Cortex-M3, SPI with USART, RXRDY doesn't work?

Hallo,

I use an SAM3X8 (Arduino-DUE) and use USART0 as SPI-Interface in slave-mode. In a test circuit I use the standard SPI as master that sends data to the USART-SPI (as slave). It works in principle with one exception: The RXRDY-Bit in the USART Channel Status Register (US_CSR) is always 0, even if the sent data have been arrived  a ms ago in the USART Receive Holding Register (US_RHR). I tried it with interrupts, but no interrupt was triggered.

(In my test circuit I now, when data shoul be arrived in the US-RHR of the slave, but typically, a slave can only wait.)

Does anybody have an idea?

Below the kernel of my code.

Greatings
GuaAck

void USART0config(uint16_t divider) {
  USART0->US_WPMR = 0x55534100;   // Unlock the USART Mode register
  USART0->US_MR |= 0x409CF;       // Set Mode to CLK0=1, 8_BIT, SPI_SLAVE (CPOL = 1 0x508CE, CPHA = 1 0x409CE, both set to 1 0x509CE)
  PIOA->PIO_WPMR = 0x50494F00;    // Unlock PIOA Write Protect Mode Register
  PIOB->PIO_WPMR = 0x50494F00;    // Unlock PIOB Write Protect Mode Register
  PIOB->PIO_ABSR |= (0u << 26);   // CS: Assign PB26 I/O to the Peripheral A function
  PIOB->PIO_PDR |= (1u << 26);    // CS: Disable PIO control, enable peripheral control
  PIOA->PIO_ABSR |= (1u << 17);   // SCK: Assign PA17 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 PA10 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 PA11 I/O to the Peripheral A function
  PIOA->PIO_PDR |= (1u << 11);    // MISO: Disable PIO control, enable peripheral control
}

void loop() {
  volatile word US_CSRWert;
  byte swert;
  byte ewert;
  swert = 0;
  while (true) {
  // Zeichen Senden
    pinport -> PIO_CODR = pinmask ;  // clear SS-pin
    SPI.transfer(swert);
    pinport -> PIO_SODR = pinmask ;   // set SS-pin
    delay(1);
  
// Zeichen sollte empfangen sein
    US_CSRWert = USART0->US_CSR; // Reading may change content
    Serial.print(swert);
    Serial.print(" --> ");
    delay(100);
    if (true) { // statt if ((USART0->US_CSR & 0x1)!=0) o. ä.
      Serial.print("US_CSR  \t");
      Serial.print(US_CSRWert, HEX);
      Serial.print("; ");
      US_CSRWert = USART0->US_CSR; // Reading may change content
      Serial.print(US_CSRWert, HEX);
      Serial.print("; ");

      ewert = USART0->US_RHR & 0xFF;
      Serial.print(ewert);
      Serial.print("; US_CSR  \t");
      US_CSRWert = USART0->US_CSR; // Reading may change content
      Serial.println(US_CSRWert, HEX);
    }
    else {
      Serial.println("-?-");
    }
    delay(1000);
    swert = (swert + 1);
  }
}