[example] Add Feather M4 neopixel example

examples/feather_m4/neopixel/main.cpp

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+/*
+ * Copyright (c) 2016-2017, Niklas Hauser
+ * Copyright (c) 2023, Thomas Rush
+ *
+ * This file is part of the modm project.
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+// ----------------------------------------------------------------------------
+
+#include <modm/board.hpp>
+
+using namespace Board;
+
+/*
+ * Output a color (GRB) to the Neopixel
+ *
+ * 'Neopixel' is Adafruit's name for what is essentially a WS2812 RGB LED.
+ * The timing, which runs at 800kHz, is basically a long high pulse followed
+ * by a short low pulse for a '1' bit, or a short high pulse followed by a
+ * long low pulse for a '0' bit. The idle state of the signal is low. The
+ * data is 24 bits long (three 8-bit color values sent out in green-red-blue
+ * order, MSB first).
+ *
+ * The long and short pulse times chosen here are nominally 875ns and 375ns,
+ * respectively, totalling 1.25us per bit. Using modm::delay_ns() with these
+ * values means that the actual times will be slightly longer because of the
+ * setup times for each bit, but the cycle time can be longer (within limits)
+ * without affecting the operation. (See
+ * https://cpldcpu.wordpress.com/2014/01/14/light_ws2812-library-v2-0-part-i-understanding-the-ws2812/
+ * for more information about the signal timing for these devices.)
+ *
+ * This function is not intended to be the best solution as a driver for the
+ * WS2812, just an example of a simple application of the modm library.
+ */
+
+void
+setPixel(uint32_t data)
+{
+	constexpr uint32_t neoPeriod = 1250;
+	constexpr uint32_t neoOneHi  = 875;
+	constexpr uint32_t neoZeroHi = 375;
+
+	uint32_t bitMask = (1UL << 23);
+
+	{
+		modm::atomic::Lock lock;
+
+		do
+		{
+			uint32_t hiTime = data & bitMask ? neoOneHi : neoZeroHi;
+			Neopixel::toggle(); // toggle high
+			modm::delay_ns(hiTime);
+			Neopixel::toggle(); // toggle low
+			modm::delay_ns(neoPeriod - hiTime);
+			bitMask >>= 1;
+		}
+		while (bitMask);
+	}
+}
+
+int
+main()
+{
+	initialize();
+
+	// Use the logging streams to print some messages.
+	// Change MODM_LOG_LEVEL above to enable or disable these messages
+	MODM_LOG_DEBUG   << "debug"   << modm::endl;
+	MODM_LOG_INFO    << "info"    << modm::endl;
+	MODM_LOG_WARNING << "warning" << modm::endl;
+	MODM_LOG_ERROR   << "error"   << modm::endl;
+
+	uint32_t counter(0);
+
+	const char *log_str[] = { "green ", "red ", "blue ", "loop: " };
+
+	while (true)
+	{
+		// color values are reduced to decrease the brightness of the LED
+		// (full intensity can be uncomfortable for the eyes!)
+		for (uint32_t i = 0, color = 0x110000; i < 4; i++, color >>= 8) {
+			modm::delay(1s);
+			setPixel(color);
+			MODM_LOG_INFO << log_str[i];
+		}
+		MODM_LOG_INFO << counter++ << modm::endl;
+	}
+
+	return 0;
+}
+

examples/feather_m4/neopixel/project.xml

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+<library>
+  <extends>modm:feather-m4</extends>
+  <options>
+    <option name="modm:build:build.path">../../../build/feather_m4/neopixel</option>
+  </options>
+  <modules>
+    <module>modm:build:scons</module>
+    <module>modm:architecture:delay</module>
+  </modules>
+</library>