RTL Design #10 · 2026-10-03 · Flow control · Medium · about 15 min

A credit counter that leaks credits

credit_counter is the sender's side of credit-based flow control. The receiver has 4 buffer slots, and credits is how many of them the sender knows to be free.

• send is send_req whenever credits is not 0. It looks only at the registered count: a credit returned in this cycle can't be spent until the next one.
• Every cycle with send high spends one credit. Every cycle with credit_ret high gives one back. Both can happen in the same cycle.
• Reset sets credits to 4. The receiver never returns a credit that wasn't spent, so credits never needs to go above 4.

A burst of beats goes through fine. Under steady traffic the link slows down and finally stops, with the receiver's buffer empty. Fix credit.sv.

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