Generic function to send commands over the SPI interface of an AVR uC

gistfile1.c

// defines for all commands and the max length of spi messages
#define CHIPERASE "\xc7\x94\x80\x9a"
#define SPILENGTH 32

// array to receive data
unsigned char spi_rxdata[SPILENGTH];

// transfer a byte on spi
static unsigned char spi_transfer (unsigned char c) {
  SPDR = c;
  while (!(SPRS & (1<<SPIF))) {}
  return SPDR;
}

// function to send commands & receive data over the spi if
static int spi_sendcmd (unsigned char *received, unsigned char *command, uint8_t length) {
	if (length>SPILENGTH) {
		return -1;
	}
        // activate slave select
	PORTB &= ~(1<<PB2);
	for (int index=0; index<length; index++) {
		received[index] = spi_transfer(command[index]);
	}
        // deactivate slave select
	PORTB |= (1<<PB2);
	return index;
}

// call the function
spi_rxdata_length = spi_sendcmd(spi_rxdata, CHIPERASE, sizeof(CHIPERASE)-1);