Komunikacja SPI w Arduino

#include "SPI.h"                       

pinMode(SS, OUTPUT);                    
digitalWrite(SS, HIGH);
SPI.begin();  //rozpoczęcie komunikacji SPI
SPI.beginTransaction(SPISettings(clockSpeed, bitOrder, dataMode)); 
        //ustawienia komunikacji
        // clockSpeed: Prędkość zegara SPI, np. 500000 dla 500 kHz.
        // bitOrder: Kolejność przesyłania bitów. MSBFIRST lub LSBFIRST 
        // dataMode: Tryb SPI (kombinacja CPOL i CPHA). Dostępne tryby to:
        // SPI_MODE0: CPOL = 0, CPHA = 0,
        // SPI_MODE1: CPOL = 0, CPHA = 1,
        // SPI_MODE2: CPOL = 1, CPHA = 0,
        // SPI_MODE3: CPOL = 1, CPHA = 1.
digitalWrite(SS, LOW);                  
SPI.transfer(byte);        //przesyłanie danych, linia SS w stanie LOW
digitalWrite(SS, HIGH);                 
SPI.endTransaction();     // zakończenie transakcji, zwolnienie magistrali dla innych urządzeń

//MASTER ARDUINO

#include<SPI.h>
int counter = 0;
int btnPin = 2;
bool btnPrevState = false;

void setup(){
  Serial.begin(9600);
  SPI.begin();
  SPI.beginTransaction(SPISettings(125000, MSBFIRST , SPI_MODE0));
  pinMode(SS, OUTPUT);   
}

void onClick() {
  bool btnNowState = digitalRead(btnPin);
  if (btnNowState == 1 && btnPrevState == 0) {
    digitalWrite(SS, LOW);
    SPI.transfer(counter);
    digitalWrite(SS, HIGH);
    SPI.endTransaction();     
    counter++;
  }
  btnPrevState = btnNowState;
}

void loop(){
  onClick();
}

//SLAVE ARDUINO

#include<SPI.h>
volatile bool flag1 = false;

void setup(){
  Serial.begin(9600);
  pinMode(MOSI, OUTPUT);     //should be made output to send data to Master
  SPCR |= _BV(SPE);          //SPI Port is enabled or SPCR |= (1 << SPE);
  SPI.attachInterrupt();
  pinMode(SS, INPUT_PULLUP);
}

void loop(){
  if (flag1 == true){
    Serial.println(SPDR);
    flag1 = false;
  }
}

ISR (SPI_STC_vect){
  flag1 = true;  
}

//master
#include<SPI.h>
char txMsg[] = "Arduino!";
char rxMasg[20] = "";

void setup(){
  Serial.begin(9600);
  SPI.begin();
  SPI.setClockDivider(SPI_CLOCK_DIV128); //16 MHz/128
  pinMode(SS, OUTPUT);    //SS is DPin-10
  digitalWrite(SS, LOW);   //Slave is selected
}

void loop(){
  SPI.transfer('<'); //start mark
  for(int i=0; i< sizeof(txMsg); i++)  {
    SPI.transfer(txMsg[i]);//SPI transfer is byte-by-byte
  }
  SPI.transfer('>');  //end mark 
  delay(1000);         //test interval
}

//slave
#include<SPI.h>
char rxMsg[20] = "";
volatile bool flag1 = false;
bool flag2 = false;
int i = 0;

void setup(){
  Serial.begin(9600);
  pinMode(SS, INPUT_PULLUP);  //SPI.h helps to get the meaning of SS
  pinMode(MOSI, OUTPUT);      //should be made output to send data to Master
  SPCR |= _BV(SPE);         //SPI Port is enabled
  SPI.attachInterrupt();    //SPI interrupt is enabled
}

void loop(){
  if (flag1 == true) {  //SPDR has data  
    if (flag2 == false) //start mark of message not yet received
    {
      char q = SPDR;
      if (q == '<')
      {
        flag2 = true;   //start mark is detected
        flag1 = false;  //must be made false for the next cycle
      }
    }
    else    {
      if (SPDR == '>')  //checking if end mark has arived
      {
        flag1 = false;
        flag2 = false;
        i = 0;
        Serial.print(rxMsg);  //end mark has arrived; show the received message
        Serial.println();     //insert newline
      }
      else
      {
        rxMsg[i] = SPDR;    //save received charcater/data byte in array
        i++;                //adjust array pointer
        flag1 = false;      //must be false for next cycle
      }
    }
  }
}

ISR (SPI_STC_vect) {  //MCU comes here when there is a data byte in SPDR
  flag1 = true;   //flag to indicate there is data in the SPDR register
}