavior for invalid states. The next-state logic for a fault-tolerant mod-6 counter must explicitly route any illegal state back to the reset state to prevent system lockup from single-event upsets.
The circuit computes the next state (D2, D1, D0) given the current state (Q2, Q1, Q0). The counter increments normally from 0 to 5. If the current state is 6 or 7, the next state must immediately evaluate to 0.
| Q2 | Q1 | Q0 | D2 | D1 | D0 | |------|------|------|------|------|------| | 0 | 0 | 0 | 0 | 0 | 1 | | 0 | 0 | 1 | 0 | 1 | 0 | | 0 | 1 | 0 | 0 | 1 | 1 | | 0 | 1 | 1 | 1 | 0 | 0 | | 1 | 0 | 0 | 1 | 0 | 1 | | 1 | 0 | 1 | 0 | 0 | 0 | | 1 | 1 | 0 | 0 | 0 | 0 | | 1 | 1 | 1 | 0 | 0 | 0 |
Constraints
- Implement combinational logic only.
- Route states 6 and 7 explicitly to state 0.
- Do not use sequential elements.
Topics
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