ice40: Add examples folder including floorplan example

Signed-off-by: David Shah <dave@ds0.me>

ice40/examples/floorplan/.gitignore

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+*.json
+*.asc
+*.bin
+__pycache__

ice40/examples/floorplan/floorplan.py

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+ctx.createRectangularRegion("osc", 1, 1, 1, 4)
+for cell, cellinfo in ctx.cells:
+    if "ringosc" in cellinfo.attrs:
+        print("Floorplanned cell %s" % cell)
+        ctx.constrainCellToRegion(cell, "osc")

ice40/examples/floorplan/floorplan.sh

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+#!/usr/bin/env bash
+set -ex
+yosys -p "synth_ice40 -top top -json floorplan.json" floorplan.v
+../../../nextpnr-ice40 --up5k --json floorplan.json --pcf icebreaker.pcf --asc floorplan.asc --ignore-loops --pre-place floorplan.py
+icepack floorplan.asc floorplan.bin
+iceprog floorplan.bin

ice40/examples/floorplan/floorplan.v

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+module top(output LED1, LED2, LED3, LED4, LED5);
+    localparam N = 31;
+    wire [N:0] x;
+    assign x[0] = x[N];
+
+    genvar ii;
+    generate
+
+    for (ii = 0; ii < N; ii = ii + 1) begin
+        (* ringosc *)
+        SB_LUT4 #(.LUT_INIT(1)) lut_i(.I0(x[ii]), .I1(), .I2(), .I3(), .O(x[ii+1]));
+    end
+    endgenerate
+
+    assign clk = x[N];
+
+
+    reg [19:0] ctr;
+    always @(posedge clk)
+        ctr <= ctr + 1'b1;
+    assign {LED5, LED4, LED3, LED2, LED1} = ctr[19:15];
+endmodule

ice40/examples/floorplan/icebreaker.pcf

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+set_io -nowarn LED1       26
+set_io -nowarn LED2       27
+set_io -nowarn LED3       25
+set_io -nowarn LED4       23
+set_io -nowarn LED5       21