My AT91SAM7X512's SPI peripheral gets disabled on the X time (X varies) that I write to SPI_TDR. As a result, the processor hangs on the while loop that checks the TDRE flag in SPI_SR. This while loop is located in the function SPI_Write() that belongs to the software package/library provided by ATMEL. The problem occurs arbitrarily - sometimes everything works OK and sometimes it fails on repeated attempts (attemp = downloading the same binary to the MCU and running the program). Configurations are (defined in the order of writing): SPI_MR: MSTR = 1 PS = 0 PCSDEC = 0 PCS = 0111 DLYBCS = 0 SPI_CSR[3]: CPOL = 0 NCPHA = 1 CSAAT = 0 BITS = 0000 SCBR = 20 DLYBS = 0 DLYBCT = 0 SPI_CR: SPIEN = 1 After setting the configurations, the code verifies that the SPI is enabled, by checking the SPIENS flag. I perform a transmission of bytes as follows: const short int dataSize = 5; // Filling array with random data unsigned char data[dataSize] = {0xA5, 0x34, 0x12, 0x00, 0xFF}; short int i = 0; volatile unsigned short dummyRead; SetCS3(); // NPCS3 == PIOA15 while(i-- < dataSize) { mySPI_Write(data[i]); while((AT91C_BASE_SPI0->SPI_SR & AT91C_SPI_TXEMPTY) == 0); dummyRead = SPI_Read(); // SPI_Read() from Atmel's library } ClearCS3(); /**********************************/ void mySPI_Write(unsigned char data) { while ((AT91C_BASE_SPI0->SPI_SR & AT91C_SPI_TXEMPTY) == 0); AT91C_BASE_SPI0->SPI_TDR = data; while ((AT91C_BASE_SPI0->SPI_SR & AT91C_SPI_TDRE) == 0); // <-- This is where // the processor hangs, because that the SPI peripheral is disabled // (SPIENS equals 0), which makes TDRE equal to 0 forever. } Questions: What's causing the SPI peripheral to become disabled on the write to SPI_TDR? Should I un-comment the line in SPI_Write() that reads the SPI_RDR register? Means, the 4th line in the following code: (The 4th line is originally marked as a comment) void SPI_Write(AT91S_SPI *spi, unsigned int npcs, unsigned short data) { // Discard contents of RDR register //volatile unsigned int discard = spi->SPI_RDR; /* Send data */ while ((spi->SPI_SR & AT91C_SPI_TXEMPTY) == 0); spi->SPI_TDR = data | SPI_PCS(npcs); while ((spi->SPI_SR & AT91C_SPI_TDRE) == 0); } Is there something wrong with the code above that transmits 5 bytes of data? Please note: The NPCS line num. 3 is a GPIO line (means, in PIO mode), and is not controlled by the SPI controller. I'm controlling this line by myself in the code, by de/asserting the ChipSelect#3 (NPCS3) pin when needed. The reason that I'm doing so is because that problems occurred while trying to let the SPI controller to control this pin. I didn't reset the SPI peripheral twice, because that the errata tells to reset it twice only if I perform a reset - which I don't do. Quoting the errata: If a software reset (SWRST in the SPI Control Register) is performed, the SPI may not work properly (the clock is enabled before the chip select.) Problem Fix/Workaround The SPI Control Register field, SWRST (Software Reset) needs to be written twice to be cor- rectly set. I noticed that sometimes, if I put a delay before the write to the SPI_TDR register (in SPI_Write()), then the code works perfectly and the communications succeeds. Useful links: AT91SAM7X Series Preliminary.pdf ATMEL software package/library spi.c from Atmel's library spi.h from Atmel's library An example of initializing the SPI and performing a transfer of 5 bytes is highly appreciated and helpful.