RTL Design #15 · 2026-10-08 · Gray code · Easy · about 10 min

A Gray counter that changes two bits

gray_counter is a W-bit counter whose output is in Gray code, so it can be sampled safely from another clock domain.

• After reset, gray is 0.
• At each rising edge with en = 1 it moves to the Gray code of the next count: when the count is n, gray is n ^ (n >> 1). The count wraps from 2^W − 1 to 0.
• With en = 0 it holds.

So exactly one bit of gray changes at every step, including the wrap. The module must work for any W.

The current design starts well, 0000, 0001, 0011, 0010, 0110, and then goes wrong. Fix gray_counter.sv.

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