SPI Program for AVR

An example of SPI, using SPI to realize two-machine communication, using interrupt mode to realize full-duplex communication.
 This example is implemented with two MEGA8515, and the connection is:
   MISO----MISO
   MOSI----MOSI
   SCK ----SCK
   /SS ----/SS
 The function fill_tx_buffer that loads the data to be sent into the send buffer and the function read_rx_buffer that reads the data from the receive buffer are not given, please complete them according to your own needs.

#define SPI_RX_BUFFER_SIZE 10
#define SPI_RX_BUFFER_MASK ( SPI_RX_BUFFER_SIZE - 1 )
#define SPI_TX_BUFFER_SIZE 10
#define SPI_TX_BUFFER_MASK ( SPI_TX_BUFFER_SIZE - 1 )
#define SET_SPI_MODE PORTB.4
#define SPI_MODE PINB.4

static unsigned char SPI_RxBuf[SPI_RX_BUFFER_SIZE];
static volatile unsigned char SPI_RxHead;
static unsigned char SPI_TxBuf[SPI_TX_BUFFER_SIZE];
static volatile unsigned char SPI_TxHead;
//******************************************
// SPI interrupt service routine
//******************************************
interrupt [SPI_STC] void spi_isr(void)
{
unsigned char data;
if(spi_m==0)                        //If spi_m is 0, it indicates receiving state
{
 data = SPDR;                      //Read received
   dataSPI_RxBuf[SPI_RxHead-1] = data;        //Store received data into receiving bufferif
   ( SPI_RxHead == SPI_RX_BUFFER_MASK )   //If it is the last data of receiving frame  
   {
     SPI_RxHead = 0;                //Restore received dataMSTR
     =1;                      //Receiving completed, set SPI back to main modespi_trans_com
     =1;                //Set receiving completed flag
   }
   else
   {  
     SPI_RxHead++;                  //Received data counter plus 1
   }
 
}
else                              //If spi_m is 1, it indicates sending state
 {
                         
   if ( SPI_TxHead <= SPI_TX_BUFFER_MASK)   //If the data to be sent has not been sent completely
   {  
     SPDR = SPI_TxBuf[SPI_TxHead];      //Get data from the transmit buffer and send
     SPI_TxHead++;                  //Sent data counter plus 1
   }
   else                        //If all the data to be sent have been sent
     {
     SPI_TxHead=0;                //Sent data counter restored
     DDRB.4=0;
     SET_SPI_MODE=1;              //Release the bus so that the receiver can enter the main transmission.
     spi_m=0;                    
     spi_sending=0;                //Clear the sending flag
     }
 
 }

}

//**********************************************
// SPI initialization
//**********************************************
void InitSPI(void)
{

SPCR=0x52;
SPI_RxHead = 0;
SPI_TxHead = 0;

}

//**********************************************
//Send data
//**********************************************
void spi_send(void)
{
if(spi_sending==0)    //The sending flag is 0, indicating that spi sending is idle
 {
 
 fill_tx_buffer();   //Call fill_tx_buffer function to load the data to be sent into the send bufferwhile
 (PINB.4==0)  //If PINB.4 is low, it indicates that the bus is occupied by the receiver. Wait until the receiver completes sending.
 {;}
 
 InitSPI();      //Initialize SPI to master mode
 DDRB.4=1;
 SET_SPI_MODE=0;   //Pull PORTB.4 low to force the receiver to enter slave mode
 spi_m=1;        //Set spi_m flag to indicate sending state
 delay_us(10);
 spi_sending=1;    //Set spi_sending flag to indicate sending in progress
 SPDR=0xFF;      //Start sending, the first data received by the receiver is 0xFF and should be ignored
 SPIE=1;        //Open SPI interrupt,
 SPI_TxHead = 0;   //Clear the sent data counter
 }
}

void main(void)
{

...
while(1)
{
 ...
 
 if(spi_trans_com==1) //If the receive completion flag is 1, it indicates that some data has been received
 {
   read_rx_buffer(); //Call read_rx_buffer function to read the received data from the receive buffer
   spi_trans_com=0; //Clear the receive completion flag after reading
 }
 ...

}

}