Update GigaDisplayRGB.h: Prevent Digital Mode, Implemented PDM mode, to make the backlight dimming working

Author: alblib

src/GigaDisplayRGB.h

@@ -17,37 +17,84 @@ class GigaDisplayRGB {
 using namespace std::chrono_literals;
 using namespace std::chrono;
 
+
 class GigaDisplayBacklight {
-    public:
-        GigaDisplayBacklight() {}
-        void begin(int target_percent = 100) {
-            pin = new mbed::DigitalOut(PB_12);
-            ticker = new mbed::Ticker();
-            ticker->attach(mbed::callback(this, &GigaDisplayBacklight::cb), 2ms);
-            set(target_percent);
+private:
+    mbed::Ticker* ticker = nullptr;
+    mbed::DigitalOut* pin = nullptr;
+
+    // Exponential Lightness Model: This constants are determined 
+    // by Table II of the OSA report (exponent = 0.426, wikipedia: lightness)
+    // and empirical measurement of Giga Display KD040WVFID026-01-C025A by Albertus Liberius
+    static constexpr float min_pwm_percent = 3.36136f;
+    static constexpr float max_pwm_percent = 100.f;
+    static inline float pwm_percent_from_lightness_percent(float lightness_percent) {
+        return min_pwm_percent * pow(max_pwm_percent / min_pwm_percent, lightness_percent / 100);
+    }
+
+    // Specification by LED Driver chip LV52204MT
+    inline static constexpr std::chrono::microseconds tick = 50us;    // Ton(20us) < tick < Tw0(100us)
+                                                            // to prevent digital mode and goto pwm mode
+    inline static constexpr uint16_t start_sequence_size = 50;  // more than Tsel = 2.2ms 
+                                                            // (mode selection determination time period)
+
+    // PDM counter
+    uint16_t intensity = 0xFFFF; //maxIntensity
+    uint16_t counter = 0;
+    uint16_t start_seq_cnt = 0;
+
+    void callback() {
+        if (start_sequence_size > start_seq_cnt) {
+            *pin = start_seq_cnt++ & 1;
+        } else {
+            // PDM
+            static uint16_t newcounter = 0;
+            newcounter = (counter + intensity) & 0xFFFF;
+            *pin = (counter > newcounter) ? 1 : 0;
+            counter = newcounter;
         }
-        void cb() {
-            static int counter = 0;
-            if (counter >= intensity) {
-                *pin = 0;
-            } else {
-                *pin = 1;
-            }
-            counter += 10;
-            if (counter == 100) {
-                counter = 0;
-            }
+    }
+public:
+    GigaDisplayBacklight() {}
+
+    void begin(float target_percent=100.f) {
+        if (pin) delete pin;
+        if (ticker) {
+            ticker->detach();
+            delete ticker;
         }
-        void set(int target_percent) {
-            intensity = target_percent;
+        pin = new mbed::DigitalOut(PB_12);
+        ticker = new mbed::Ticker();
+        ticker->attach(mbed::callback(this, &GigaDisplayBacklight::callback), tick); 
+        set(target_percent);
+    }
+
+    void set(float target_percent) {
+        if (0 == intensity) start_seq_cnt = 0; // restart initialization sequence
+        intensity = static_cast<uint16_t>(
+            constrain(
+                pwm_percent_from_lightness_percent(target_percent),
+                min_pwm_percent, 
+                max_pwm_percent
+            ) / 100.f * 65535.f
+        );
+    }
+
+    void off() {
+        intensity = 0; // if it is longer than 8.9ms, it is shutdown mode.
+    }
+
+    inline virtual ~GigaDisplayBacklight() {
+        if (ticker) {
+            ticker->detach();
+            delete ticker;
         }
-        void off() {
-            set(0);
+        if (pin) {
+            delete pin;
         }
-    private:
-        mbed::Ticker* ticker;
-        mbed::DigitalOut* pin;
-        int intensity;
+    }
+
 };
 
+
 #endif