AVR Freaks

Hi, I am working on utilising the USART as SPI on Arduino Due. Tested 8-bit communication and is working as expected. Now working on 16-bit data transfer so what are the changes to be done and which are the register bits need to be monitored/altered. 

The Arduino code is as below.

unsigned long previousMicros = 0, interval = 50;

void setup() {
  Serial.begin(9600);   // diagnostics
  Serial1.begin(9600);  // pre-initialize USART0
  USART0config();       // setup mode, clock and pins
  USART0status();
}

void loop() {
  // Sends 8-bit data every 50us
  if (micros() - previousMicros >= interval) {
    previousMicros = micros();
    USART0->US_THR = 0xAA; // Sends 8-bit data
  }
}

void USART0config() {
  USART0->US_WPMR = 0x55534100;   // Unlock the USART Mode register
  USART0->US_MR |= 0x408CE;       // Set Mode to CLK0=1, 8_BIT, SPI_MASTER
  USART0->US_BRGR = 21;           // Clock Divider (SCK = 4MHz)

  PIOA->PIO_WPMR = 0x50494F00;    // Unlock PIOA Write Protect Mode Register
  PIOB->PIO_WPMR = 0x50494F00;    // Unlock PIOB Write Protect Mode Register
  PIOB->PIO_ABSR |= (0u << 25);   // CS: Assign A14 I/O to the Peripheral A function
  PIOB->PIO_PDR |= (1u << 25);    // CS: Disable PIO control, enable peripheral control
  PIOA->PIO_ABSR |= (1u << 17);   // SCK: Assign A16 I/O to the Peripheral B function
  PIOA->PIO_PDR |= (1u << 17);    // SCK: Disable PIO control, enable peripheral control
  PIOA->PIO_ABSR |= (0u << 10);   // MOSI: Assign PA13 I/O to the Peripheral A function
  PIOA->PIO_PDR |= (1u << 10);    // MOSI: Disable PIO control, enable peripheral control
  PIOA->PIO_ABSR |= (0u << 11);   // MISO: Assign A12 I/O to the Peripheral A function
  PIOA->PIO_PDR |= (1u << 11);    // MISO: Disable PIO control, enable peripheral control
}

void USART0status() {
  Serial.print("PIOA_ABSR \t");
  Serial.println(PIOA->PIO_ABSR, HEX);
  Serial.print("PIOB_ABSR \t");
  Serial.println(PIOB->PIO_ABSR, HEX);
  Serial.print("US_MR   \t");
  Serial.println(USART0->US_MR, HEX);
  Serial.print("US_CSR  \t");
  Serial.println(USART0->US_CSR, HEX);
  Serial.print("US_BRGR (DEC)\t");
  Serial.println(USART0->US_BRGR, DEC);
  Serial.print("US_RTOR \t");
  Serial.println(USART0->US_RTOR, HEX);
  Serial.print("US_TTGR \t");
  Serial.println(USART0->US_TTGR, HEX);
  Serial.print("US_FIDI \t");
  Serial.println(USART0->US_FIDI, HEX);
  Serial.print("US_NER  \t");
  Serial.println(USART0->US_NER, HEX);
  Serial.print("US_IF   \t");
  Serial.println(USART0->US_IF, HEX);
  Serial.print("US_MAN  \t");
  Serial.println(USART0->US_MAN, HEX);
  Serial.print("US_LINMR \t");
  Serial.println(USART0->US_LINMR, HEX);
  Serial.print("US_LINIR \t");
  Serial.println(USART0->US_LINIR, HEX);
  Serial.print("US_WPSR \t");
  Serial.println(USART0->US_WPSR, HEX);
}