Rule 90 is a one-dimensional cellular automaton with interesting properties.
The rules are simple. There is a one-dimensional array of cells (on or off). At each time step, the next state of each cell is the XOR of the cell's two current neighbours. A more verbose way of expressing this rule is the following table, where a cell's next state is a function of itself and its two neighbours:
(The name "Rule 90" comes from reading the "next state" column: 01011010 is decimal 90.)
In this circuit, create a 512-cell system (q[511:0]), and advance by one time step each clock cycle. The load input indicates the state of the system should be loaded with data[511:0]. Assume the boundaries (q[-1] and q[512]) are both zero (off).
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module top_module(
input clk,
input load,
input [511:0] data,
output [511:0] q );
wire [511:0] left_data;
wire [511:0] right_data;
assign left_data = {q[510:0],1'b0 };
assign right_data = {1'b0, q[511:1]};
always @(posedge clk)
if(load)
q <= data;
else
q <= left_data ^ right_data;
endmodule