Fix I2C communication issue with MCP23008, where STOP-condition was failing due to SDA non-LOW possibly because of a late ACK.

QCX-SSB.ino

@@ -1115,6 +1115,8 @@ ISR(PCINT2_vect){  // Interrupt on rotary encoder turn
   enc.event();
 }*/
 
+// I2C communication starts with a START condition, multiple single byte-transfers (MSB first) followed by an ACK/NACK and stops with a STOP condition;
+// during data-transfer SDA may only change when SCL is LOW, during a START/STOP condition SCL is HIGH and SDA goes DOWN for a START and UP for a STOP.
 class I2C {
 public:
 #if(F_MCU > 20900000)
@@ -1194,10 +1196,11 @@ public:
     SendBit(data, 1 << 2);
     SendBit(data, 1 << 1);
     SendBit(data, 1 << 0);
-    I2C_SDA_HI();  // recv ACK
+    SendBit(   0, 1 << 0);  // ACK/NAK: instead of assuming slave will set SDA is LOW (ACK), force SDA is LOW so that at least STOP condition will succeed (in case ACK is delayed, e.g. with MCP23008).
+    /*I2C_SDA_HI();  // recv ACK
     DELAY(I2C_DELAY);
     I2C_SCL_HI();
-    I2C_SCL_LO();
+    I2C_SCL_LO();*/
   }
   inline uint8_t RecvBit(uint8_t mask){
     I2C_SCL_HI();
@@ -4368,6 +4371,12 @@ clear; echo ";UA1;UD;" > /tmp/ttyS0; cat /tmp/ttyS0 | while IFS= read -d \; c; d
 
 Use pavumeter to set the correct mixer settings
 
+For an Arduino board at 16 MHz, the following simple script will start streaming audio:
+
+cat /dev/ttyACM0 | aplay -c 1 -r 6249 -f U8
+stty -F /dev/ttyACM0 raw -echo -echoe -echoctl -echoke 115200;
+echo ";UA1;" > /dev/ttyACM0
+
 */
 #ifdef CAT_EXT
   else if((CATcmd[0] == 'U') && (CATcmd[1] == 'K') && (CATcmd[4] == ';'))  // remote key press