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/* Arbiter
* A module that grants access to a shared ressource (e.g a bus)
* to an arbitrary amount of drivers. A driver of the shared resource
* has a bit on the `request` line and fires it up when it wants to use
* the resource. The arbiter fires the corresponding `grant` bit when
* the driver is allowed to use the resource */
module arbiter #(
parameter NUM_CLIENTS = 4
) (
input clk,
// Request bits, each driver has its request line
input[NUM_CLIENTS-1:0] request,
// Grant bits, each driver has its grant line
output[NUM_CLIENTS-1:0] grant
);
// The current round of the arbiter
reg[NUM_CLIENTS-1:0] round = 1;
// A circle of wires used by the arbiter to
// indicate that a driver that has the priority
// to use the resource yields it to the next driver
// because it doesn't need to use the resource
wire pass_on_rollover;
/* verilator lint_off UNOPTFLAT */
logic pass_on;
assign pass_on_rollover = pass_on; // NOTE: assign is applied on last assignment to wire
// Logic bitfield used to generate the grant bus
logic[NUM_CLIENTS-1:0] grant_gen;
assign grant = grant_gen;
always @(posedge clk) begin
// Keep current grant as round, allowing a selected driver
// to freeze the arbiter during the entire duration of its
// resource utilization
round <= |grant ?
grant : { round[NUM_CLIENTS-2:0], round[NUM_CLIENTS-1] };
end
always_comb begin
pass_on = pass_on_rollover;
for (integer i = 0; i < NUM_CLIENTS; i++) begin
logic has_priority;
// Current driver has priority if it's its round
// or previous driver has passed on its
has_priority = round[i] || pass_on;
// By default, do not grant to driver
grant_gen[i] = 0;
// By default, do not pass on priority
// to next driver (current may not have it)
pass_on = 0;
if (has_priority) begin
if (request[i])
grant_gen[i] = 1;
else
pass_on = 1;
end
end
end
endmodule
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