SPI communication between 2 UNOs

hi,

i have written codes below. use spi between 2 arduino unos first trial based on nick gammon's tutorial.

i send commands 'i' , 'o' master on serial. slave receives commands on spi. 'i' turns on , led. 'o' turns led off.

i've set protocol spi communitcation:



the code bothers me seems need have "delaymicroseconds (20)" inbetween transfers else things don't work , stop responding!

the spdr register should updated time spi.transfer complete. why need wait 20us before reading data.

and make things more fun, if use serial.print try , debug, work fine!

any ideas why, anyone?

master:

code: [select]

#include <spi.h>

volatile char req[9];
volatile uint8_t a, = 0;

byte transferandwait (byte what)
{
  byte x = spi.transfer (what);
  delaymicroseconds (20); //wait slave process , reply
  return x;
}

void setup (void)
{
  serial.begin (9600);

  digitalwrite(ss, high);  // ensure ss stays high now

  spi.begin ();

  spi.begintransaction(spisettings(4000000, msbfirst, spi_mode0));

  serial.println ("ready");
}

void loop (void)
{
  uint8_t j;

  if (serial.available()) {
    req[i] = serial.read();
    ++i;
  }


  if (req[i - 1] == ';') {
    if (req[0] == 'o') { //turn led off
      // enable slave select
      digitalwrite(ss, low);

      a = transferandwait(0); //start spi comms
      //serial.println (string(a,hex));
      a = transferandwait('o'); //send command
      //serial.println (string(a,hex));
      a = transferandwait(0); //end spi comms

      if (a == 'o') {
        serial.print("led off!");
        serial.println (char(a));
      }
      else {
        serial.print("failed! ");
        serial.println (string(a, hex));
      }

      // disable slave select
      digitalwrite(ss, high);
    }
    else if (req[0] == 'i') { //turn led on
      // enable slave select
      digitalwrite(ss, low);

      a = transferandwait(0); //start spi comms
      //serial.println (string(a,hex));
      a = transferandwait('i'); //send command
      //serial.println (string(a,hex));
      a = transferandwait(0); //end spi comms

      if (a == 'i') {
        serial.print("led on!");
        serial.println (char(a));
      }
      else {
        serial.print("failed! ");
        serial.println (string(a, hex));
      }

      // disable slave select
      digitalwrite(ss, high);
    }

    i = 0;
  }

}


slave:

code: [select]

#define spi_timeout 25

#include <spi.h>

volatile uint8_t start = 0, command = 0;

// spi interrupt routine
isr (spi_stc_vect)
{
  byte spi = spdr;

  if (spi == 0 && start == 0) { //start of spi comms
    if (command != 0xa3) {
      start = 1;
      spdr = 0xa4; //acknowledge start of comms
    }
    else {
      command = 0;
      spdr = 0;
    }
  }
  else if (start > 0) {

    if (start == 1) {
      start = 2;
      command = spi;
    }

    if (command == 'o') {
      digitalwrite(2, low);
      command = 0xa3; //end of command
      start = 0; //end of comms
      spdr = 'o';
    }
    else if (command == 'i') {
      digitalwrite(2, high);
      command = 0xa3; //end of command
      start = 0; //end of comms
      spdr = 'i';
    }
    else { //reset spi comms
      start = 0;
      command = 0;
    }
  }

  timeout = micros();
}

void setup (void)
{

  pinmode(2, output); //led output
  digitalwrite(2, low);

  // have send on master in, *slave out*
  pinmode(miso, output);

  // turn on spi in slave mode
  spcr |= _bv(spe);

  // turn on interrupts
  spcr |= _bv(spie);

  spdr = 0; //initialise spi register

}  // end of setup

void loop (void)
{
  //re-initialise spi variables on timeout
  if (spdr != 0 && (micros() - timeout) > spi_timeout) {
    start = 0;
    command = 0;
    spdr = 0;
  }
}

edit - gammon says,

quote

otherwise slave doesn't have chance react incoming data , it.

how relate problem?

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