I2C Slaveimplementierung auf einem ATTINY 2313

  while (  ( PIN_USI & ( 1 << PIN_USI_SCL ) ) &&  !( ( PIN_USI & ( 1 << PIN_USI_SDA ) ) ));// SCL his high and SDA is low

  if ( !( PIN_USI & ( 1 << PIN_USI_SDA ) ) )

    {  // A Stop Condition did not occur

    USICR =

    ( 1 << USISIE ) |                // Keep Start Condition Interrupt enabled to detect RESTART

    ( 1 << USIOIE ) |                // Enable Overflow Interrupt

    ( 1 << USIWM1 ) | ( 1 << USIWM0 ) |          // Set USI in Two-wire mode, hold SCL low on USI Counter overflow

    ( 1 << USICS1 ) | ( 0 << USICS0 ) | ( 0 << USICLK ) |  // 4-Bit Counter Source = external, both edges; Clock Source = External, positive edge  

    ( 0 << USITC );                  // No toggle clock-port pin

    }

  else

    {  // A Stop Condition did occur

    USICR =

    ( 1 << USISIE ) |                // Enable Start Condition Interrupt

    ( 0 << USIOIE ) |                // Disable Overflow Interrupt

    ( 1 << USIWM1 ) | ( 0 << USIWM0 ) |          // Set USI in Two-wire mode, no USI Counter overflow hold

    ( 1 << USICS1 ) | ( 0 << USICS0 ) | ( 0 << USICLK ) |    // 4-Bit Counter Source = external, both edges; Clock Source = external, positive edge

    ( 0 << USITC );                  // No toggle clock-port pin

    }