verilog/rtl/digital_pll_controller.v

// (True) digital PLL
//
// Output goes to a trimmable ring oscillator (see documentation).
// Ring oscillator should be trimmable to above and below maximum
// ranges of the input.
//
// Input "osc" comes from a fixed clock source (e.g., crystal oscillator
// output).
//
// Input "div" is the target number of clock cycles per oscillator cycle.
// e.g., if div == 8 then this is an 8X PLL.
//
// Clock "clock" is the PLL output being trimmed.
// (NOTE:  To be done:  Pass-through enable)
//
// Algorithm:
//
// 1) Trim is done by thermometer code.  Reset to the highest value
//    in case the fastest rate clock is too fast for the logic.
//
// 2) Count the number of contiguous 1s and 0s in "osc"
//    periods of the master clock.  If the count maxes out, it does
//    not roll over.
//
// 3) Add the two counts together.
//
// 4) If the sum is less than div, then the clock is too slow, so
//    decrease the trim code.  If the sum is greater than div, the
//    clock is too fast, so increase the trim code.  If the sum
//    is equal to div, the the trim code does not change.
//

module digital_pll_controller(reset, clock, osc, div, trim);
    input reset;
    input clock;
    input osc;
    input [4:0] div;
    output [25:0] trim;		// Use ring_osc2x13, with 26 trim bits

    wire [25:0] trim;
    reg [2:0] oscbuf;
    reg [2:0] prep;

    reg [4:0] count0;
    reg [4:0] count1;
    reg [6:0] tval;	// Includes 2 bits fractional
    wire [4:0] tint;	// Integer part of the above

    wire [5:0] sum;

    assign sum = count0 + count1;
 
    // Integer to thermometer code (maybe there's an algorithmic way?)
    assign tint = tval[6:2];
                                     // |<--second-->|<-- first-->|
    assign trim = (tint == 5'd0)  ? 26'b0000000000000_0000000000000 :
          (tint == 5'd1)  ? 26'b0000000000000_0000000000001 :
          (tint == 5'd2)  ? 26'b0000000000000_0000001000001 :
          (tint == 5'd3)  ? 26'b0000000000000_0010001000001 :
          (tint == 5'd4)  ? 26'b0000000000000_0010001001001 :
          (tint == 5'd5)  ? 26'b0000000000000_0010101001001 :
          (tint == 5'd6)  ? 26'b0000000000000_1010101001001 :
          (tint == 5'd7)  ? 26'b0000000000000_1010101101001 :
          (tint == 5'd8)  ? 26'b0000000000000_1010101101101 :
          (tint == 5'd9)  ? 26'b0000000000000_1011101101101 :
          (tint == 5'd10) ? 26'b0000000000000_1011101111101 :
          (tint == 5'd11) ? 26'b0000000000000_1111101111101 :
          (tint == 5'd12) ? 26'b0000000000000_1111101111111 :
          (tint == 5'd13) ? 26'b0000000000000_1111111111111 :
          (tint == 5'd14) ? 26'b0000000000001_1111111111111 :
          (tint == 5'd15) ? 26'b0000001000001_1111111111111 :
          (tint == 5'd16) ? 26'b0010001000001_1111111111111 :
          (tint == 5'd17) ? 26'b0010001001001_1111111111111 :
          (tint == 5'd18) ? 26'b0010101001001_1111111111111 :
          (tint == 5'd19) ? 26'b1010101001001_1111111111111 :
          (tint == 5'd20) ? 26'b1010101101001_1111111111111 :
          (tint == 5'd21) ? 26'b1010101101101_1111111111111 :
          (tint == 5'd22) ? 26'b1011101101101_1111111111111 :
          (tint == 5'd23) ? 26'b1011101111101_1111111111111 :
          (tint == 5'd24) ? 26'b1111101111101_1111111111111 :
          (tint == 5'd25) ? 26'b1111101111111_1111111111111 :
                    26'b1111111111111_1111111111111;
   
    always @(posedge clock or posedge reset) begin
    if (reset == 1'b1) begin
        tval <= 7'd0;	// Note:  trim[0] must be zero for startup to work.
        oscbuf <= 3'd0;
        prep <= 3'd0;
        count0 <= 5'd0;
        count1 <= 5'd0;

    end else begin
        oscbuf <= {oscbuf[1:0], osc};

        if (oscbuf[2] != oscbuf[1]) begin
        count1 <= count0;
        count0 <= 5'b00001;
        prep <= {prep[1:0], 1'b1};

        if (prep == 3'b111) begin
            if (sum > div) begin
		if (tval < 127) begin
            	    tval <= tval + 1;
		end
            end else if (sum < div) begin
		if (tval > 0) begin
            	    tval <= tval - 1;
		end
            end
        end
        end else begin
        if (count0 != 5'b11111) begin
                count0 <= count0 + 1;
        end
        end
    end
    end

endmodule	// digital_pll_controller