Added pwm generator with a gamma correction table.

This example fades two LEDs on the iCEBreaker board but uses a gamma
correction lookup table. This makes the LEDs fade more evenly. Our
eyesight is not linear but exponential, this means that if we fade the
LEDs linearelly they will go from off to on very quickly and seem to be
at a high brightness level for a very long time. The gamma correction
makes it seem that we fade the led value more linearelly. :)

Author: esden

main.mk

@@ -1,10 +1,10 @@
 
 all: $(PROJ).rpt $(PROJ).bin
 
-%.blif: %.v
+%.blif: %.v $(ADD_SRC) $(ADD_DEPS)
 	yosys -p 'synth_ice40 -top top -blif $@' $<
 
-%.json: %.v
+%.json: %.v $(ADD_SRC) $(ADD_DEPS)
 	yosys -p 'synth_ice40 -top top -json $@' $<
 
 ifeq ($(USE_NEXTPNR),)
@@ -45,7 +45,7 @@ sudo-prog: $(PROJ).bin
 	sudo iceprog $<
 
 clean:
-	rm -f $(PROJ).blif $(PROJ).asc $(PROJ).rpt $(PROJ).bin $(PROJ).json
+	rm -f $(PROJ).blif $(PROJ).asc $(PROJ).rpt $(PROJ).bin $(PROJ).json $(ADD_CLEAN)
 
 .SECONDARY:
 .PHONY: all prog clean

pwm_fade_gamma/.gitignore

@@ -0,0 +1,2 @@
+gen_gamma_table*
+gamma_table.hex

pwm_fade_gamma/COPYING

@@ -0,0 +1,13 @@
+Copyright (C) 2018 Piotr Esden-Tempski <[email protected]>
+
+Permission to use, copy, modify, and/or distribute this software for any
+purpose with or without fee is hereby granted, provided that the above
+copyright notice and this permission notice appear in all copies.
+
+THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

pwm_fade_gamma/Makefile

@@ -0,0 +1,14 @@
+PROJ = gamma_pwm
+ADD_DEPS = gamma_table.hex
+ADD_CLEAN = *.o gen_gamma_table *.hex
+
+PIN_DEF = ../icebreaker.pcf
+DEVICE = up5k
+
+include ../main.mk
+
+gen_gamma_table: gen_gamma_table.o
+	gcc $< -o $@ -lm
+
+gamma_table.hex: gen_gamma_table
+	./gen_gamma_table > gamma_table.hex

pwm_fade_gamma/gamma_pwm.v

@@ -0,0 +1,103 @@
+/*
+ *  icebreaker examples - gamma pwm demo 
+ *
+ *  Copyright (C) 2018 Piotr Esden-Tempski <[email protected]>
+ *
+ *  Permission to use, copy, modify, and/or distribute this software for any
+ *  purpose with or without fee is hereby granted, provided that the above
+ *  copyright notice and this permission notice appear in all copies.
+ *
+ *  THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ *  WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ *  MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ *  ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ *  WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ *  ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ *  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ *
+ */
+
+// This example generates PWM with gamma correction
+// The intended result is opposite pulsating Red and Green LEDs
+// on the iCEBreaker. The intended effect is that the two LED "breathe" evenly
+// and don't stay at a perceptable brightness level longer than others.
+//
+// For more information about gamma correction:
+// https://en.wikipedia.org/wiki/Gamma_correction
+
+module top (
+	input CLK,
+	output LEDR_N,
+	output LEDG_N,
+	output P1A7,
+	output P1A8,
+	output P1A9
+);
+
+// Gamma value lookup table parameters
+parameter G_PW = 8; // Number of input bits
+parameter G_OW = 16; // Number of output bits
+
+// Load the gamma value lookup table
+reg [(G_OW-1):0] gamma_lut [0:((1<<(G_PW))-1)];
+initial $readmemh("gamma_table.hex", gamma_lut);
+
+// Very simple dual output PWM generator
+// We need two outputs as we can't just invert the gamma corrected PWM output,
+// as this would also invert the gamma curve.
+reg [15:0] pwm_counter = 0;
+reg [15:0] pwm_compare_0 = (1 << 15) / 2;
+reg [15:0] pwm_compare_1 = (1 << 15) / 2;
+reg pwm_out_0;
+reg pwm_out_1;
+always @(posedge CLK) begin
+	pwm_counter <= pwm_counter + 1;
+
+	if (pwm_counter < pwm_compare_0) begin
+		pwm_out_0 <= 1;
+	end else begin
+		pwm_out_0 <= 0;
+	end
+
+	if (pwm_counter < pwm_compare_1) begin
+		pwm_out_1 <= 1;
+	end else begin
+		pwm_out_1 <= 0;
+	end
+end
+
+// PWM compare value generator
+// Fade through the values using the gamma correction
+reg [16:0] pwm_inc_counter = 0;
+reg [G_PW-1:0] pwm_value = 0;
+reg pwm_dir = 1;
+always @(posedge CLK) begin
+	// Divide clock by 131071
+	pwm_inc_counter <= pwm_inc_counter + 1;
+
+	// increment/decrement the index
+	if (pwm_inc_counter == 0)
+		if (pwm_dir)
+			pwm_value <= pwm_value + 1;
+		else
+			pwm_value <= pwm_value - 1;
+
+	// Reverse directions when reaching the index extremes
+	if (pwm_value == ((1 << G_PW) - 1))
+		pwm_dir <= 0;
+	if (pwm_value == 0)
+		pwm_dir <= 1;
+
+	if (pwm_inc_counter == 1) begin
+		pwm_compare_0 <= gamma_lut[pwm_value];
+		pwm_compare_1 <= gamma_lut[((1 << G_PW) - 1) - pwm_value];
+	end
+end
+
+assign LEDG_N = ~pwm_out_0;
+assign LEDR_N = ~pwm_out_1;
+assign P1A7 = pwm_counter[15];
+assign P1A8 = pwm_out_0;
+assign P1A9 = pwm_out_1;
+
+endmodule

pwm_fade_gamma/gen_gamma_table.c

@@ -0,0 +1,49 @@
+/*
+ *  icebreaker examples - gamma pwm demo 
+ *
+ *  Copyright (C) 2018 Piotr Esden-Tempski <[email protected]>
+ *
+ *  Permission to use, copy, modify, and/or distribute this software for any
+ *  purpose with or without fee is hereby granted, provided that the above
+ *  copyright notice and this permission notice appear in all copies.
+ *
+ *  THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ *  WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ *  MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ *  ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ *  WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ *  ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ *  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ *
+ */
+
+// This program generates a gamma correction table. This table is then loaded
+// into bram of the FPGA to provide a lookup table.
+
+#include <stdio.h>
+#include <math.h>
+
+#define GAMMA 2.2
+
+int main()
+{
+	fprintf(stderr, "Generating the gamma lookup table.\n");
+
+	for (int i = 0; i < 256; i++) {
+		double dvalue = pow((1.0 / 256.0) * i, GAMMA);
+		long lvalue = 0xFFFFl * dvalue;
+
+		fprintf(stderr, ".");
+
+		if ((i % 8) == 0) {
+			printf("%s@%08x ", (i != 0) ? "\n" : "", i);
+		}
+		printf("%04lX ", lvalue);
+		//printf("%f\n", value);
+		//printf("%5i | %f\n", i, value);
+	}
+
+	fprintf(stderr, "\ndone\n");
+
+	return 0;
+}