Method begin

avr/libraries/SPI/src/SPI.cpp:26–64

#endif

void SPIClass::begin()
{
  uint8_t sreg = SREG;
  noInterrupts(); // Protect from a scheduler and prevent transactionBegin
  if (!initialized) {
    // Set SS to high so a connected chip will be "deselected" by default
    uint8_t port = digitalPinToPort(SS);
    uint8_t bit = digitalPinToBitMask(SS);
    volatile uint8_t *reg = portModeRegister(port);

    // if the SS pin is not already configured as an output
    // then set it high (to enable the internal pull-up resistor)
    if(!(*reg & bit)){
      digitalWrite(SS, HIGH);
    }

    // When the SS pin is set as OUTPUT, it can be used as
    // a general purpose output port (it doesn't influence
    // SPI operations).
    pinMode(SS, OUTPUT);

    // Warning: if the SS pin ever becomes a LOW INPUT then SPI
    // automatically switches to Slave, so the data direction of
    // the SS pin MUST be kept as OUTPUT.
    SPCR |= _BV(MSTR);
    SPCR |= _BV(SPE);

    // Set direction register for SCK and MOSI pin.
    // MISO pin automatically overrides to INPUT.
    // By doing this AFTER enabling SPI, we avoid accidentally
    // clocking in a single bit since the lines go directly
    // from "input" to SPI control.
    // http://code.google.com/p/arduino/issues/detail?id=888
    pinMode(SCK, OUTPUT);
    pinMode(MOSI, OUTPUT);
  }
  initialized++; // reference count
  SREG = sreg;
}

void SPIClass::end() {
  uint8_t sreg = SREG;