added QuadTari controller switch timer

src/emucore/QuadTari.cxx

@@ -61,8 +61,17 @@ bool QuadTari::read(DigitalPin pin)
   // can switch the controller multiple times per frame
   // (we can't just read 60 times per second in the ::update() method)
 
-  // If bit 7 of VBlank is not set, read first, else second controller
+  constexpr int MIN_CYCLES = 30 * 76; // minimal cycles required for stable input switch (TODO: define cycles)
-  if(!(mySystem.tia().registerValue(VBLANK) & 0x80))
+  bool readFirst;
+
+  if(mySystem.tia().dumpPortsCycles() < MIN_CYCLES)
+    // Random controller if read too soon after dump ports changed
+    readFirst = mySystem.randGenerator().next() % 2;
+  else
+    // If bit 7 of VBlank is not set, read first, else second controller
+    readFirst = !(mySystem.tia().registerValue(VBLANK) & 0x80);
+
+  if(readFirst)
     return myFirstController->read(pin);
   else
     return mySecondController->read(pin);

src/emucore/tia/TIA.cxx

@@ -523,6 +523,7 @@ bool TIA::poke(uInt16 address, uInt8 value)
 
       for (PaddleReader& paddleReader : myPaddleReaders)
         paddleReader.vblank(value, myTimestamp);
+      updateDumpPorts(value);
 
       myDelayQueue.push(VBLANK, value, Delay::vblank);
 
@@ -1981,6 +1982,24 @@ void TIA::toggleCollBLPF()
   myCollisionMask ^= (CollisionMask::ball & CollisionMask::playfield);
 }
 
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+void TIA::updateDumpPorts(uInt8 value)
+{
+  bool newIsDumped = value & 0x80;
+
+  if(myArePortsDumped != newIsDumped)
+  {
+    myArePortsDumped = newIsDumped;
+    myDumpPortsTimestamp = myTimestamp;
+  }
+}
+
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+Int64 TIA::dumpPortsCycles()
+{
+  return (myTimestamp - myDumpPortsTimestamp) / 3;
+}
+
 #ifdef DEBUGGER_SUPPORT
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 void TIA::createAccessArrays()

src/emucore/tia/TIA.hxx

@@ -334,6 +334,13 @@ class TIA : public Device
       return uInt32(mySystem->cycles() - myCyclesAtFrameStart);
     }
 
+    /**
+     * Get the CPU cycles since the last dump ports change.
+     *
+     * @return  The number of CPU cycles since the last dump ports change
+    */
+    Int64 dumpPortsCycles();
+
     /**
       Answers whether the TIA is currently in being rendered
       (we're in between the start and end of drawing a frame).
@@ -695,6 +702,13 @@ class TIA : public Device
      */
     void applyDeveloperSettings();
 
+    /**
+     * Updates the dump ports state with the time of change.
+     *
+     * @param value  The value to be stored at VBLANK
+    */
+    void updateDumpPorts(uInt8 value);
+
   #ifdef DEBUGGER_SUPPORT
     void createAccessArrays();
 
@@ -894,6 +908,16 @@ class TIA : public Device
      */
     uInt64 myTimestamp{0};
 
+    /**
+     * The number of CPU clocks since the last dump ports state change.
+     */
+    uInt64 myDumpPortsTimestamp{0};
+
+    /**
+     * The current dump ports state.
+    */
+    bool myArePortsDumped{false};
+
     /**
      * The "shadow registers" track the last written register value for the
      * debugger.