====== Anfragen von Daten vom Slave per SPI ======

Ein einfaches Beispiel wie ein SPI-Master von einem SPI-Slave Daten anfragen kann ist hier zu finden: {{microcontrollertechnik:spi_slavetest.zip}} \\
In der Datei ist auch das passende Simulide File vorhanden

==== Code des Master ====
<sxh c; first-line: 1>
#define F_CPU				16000000UL

#include <avr/io.h>
#include <util/delay.h>
#include <stdint.h>

#define SET_BIT(BYTE, BIT)  ((BYTE) |=  (1 << (BIT)))
#define CLR_BIT(BYTE, BIT)  ((BYTE) &= ~(1 << (BIT)))

#define SLAVE_REQUEST		0x11			// The value can be changed

void SPI_MasterTransmit(uint8_t data);

int main() 
{											// Set the following Pins as output
	DDRB =	 (1<<DDB2)						//  - !SS 
			|(1<<DDB3)						//  - MOSI
			|(1<<DDB4)						//  - MOSI
			|(1<<DDB5); 					//  - SCK
	SPCR =   (1<<SPE)|(1<<MSTR)|(1<<SPR0);	// Enable SPI, Master mode, clock rate fck/16
	
	DDRD = 0xFF;							// Set PORTD as output
	
	while(1) {
		SPI_MasterTransmit(SLAVE_REQUEST);	// Send request to slave

		uint8_t dataFromSlave = SPDR;		// Receive the answer from the slave
		PORTD = dataFromSlave^0xFF;			// Output the answer on PORTC, but all Bits inverted 
	}
}

void SPI_MasterTransmit(uint8_t data) 
{
	CLR_BIT(PORTB, PORTB2);					// clear !SS to activate Slave
	SPDR = data;							// Start transmission
	while(!(SPSR & (1<<SPIF)));				// Wait for transmission complete
	SET_BIT(PORTB, PORTB2);					// Set !SS to deactivate Slave
}
</sxh>

==== Code des Slave ====
<sxh c; first-line: 1>
#include <avr/io.h>
#include <avr/interrupt.h>

#define SLAVE_REQUEST		0x11		// The value can be changed

volatile uint8_t dataToMaster	= 0x00;
volatile uint8_t dataFromMaster = 0x00;

int main() {
	DDRB = (1<<DDB4);					// MISO as output
	PORTD = 0xFF;						// Activate all pull-up resistors for PIND, to read switch states correctly
	SPCR = (1<<SPE)|(1<<SPIE);			// Enable SPI and SPI interrupt
	sei();								// Enable global interrupts

	while(1) 
	{
										// Main application code
	}
}

ISR(SPI_STC_vect) {
	dataFromMaster = SPDR;
	if (dataFromMaster == SLAVE_REQUEST)
	{
		dataToMaster=PIND;
		SPDR = dataToMaster;			// Load data into the buffer		
	}
}
</sxh>