RTL Design #31 · 2026-10-24 · Beat counting · Easy · about 10 min

A packet marker that counts stalled beats

beat_last marks the last beat of a fixed-length packet on a valid/ready stream. A packet is LEN beats long, and idx is the position of the beat currently offered, from 0 to LEN-1.

• last is high while valid is high and the beat offered is the last of its packet (idx == LEN-1).
• A beat is transferred in a cycle where valid and ready are both high. After the last beat is transferred, the next one is position 0.
• A beat that is offered but not accepted is offered again, in the same position.
• Reset (synchronous, active low) sets idx to 0.

With no back-pressure the packets come out the right length. Once the receiver stalls, last starts showing up in the wrong places. Fix beat_last.sv.

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