How can i solve data transferred from slave to master using SPI? - embedded

Question

I tried to make a communication between two AVR (ATmega128) using SPI.
Data transferred correctly from master to slave but data transferred wrong from slave to master, the first sampled bit is always wrong.
Slave sends (0X7E) to master but the received data is (0X3F).
Where is the mistake?
Code of MASTER
#include <avr/io.h>
#define F_CPU 16000000UL
#include <util/delay.h>
#define ACK 0x7E
void spi_init_master(void)
{
DDRB = (1<<0)|(1<<1)|(1<<2); //Set MOSI, SCK, SS as Output
PORTB |= 1;
SPCR = (1<<SPE)|(1<<MSTR)|(1<<SPR0); //Enable SPI, Set as Master, Prescaler: Fosc/16
}
//Function to send and receive data
unsigned char spi_tranceiver(unsigned char data)
{
PORTB &= ~(1<<0);
SPDR = data; //Load data into the buffer
while (!(SPSR & (1<<SPIF) )); //Wait until transmission complete
PORTB |= (1<<0);
return (SPDR); //Return received data
}
int main(void)
{
spi_init_master(); //Initialize SPI Master
unsigned char data;
unsigned char ret;
while (1)
{
ret = spi_tranceiver(data);
}
}
Code of SLAVE
#include <avr/io.h>
#define F_CPU 16000000UL
#include <util/delay.h>
#define ACK 0x7E
void spi_init_slave(void)
{
DDRB = (1<<3); //MISO as OUTPUT
SPCR = (1<<SPE); //Enable SPI
}
//Function to send and receive data
unsigned char spi_tranceiver(unsigned char data)
{
SPDR = data; //Load data into buffer
while (!(SPSR & (1<<SPIF) )); //Wait until transmission complete
return (SPDR); //Return received data
}
int main(void)
{
spi_init_slave(); //Initialize slave SPI
unsigned char data;
while (1)
{
data = spi_tranceiver(ACK);
}
}

Answer 1

Maybe the problem is at reloading the data register #SLAVE. In your code this is done via polling. So if you have a high clock speed the slave does not get enough time to reload the register corectly.
Important notice:
Clock peed of SPI has to be F_CPU/4 on slave (see the datasheet on Page 168)
Try to put in a delay at master. So the slave got some time to reload the register:
#define F_CPU ?????????UL // Clock speed of system in Hz
unsigned char spi_tranceiver(unsigned char data);
#include <util/delay.h>
int main(void)
{
// ...
while (1)
{
ret = spi_tranceiver(data);
}
}
unsigned char spi_tranceiver(unsigned char data)
{
_delay_ms(10);
PORTB &= ~(1<<SS);
SPDR = data;
while (!(SPSR & (1<<SPIF)));
PORTB |= (1<<SS);
return (SPDR);
}
Try to start the slave before master is started!