RTL Design #35 · 2026-10-28 · Remainder FSMs · Easy · about 10 min

A remainder tracker that drops the first bit

mod3 receives a number one bit at a time, most significant bit first, and reports its remainder when divided by 3.

• On a rising edge with valid = 1, din is the next bit. With start = 1 as well, it is the first bit of a new number, and whatever came before is forgotten.
• After each accepted bit, rem is the remainder of all the bits of the current number so far (0, 1 or 2). rem holds while valid = 0, and start is ignored without valid.
• rst_n = 0 (synchronous) clears rem to 0.
• Numbers can follow each other with no idle cycle in between, and a number can be a single bit.

Run sends four numbers and some of the remainders come out wrong. Fix mod3.sv. Grading uses three hidden checks.

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