Attention

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cic_interp Source File

`timescale 1ns / 1ns

// First-order CIC interpolator
module cic_interp(
     input clk,
     input  [17:0] d_in,
     input strobe,
     output [17:0] d_out
);

// span should be log2(update interval), right?

// Coarse stepwise frequency changes are bad, as we discovered
// experimentally in hardware at APEX.  But linear small-steps
// like this should be fine.  The consequences get integrated
// one more time (to get phase) before they show up on an "ADC".
parameter span=6;

// Differentiate
reg signed [17:0] d_last=0, diff=0;
always @(posedge clk) if (strobe) begin
     d_last <= d_in;
     diff <= d_in - d_last;
end

// Integrate
reg signed [17+span:0] i1=0;
always @(posedge clk) begin
     i1 <= i1+diff;
end
assign d_out = i1[17+span:span];

endmodule