diff --git a/C64_4joy_adapter/4joy_adapter.ino b/C64_4joy_adapter/4joy_adapter.ino
index d3e1d36..1593959 100644
--- a/C64_4joy_adapter/4joy_adapter.ino
+++ b/C64_4joy_adapter/4joy_adapter.ino
@@ -5,10 +5,12 @@
 // http://cloud.cbm8bit.com/penfold42/joytester.zip
 // https://arduino.stackexchange.com/questions/8758/arduino-interruption-on-pin-change/8926
 // http://www.pighixxx.net/wp-content/uploads/2016/07/pro_micro_pinout_v1_0_red.png
+// https://opencircuit.shop/ProductInfo/1000378/ATmega32U4-Datasheet.pdf
+// https://cdn.sparkfun.com/datasheets/Dev/Arduino/Boards/Pro_Micro_v13b.pdf
 
-// TXD (INT3,PD3) and RXD (INT2,PD2) to userport L
-// INT2(RXD) is used for rising edge and INT3(TXD) for falling edge
-
+// TXD (INT3,PD3) and RXD (INT2,PD2) to userport L.
+// INT2(RXD) is used for rising edge and INT3(TXD) for falling edge.
+// Interrupts takes only less than 1us to change output port state after select-signal.
 
 // Joystick port 3
 // GND = GND (8)
@@ -31,18 +33,23 @@
 // GND = GND (A)
 // TXD+RXD = Select = PB7 (L)
 #define upC 1 // 15,PB1 = PB0 (C) 
-#define downC 3 // 14,PB3 = PB1(D)
+#define downC 3 // 14,PB3 = PB1 (D)
 #define leftC 2 // 16,PB2 = PB2 (E)
 #define rightC 6 // 10,PB6 = PB3 (F)
 #define fire1C 4 // 8,PB4 = PB4 (H)
 #define fire2C 5 // 9,PB5 = PB5 (J)
 
+// LEDS:
+// RX = D17,PB0
+// TX = -,PD5
+
 volatile uint8_t output1;
 volatile uint8_t output2;
 volatile uint16_t last_interrupt;
 volatile uint8_t mode = 0; // 0 = 3-joystick mode, 1 = 4-joystick mode
+volatile uint8_t last_joystick = 1;
 
-ISR(INT2_vect, ISR_NAKED) {
+ISR(INT2_vect, ISR_NAKED) { // rising edge, output joystick 3
     asm volatile(
     "    push r0                \n"  // save register r0
     "    lds r0, output1        \n"
@@ -52,9 +59,9 @@ ISR(INT2_vect, ISR_NAKED) {
     :: [pin] "I" (_SFR_IO_ADDR(PORTB)));
 }
 
-ISR(INT2_vect_part_2) { mode = 1; last_interrupt = TCNT1; }
+ISR(INT2_vect_part_2) { mode = 1; last_joystick = 1; last_interrupt = TCNT1; }
 
-ISR(INT3_vect, ISR_NAKED) {
+ISR(INT3_vect, ISR_NAKED) { // falling edge, output joystick 4
     asm volatile(
     "    push r0                \n"  // save register r0
     "    lds r0, output2        \n"
@@ -64,32 +71,36 @@ ISR(INT3_vect, ISR_NAKED) {
     :: [pin] "I" (_SFR_IO_ADDR(PORTB)));
 }
 
-ISR(INT3_vect_part_2) { mode = 1; last_interrupt = TCNT1; }
+ISR(INT3_vect_part_2) { mode = 1; last_joystick = 2; last_interrupt = TCNT1; }
 
 void setup() {
   pinMode(5, INPUT_PULLUP); // pin5 (PC6) is input
   pinMode(7, INPUT_PULLUP); // pin7 (PE6) is input
   DDRB = 0xff; //all PB-ports are outputs
   DDRF = 0; PORTF = 0xff; // all PF-ports are inputs with pullups
-  DDRD = 0; PORTD = B11010011; // all PD-ports are inputs with pullups (PD2,PD3,PD5 without pullup)
+  DDRD = B00100000; PORTD = B11110011; // all PD-ports are inputs (except PD5) with pullups (PD2,PD3 without pullup)
+  
+  TIMSK0 = 0; // disable timer0 interrupts (Arduino Uno/Pro Micro millis()/micros() update ISR)
   
-  TIMSK0 = 0; // disable timer0 interrupts (Arduino Uno/Pro Micro millis() update ISR)
   EICRA = B10110000;  // INT2 – rising edge on RXD (Bxx11xxxx), INT3 - falling edge on TXD (B10xxxxxx)
-  EIMSK = B1100;  // enable int2 (Bx1xx) and int3 (B1xxx)
+  EIMSK = B1100;  // enable INT2 (Bx1xx) and INT3 (B1xxx)
 
   //Serial.begin(115200);
+  //PORTD &= ~_BV(5);
 
+  // We can't use millis() or micros() because Timer0 interrupts are disabled. We use 16-bit Timer1 with 1024 prescaler as "clock".
   TIMSK1 = 0; // disable timer1 interrupts
   TCCR1A = 0;
   TCCR1B = B00000101; // Timer1, normal mode, prescaler 1024. One tick is 64us.
-  TCNT1 = 0;
+  TCNT1 = 0; // reset Timer1 counter
 }
 
-uint8_t joy1, joy2, PF, PD, PC, PE;
+volatile uint8_t joy1, joy2;
 
 void loop() {
+  uint8_t PF, PD, PC, PE;
   PF = PINF; PD = PIND; PC = PINC; PE = PINE;
-  joy1 = 0xff; joy2 = 0xff;
+  joy1 = 0xff; joy2 = 0xff; // all signals are inverted (also LED)
   if (up1) bitClear(joy1,upC);
   if (down1) bitClear(joy1,downC);
   if (left1) bitClear(joy1,leftC);
@@ -100,20 +111,24 @@ void loop() {
   if (right2) bitClear(joy2,rightC);
   if (fire1) { bitClear(joy1,fire1C); bitClear(joy2,fire1C); }
   if (fire2) { bitClear(joy1,fire2C); bitClear(joy2,fire2C); }
-  if (mode) { bitClear(joy1,0); bitClear(joy2,0); }
-  /*if (now > 50000) {
-    bitSet(joy1,0); bitSet(joy2,0);
-  }*/
+  //mode = 1;
+  //if (mode) { bitClear(joy1,0); bitClear(joy2,0); } // RX LED is in PB0
+  if (mode) { PORTD &= ~_BV(5); } else { PORTD |= _BV(5); } // TX LED is in PD5
+//bitClear(joy1,0); bitClear(joy2,0);
+//joy1 = 0; joy2 = 0;
+//PORTD |= _BV(5);
   output1 = joy1; output2 = joy2;
 
-  uint16_t now = TCNT1;
   if (mode == 1) {
+    uint16_t now = TCNT1; // TCNT1 is special 16-bit register, so it must be copied to variable before it can be used
     if ((now - last_interrupt) > 15625) mode = 0; // if there is no select-signal in 1s, fallback to 3-joystick mode 
   } else {
-    PORTB = output1;  // 3-joystick mode, update repeatedly and only joystick 3
+    if (last_joystick == 1) PORTB = output1;  // 3-joystick mode, update repeatedly and only joystick 3
+    if (last_joystick == 2) PORTB = output2;  // 3-joystick mode, update repeatedly and only joystick 4
   }
   //PORTB = output1;  // 3-joystick mode, update repeatedly and only joystick 3
   //Serial.print(joy1, BIN); Serial.print(" "); Serial.println(joy2, BIN); delay(100);
   //Serial.print(mode, BIN); Serial.print(" "); Serial.println(last_interrupt);
   //delayMicroseconds(50);
+  delayMicroseconds(10);
 }