SPI MOSI receive error

Question

I have a master and a slave (both are arduino UNO) connected through SPI connected. I want to transmit a character to the slave and receive true or false to the master. The transmission is working perfectly. If I send '2', slave is receiving '2'. When I try to transmit an integer from slave to master, the master is always receiving some arbitrary number like 128 or 255.

Master Code:

void setup() {
 Serial.begin(9600);
 Serial.println("Master");
 digitalWrite(SS,HIGH);
 SPI.begin();
 SPI.setClockDivider(SPI_CLOCK_DIV8);
}
void loop() {
 digitalWrite(SS,LOW); // Slave select to low
 received1 = SPI.transfer(test); // test is a variable which is '2'. The data received by master is stored in received1 variable which is char.
 Serial.println(received1); 
 digitalWrite(SS,HIGH); 
}

Slave code:

void setup() {
 Serial.begin(9600);
 Serial.println("Slave");
 pinMode(MISO,OUTPUT);
 SPCR |= _BV(SPE);
 process = false;
 SPI.attachInterrupt(); 
}
ISR(SPI_STC_vect){
 received = SPDR; // the data received by slave is slored in **received** variable(the variable is char type)
 Serial.println(received);
 process = true;
}
void loop() {
 if(process){
 SPDR = jj; // Updating SPDR to send data from slave. jj is char type
 process = false;
 } 
}

• KIIV: The most important part, declaration of variable process, is not here. Is it volatile? Also do you realize that SPDR on slave must be set before the master starts transmission?
• Sri Harsha: Sorry for not mentioning about process. It's a boolean variable which will be true when something is received by slave.

Answer 1

I have found a solution for this problem in a blog.

Link: http://www.gammon.com.au/forum/?id=10892

I modified my master code and got my desired result.

Master code:

byte transferAndWait (const byte what)
{
 byte a = SPI.transfer (what);
 delayMicroseconds (2000);
 return a;
} // end of transferAndWait
void loop() {
 digitalWrite(SS,LOW);
 transferAndWait (10); 
 received1 = transferAndWait (11);
 digitalWrite(SS,HIGH);
 Serial.println(received1);
}

The response will be "collected" next time through the loop. That's why we need to keep a dummy transferAndWait() function first and then the result of that transfer will be stored in received1 variable.

• Nick Gammon: Please accept your own answer if it solved your problem, otherwise Stack Exchange throws it up from time to time as an unsolved question.