SPI uses the following four wires −

• SCK − This is the serial clock driven by the master.
• MOSI − This is the master output / slave input driven by the master.
• MISO − This is the master input / slave output driven by the master.
• SS − This is the slave-selection wire.

The following functions are used. You have to include the SPI.h.

• SPI.begin() − Initializes the SPI bus by setting SCK, MOSI, and SS to outputs, pulling SCK and MOSI low, and SS high.
• SPI.setClockDivider(divider) − To set the SPI clock divider relative to the system clock. On AVR based boards, the dividers available are 2, 4, 8, 16, 32, 64 or 128. The default setting is SPI_CLOCK_DIV4, which sets the SPI clock to one-quarter of the frequency of the system clock (5 Mhz for the boards at 20 MHz).
• Divider − It could be (SPI_CLOCK_DIV2, SPI_CLOCK_DIV4, SPI_CLOCK_DIV8, SPI_CLOCK_DIV16, SPI_CLOCK_DIV32, SPI_CLOCK_DIV64, SPI_CLOCK_DIV128).
• SPI.transfer(val) − SPI transfer is based on a simultaneous send and receive: the received data is returned in receivedVal.
• SPI.beginTransaction(SPISettings(speedMaximum, dataOrder, dataMode)) − speedMaximum is the clock, dataOrder(MSBFIRST or LSBFIRST), dataMode(SPI_MODE0, SPI_MODE1, SPI_MODE2, or SPI_MODE3).

We have four modes of operation in SPI as follows −

• Mode 0 (the default) − Clock is normally low (CPOL = 0), and the data is sampled on the transition from low to high (leading edge) (CPHA = 0).
• Mode 1 − Clock is normally low (CPOL = 0), and the data is sampled on the transition from high to low (trailing edge) (CPHA = 1).
• Mode 2 − Clock is normally high (CPOL = 1), and the data is sampled on the transition from high to low (leading edge) (CPHA = 0).
• Mode 3 − Clock is normally high (CPOL = 1), and the data is sampled on the transition from low to high (trailing edge) (CPHA = 1).
• SPI.attachInterrupt(handler) − Function to be called when a slave device receives data from the master.

Now, we will connect two Arduino UNO boards together; one as a master and the other as a slave.

• (SS) : pin 10
• (MOSI) : pin 11
• (MISO) : pin 12
• (SCK) : pin 13

The ground is common. Following is the diagrammatic representation of the connection between both the boards −

Let us see examples of SPI as Master and SPI as Slave.

Arduino SPI as Master

Master unit sends hello world data to slave unit.

#include <SPI.h>

void setup (void) {
   Serial.begin(115200); //set baud rate to 115200 for usart
   digitalWrite(SS, HIGH); // disable Slave Select
   SPI.begin ();
   SPI.setClockDivider(SPI_CLOCK_DIV8);//divide the clock by 8
}

void loop (void) {
   char c;
   digitalWrite(SS, LOW); // enable Slave Select
   // send test string
   for (const char * p = "Hello, world!\r" ; c = *p; p++) 
   {
      SPI.transfer (c);
      Serial.print(c);
   }
   digitalWrite(SS, HIGH); // disable Slave Select
   delay(2000);
}

Arduino SPI as Slave

Slave unit waits for data as soon as data is arrived process variable becomes true, indicating there is data in buffer. in main loop we read this buffer and send to serial terminal.

#include <SPI.h>
char buff [50];
volatile byte indx;
volatile boolean process;

void setup (void) {
   Serial.begin (115200);
   pinMode(MISO, OUTPUT); // have to send on master in so it set as output
   SPCR |= _BV(SPE); // turn on SPI in slave mode
   indx = 0; // buffer empty
   process = false;
   SPI.attachInterrupt(); // turn on interrupt
}

ISR (SPI_STC_vect) // SPI interrupt routine 
{ 
   byte c = SPDR; // read byte from SPI Data Register
   if (indx < sizeof buff) {
      buff [indx++] = c; // save data in the next index in the array buff
      if (c == '\r') //check for the end of the word
      process = true;
   }
}

void loop (void) {
   if (process) {
      process = false; //reset the process
      Serial.println (buff); //print the array on serial monitor
      indx= 0; //reset button to zero
   }
}

Results

Open serial monitor of slave, you will see “Hello, World”.