Summary ->

Module	Description
Round-robin	Each arbiter rotates priority via a mask register, preventing starvation
Packet-level	Once a packet (Head) starts, the entire 3-flit sequence holds the grant until Tail exits
Deadlock-free design	No packet fragmentation → no cyclic waiting → guaranteed forward progress
Scalable conflict	5 independent arbiters (one per output) run in parallel; request transposition separates each conflict into a trivial 5-input problem
One-hot style	All signals respect one-hot encoding; the design's correctness depends on this invariant

Round-Robin Arbiter - The Atom

The round_robin_arbiter is a sub-module instantiated 5 times (once per output port). Each arbiter handles requests from all 5 input ports competing for that one output.

module round_robin_arbiter #(
    parameter PORTS = 5
)(
    input  logic             clk,
    input  logic             rst_n,
    input  logic             release_lock,    // Signal to unlock packet
    input  logic [PORTS-1:0] req,              // 5-bit request vector
    output logic [PORTS-1:0] grant             // 5-bit one-hot grant
);

Key State:

logic [PORTS-1:0] mask_reg;        // Rotating priority mask
logic [PORTS-1:0] locked_grant;    // Locked grant (during packet transfer)
logic             is_locked;       // Flag: packet in flight?

How it Works: Three States

Idle (no packet in flight, is_locked = 0) The arbiter picks a new winner each cycle:

assign masked_req   = req & mask_reg;
assign masked_grant = masked_req & ~(masked_req - 1);  // Lowest '1' bit in masked
assign unmasked_grant = req & ~(req - 1);              // Lowest '1' bit overall
assign next_grant = (masked_req == 0) ? unmasked_grant : masked_grant;

This is a fixed-priority encoder that selects the lowest-order requesting input:

req & ~(req - 1) is a bit-twiddling trick: if req = 5'b10110, then req - 1 = 5'b10101, so req & ~(req - 1) = 5'b00010 (isolates the rightmost '1').
The mask restricts this to ports that haven't recently won.
If all masked bits are 0 (everyone who hasn't won is now requesting), the mask wraps around and we pick from the raw unmasked requests, fairness resets.

Example:

req = 5'b10100 (ports 2 and 4 requesting)
mask_reg = 5'b11110 (ports 0,1,2,3 are eligible; port 4 won last cycle)
masked_req = 5'b10100 (port 2 is eligible)
next_grant = 5'b00100 → Port 2 wins