improve 10bit scaling for ESP8266

add "contributing" section to docs

README.md

@@ -54,6 +54,7 @@ void loop() {
     * [PlatformIO](#platformio-1)
     * [Arduino IDE](#arduino-ide-1)
 * [Unit tests](#unit-tests)
+* [Contributing](#contributing)
 * [Author](#author)
 * [License](#license)
 
@@ -271,6 +272,17 @@ the `File` > `Examples` > `JLed` menu.
 Info on how to run the host based provided unit tests 
 [is provided here](test/README.md).
 
+## Contributing
+
+* fork this repository
+* create your feature branch
+* add code
+* add unit test(s)
+* add documentation
+* make sure linter does not report problems (make lint)
+* commit changes
+* submit a PR
+
 ## Author
 
 Jan Delgado, jdelgado[at]gmx.net.

src/jled.cpp

@@ -21,11 +21,6 @@
 //
 #include "jled.h"  // NOLINT
 
-// uncomment to enable workaround (does not solve the problem totally) for
-// flickering LED when turning LED off.  see
-// https://arduino.stackexchange.com/questions/17946/why-does-an-led-sometimes-flash-when-increasing-brightness
-// #define PWM_FLICKER_WORKAROUND_ENABLE
-
 constexpr uint8_t JLed::kFadeOnTable[];
 
 JLed::JLed(uint8_t led_pin) : led_pin_(led_pin) {
@@ -74,16 +69,16 @@ JLed& JLed::LowActive() { return SetFlags(FL_LOW_ACTIVE, true); }
 
 bool JLed::IsLowActive() const { return GetFlag(FL_LOW_ACTIVE); }
 
+// scale an 8bit value to 10bit: 0 -> 0, ..., 255 -> 1023.
+uint16_t JLed::ScaleTo10Bit(uint8_t x) {
+    return (x == 0) ? 0 : (x << 2) + 3;
+}
+
 void JLed::AnalogWrite(uint8_t val) {
     auto new_val = IsLowActive() ? 255 - val : val;
-#ifdef PWM_FLICKER_WORKAROUND_ENABLE
-    if (new_val == 0) {
-        analogWrite(led_pin_, 1);
-    }
-#endif
 #ifdef ESP8266
     // ESP8266 uses 10bit PWM range per default, scale value up
-    new_val <<= 2;
+    new_val = JLed::ScaleTo10Bit(new_val);
 #endif
     analogWrite(led_pin_, new_val);
 }

src/jled.h

@@ -144,6 +144,8 @@ class JLed {
     // the period each.
     static uint8_t BreatheFunc(uint32_t t, uint16_t period, uintptr_t);
 
+    static uint16_t ScaleTo10Bit(uint8_t);
+
  private:
     // pre-calculated fade-on function. This table samples the function
     //   y(x) =  exp(sin((t - period / 2.) * PI / period)) - 0.36787944) * 108.

test/test_jled.cpp

@@ -38,6 +38,20 @@ TEST_CASE("jled ctor set pin mode to OUTPUT", "[jled]") {
   REQUIRE(arduinoMockGetPinMode(kTestPin) == OUTPUT);
 }
 
+TEST_CASE("properly scale 8bit to 10bit for ESP8266 support") {
+  class TestableJLed : public JLed {
+   public:
+    using JLed::JLed;
+    static void test() {
+      REQUIRE(TestableJLed::ScaleTo10Bit(0) == 0);
+      REQUIRE(TestableJLed::ScaleTo10Bit(127) == (127<<2)+3);
+      REQUIRE(TestableJLed::ScaleTo10Bit(255) == 1023);
+    }
+  };
+  TestableJLed::test();
+}
+
+
 TEST_CASE("properly initialize brightness_func", "[jled]") {
   class TestableJLed : public JLed {
    public: