Embedded: SPI Bitbanging

#define CLKLOW()       // Put SCLK GPIO to LOW

#define CLKHIGH()      // Put SCLK GPIO to HIGH

#define ASSERTCS()     // Put /CS GPIO to LOW

#define DEASSERTCS()   // Put /CS GPIO to HIGH

#define READMISO()     // MISO GPIO status?

#define MOSIHIGH()     // MOSI GPIO to HIGH

#define MOSILOW()      // MOSI GPIO to LOW

#define POWERON()      // VCC GPIO to HIGH

#define POWERDOWN      // VCC GPIO to LOW

#define SPISPPED       140

void spi_init()
{
    POWERON();
    CLKHIGH();
    DEASSERTCS();  
}

/////////////////////////////////////////////////////////////////////////////////////////////////

void spi_write_byte_SPIBitBang8BitsMode0(unsigned char byte)
{       
    unsigned char bit;
    ASSERTCS();
    for (bit = 0; bit < 8; bit++) {
        /* write MOSI on trailing edge of previous clock */
        if (byte & 0x80)
            MOSIHIGH();
        else
            MOSILOW();
        byte <<= 1;
 
        /* half a clock cycle before leading/rising edge */
        SPIDELAY(SPISPEED/2);
        CLKHIGH();
 
        /* half a clock cycle before trailing/falling edge */
        SPIDELAY(SPISPEED/2);
 
        /* read MISO on trailing edge */
        //byte |= READMISO();
        CLKLOW();
    } 

    DEASSERTCS();

}

/////////////////////////////////////////////////////////////////////////////////////////////////

void spi_write_byte(unsigned char byte)   /* Pseudo Code */

{

    TEMP = 100000000B;

    SS = 0;

    SCK = 1; 

   WHILE(TEMP)

       MOSI = (WRITE_BUFFER && TEMP);

       TEMP >> 1;

       SCK ^= 1;

   END WHILE

   SS = 1

}

/////////////////////////////////////////////////////////////////////////////////////////////////

unsigned char spi_read_byte()   /* Pseudo Code */

{

   SS = 0;

   SCK = 1;

   Count = 8;

   WHILE(count)

      READ_BUFFER | = MISO;

      READ_BUFFER << 1;

      Count--;

      SCK ^= 1;

   END WHILE

   SS = 1

}

////////////////////////

void spi_stop()

{

   POWERDOWN();

}

For more information on SPI, please visit this Wikipedia article