Day 67: Building block: priority arbiters
Building block: priority arbiters
When several requesters compete for one resource (a bus, a port, an interrupt line), an arbiter grants exactly one. A fixed-priority arbiter always favors the lowest-index requester (simple, but can starve others). A round-robin arbiter rotates priority so everyone gets a turn (fair, slightly more logic). The one-hot grant must assert for exactly one requester.
module arbiter #(parameter N = 4) (
input [N-1:0] req,
output [N-1:0] grant
);
// grant the lowest-index request: req & (~(req-1))
// isolates the least-significant set bit
assign grant = req & (~req + 1'b1);
endmodule
// e.g. req = 4'b1010 -> grant = 4'b0010 (index 1 wins)Fairness vs simplicity
Fixed priority is one line but can starve low-priority requesters under load; round-robin adds a rotating pointer to guarantee fairness. ChipX's AXI fabric (Day 91) has a single master so arbitration is trivial there, but you'll want a real arbiter thinking for interrupt prioritization and any multi-master extension. 'Design a round-robin arbiter' is a common interview ask.
Ship for Day 67
What is the main drawback of a fixed-priority arbiter compared to round-robin?
