changes: * use most recent M5Core2 library

           see https://github.com/m5stack/M5Core2/releases/tag/0.1.6
           see schreibfaul1/ESP32-audioI2S#410 needs SpeakerEnable=false in M5.begin(...)
         * fixed bat status
         * enhanced GUI; add prev, add play/pause, re-aranged other widgets to free space
         * store index of song played last to resume on power-on
           ! hack and works only upt to 255 songs (just 1 byte of 4 byte int is stored)
           ! missing is recognition whether it is the same SD
         * default volume 8 (instead of 0)

libraries:
   M5Core2 (0.1.6/cb65d23) can be installed by Library Manager (deps do not need to be installed)
   ESP32-audioI2S (c612a07, newer than 3.0.0/c395d0f) needs to be manually installed

src/battery_widget.hpp

@@ -8,7 +8,7 @@ namespace singsang
 class CBatteryWidget : public CBaseWidget
 {
 public:
-    CBatteryWidget() : CBaseWidget(270, 40, 40, 40) {}
+    CBatteryWidget() : CBaseWidget(270, 0, 40, 40) {}
 
     void update()
     {
@@ -21,23 +21,28 @@ class CBatteryWidget : public CBaseWidget
         }
         else
         {
-            const auto batteryPower = M5.Axp.GetBatPower();
+            const float batVoltage    = M5.Axp.GetBatVoltage();
+            const float batPercentage = (batVoltage < 3.2) ? 0 : (batVoltage - 3.2) * 100;
 
-            if (batteryPower > 0.20)
+            if (batPercentage > 80)
             {
-                newIconPath = "/icon-battery-1.png";
+                newIconPath = "/icon-battery-4.png";
+            }
+            else if (batPercentage > 60)
+            {
+                newIconPath = "/icon-battery-3.png";
             }
-            else if (batteryPower > 0.40)
+            else if (batPercentage > 40)
             {
                 newIconPath = "/icon-battery-2.png";
             }
-            else if (batteryPower > 0.60)
+            else if (batPercentage > 20)
             {
-                newIconPath = "/icon-battery-3.png";
+                newIconPath = "/icon-battery-1.png";
             }
-            else if (batteryPower > 0.80)
+            else
             {
-                newIconPath = "/icon-battery-4.png";
+                newIconPath = "/icon-battery.png";
             }
         }
 

src/next_song_widget.hpp

@@ -8,7 +8,7 @@ namespace singsang
 class CNextSongWidget : public CBaseWidget
 {
 public:
-    CNextSongWidget() : CBaseWidget(270, 160, 40, 40) {}
+    CNextSongWidget() : CBaseWidget(270, 200, 40, 40) {}
 
     void update() {}
 

src/pause_song_widget.hpp

@@ -0,0 +1,24 @@
+#ifndef SINGSANG_PAUSE_SONG_WIDGET_HPP
+#define SINGSANG_PAUSE_SONG_WIDGET_HPP
+
+#include "base_widget.hpp"
+
+namespace singsang
+{
+class CPauseSongWidget : public CBaseWidget
+{
+public:
+    CPauseSongWidget() : CBaseWidget(270, 120, 40, 40) {}
+
+    void update() {}
+
+    void draw(const bool f_updateOnly)
+    {
+        M5.Lcd.drawPngFile(SPIFFS, "/icon-caret-right.png", m_positionX,
+                           m_positionY, m_sizeX, m_sizeY);
+    }
+};
+
+}  // namespace singsang
+
+#endif  // SINGSANG_PAUSE_SONG_WIDGET_HPP

src/player.cpp

@@ -8,6 +8,17 @@ void CPlayer::begin()
 {
     initializeHardware();
     initializeGui();
+
+    // initialize Player (should be put into own method)
+	// hacky as int has size of 4 - should be using struct or json
+	// also store additional info to identify the SD and check whether its the same as last time
+    File status = SPIFFS.open("/status", FILE_READ);
+	if (status) {
+		m_activeSongIdx = int(status.read());  // hacky as int has size 4, works up to 255 only!
+
+        m_audio.connecttoSD(m_songFiles[m_activeSongIdx].c_str());
+	}
+    status.close();
 }
 
 void CPlayer::loop()
@@ -20,17 +31,16 @@ void CPlayer::loop()
 
     handleInactivityTimeout();
 
-    // * start next song if nothing is playing (auto-skips non-supported formats)
-    //   ! incompatible with inactivity detection or future pause/stop feature
-    //   ! needs status variable playing/stopped that holds actual requested state, currently it is hard-coded to "playing"
-    if (!m_audio.isRunning()) {  // not playing e.g. due to not supported file format
-		startNextSong();
-	}
+    if (m_isRunning) {
+        if (!m_audio.isRunning()) {  // not playing e.g. due to not supported file format (hacky work-a-round)
+		    startNextSong();
+        }
+    }
 }
 
 void CPlayer::initializeHardware()
 {
-    M5.begin();
+    M5.begin(true, true, true, false, kMBusModeOutput, false);  // SpeakerEnable = false
 
     M5.Axp.SetLed(false);
     M5.Axp.SetLcdVoltage(1800);  // dimmed, nominal value is 2800
@@ -46,7 +56,7 @@ void CPlayer::initializeHardware()
     delay(100);
 
     m_audio.setPinout(12, 0, 2);
-    m_audio.setVolume(0);
+    m_audio.setVolume(m_currentVolume);
 
     populateMusicFileList();
 
@@ -63,6 +73,8 @@ void CPlayer::initializeGui()
     m_batteryWidget.draw(false);
     m_fileSelectionWidget.draw(false);
     m_nextSongWidget.draw(false);
+    m_prevSongWidget.draw(false);
+    m_pauseSongWidget.draw(false);
     m_progressWidget.draw(false);
     m_volumeDisplayWidget.draw(false);
     m_volumeDownWidget.draw(false);
@@ -147,6 +159,18 @@ void CPlayer::handleTouchEvents()
         startNextSong();
     }
 
+    if (m_prevSongWidget.isTouched(touchPoint))
+    {
+        vibrate();
+        startPrevSong();
+    }
+
+    if (m_pauseSongWidget.isTouched(touchPoint))
+    {
+        vibrate();
+        pauseSong();
+    }
+
     if (m_volumeDownWidget.isTouched(touchPoint))
     {
         vibrate();
@@ -167,18 +191,46 @@ void CPlayer::startNextSong()
         return;
     }
 
-    m_activeSongIdx++;
-    if (m_activeSongIdx >= m_songFiles.size() || m_activeSongIdx < 0)
+    m_activeSongIdx = (m_activeSongIdx + 1) % m_songFiles.size();
+
+    if (m_audio.isRunning())
+    {
+        m_audio.stopSong();
+    }
+
+    m_audio.connecttoSD(m_songFiles[m_activeSongIdx].c_str());
+
+    File status = SPIFFS.open("/status", FILE_WRITE);
+    status.write(char(m_activeSongIdx));  // hacky as int has size 4, works up to 255 only!
+    status.close();
+}
+
+void CPlayer::startPrevSong()
+{
+    if (m_songFiles.size() == 0)
     {
-        m_activeSongIdx = 0;
+        return;
     }
 
+    m_activeSongIdx = (m_activeSongIdx - 1) % m_songFiles.size();
+
     if (m_audio.isRunning())
     {
         m_audio.stopSong();
     }
 
     m_audio.connecttoSD(m_songFiles[m_activeSongIdx].c_str());
+
+    File status = SPIFFS.open("/status", FILE_WRITE);
+    status.write(char(m_activeSongIdx));  // hacky as int has size 4, works up to 255 only!
+    status.close();
+}
+
+void CPlayer::pauseSong()
+{
+    m_audio.pauseResume();
+
+    m_isRunning = m_audio.isRunning();
 }
 
 void CPlayer::updateGui()

src/player.hpp

@@ -13,6 +13,8 @@
 #include "battery_widget.hpp"
 #include "file_selection_widget.hpp"
 #include "next_song_widget.hpp"
+#include "prev_song_widget.hpp"
+#include "pause_song_widget.hpp"
 #include "progress_widget.hpp"
 #include "volume_display_widget.hpp"
 #include "volume_down_widget.hpp"
@@ -33,6 +35,10 @@ class CPlayer
 
     void startNextSong();
 
+    void startPrevSong();
+
+    void pauseSong();
+
     void updateVolume(int f_deltaVolume);
 
     void increaseVolume();
@@ -51,15 +57,18 @@ class CPlayer
 
     Audio m_audio{};
 
-    int                 m_currentVolume{0};
+    int                 m_currentVolume{8};
     int                 m_activeSongIdx{-1};
     unsigned int        m_turnOffAfterInactiveForMilliSec{5 * 60 * 1000};
     unsigned int        m_lastActivityTimestamp{0};
     std::vector<String> m_songFiles{};
+    bool                m_isRunning{true};
 
     CBatteryWidget       m_batteryWidget;
     CFileSelectionWidget m_fileSelectionWidget;
     CNextSongWidget      m_nextSongWidget;
+    CPrevSongWidget      m_prevSongWidget;
+    CPauseSongWidget     m_pauseSongWidget;
     CProgressWidget      m_progressWidget;
     CVolumeDisplayWidget m_volumeDisplayWidget;
     CVolumeDownWidget    m_volumeDownWidget;

src/prev_song_widget.hpp

@@ -0,0 +1,24 @@
+#ifndef SINGSANG_PREV_SONG_WIDGET_HPP
+#define SINGSANG_PREV_SONG_WIDGET_HPP
+
+#include "base_widget.hpp"
+
+namespace singsang
+{
+class CPrevSongWidget : public CBaseWidget
+{
+public:
+    CPrevSongWidget() : CBaseWidget(10, 200, 40, 40) {}
+
+    void update() {}
+
+    void draw(const bool f_updateOnly)
+    {
+        M5.Lcd.drawPngFile(SPIFFS, "/icon-track-prev.png", m_positionX,
+                           m_positionY, m_sizeX, m_sizeY);
+    }
+};
+
+}  // namespace singsang
+
+#endif  // SINGSANG_PREV_SONG_WIDGET_HPP

src/volume_display_widget.hpp

@@ -8,7 +8,7 @@ namespace singsang
 class CVolumeDisplayWidget : public CBaseWidget
 {
 public:
-    CVolumeDisplayWidget() : CBaseWidget(10, 80, 40, 80) {}
+    CVolumeDisplayWidget() : CBaseWidget(10, 40, 40, 80) {}
 
     void update(const int f_newAudioVolume)
     {

src/volume_down_widget.hpp

@@ -8,7 +8,7 @@ namespace singsang
 class CVolumeDownWidget : public CBaseWidget
 {
 public:
-    CVolumeDownWidget() : CBaseWidget(10, 160, 40, 40) {}
+    CVolumeDownWidget() : CBaseWidget(10, 120, 40, 40) {}
 
     void draw(const bool f_updateOnly)
     {

src/volume_up_widget.hpp

@@ -8,7 +8,7 @@ namespace singsang
 class CVolumeUpWidget : public CBaseWidget
 {
 public:
-    CVolumeUpWidget() : CBaseWidget(10, 40, 40, 40) {}
+    CVolumeUpWidget() : CBaseWidget(10, 0, 40, 40) {}
 
     void draw(const bool f_updateOnly)
     {