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// Copyright 2020 Efabless Corporation
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// SPDX-License-Identifier: Apache-2.0
`default_nettype none
// (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
`default_nettype wire