Arduino UNO as an SPI Slave

Projects Networking, Protocols, and Devices
1

I have been working off the SPI slave example at Gammon Forum : Electronics : Microprocessors : SPI - Serial Peripheral Interface - for Arduino and am trying to implement similar logic (read MOSI and print to Serial), but with HEX numbers, specifically 2 byte HEX values that I am receiving via Bluetooth.

I have checked my SPI master out using a logic analyzer and am positive I am sending out correct values, however when I try running my arduino code, I get random numbers instead of my expected hex values.

Any help would be greatly appreciated.

#include <SPI.h>

byte buf [100];
volatile byte pos;
volatile boolean process_it;

void setup (void)
{
  Serial.begin (115200);   // debugging
  // have to send on master in, *slave out*
  pinMode(MISO, OUTPUT);
  
  // turn on SPI in slave mode
  SPCR |= _BV(SPE);
  
  // get ready for an interrupt 
  pos = 0;   // buffer empty
  process_it = false;

  // now turn on interrupts
  SPI.attachInterrupt();

}  // end of setup


// SPI interrupt routine
ISR (SPI_STC_vect)
{
byte c = SPDR;  // grab byte from SPI Data Register
  
  // add to buffer if room
  if (pos < sizeof buf)
    {
    buf [pos++] = c;
    
    // example: newline means time to process buffer
    if (pos==1)
      process_it = true;
      
    }  // end of room available
}  // end of interrupt routine SPI_STC_vect

// main loop - wait for flag set in interrupt routine
void loop (void)
{
  if (process_it)
    {
    buf [pos] = 0;  
    Serial.write (buf);
    pos = 0;
    process_it = false;
    }  // end of flag set
    
}  // end o
2

well, first I want to say sorry for my bad english.
Then, you send through serial the whole buf, which dimension is 100 (buf[100]). This mean that when you use the Serial.write(buf), it will show you 100 values (I think it will print them in dec and not hex, unless you set some format). Those 100 values are:

  1. the first byte received from spi
  2. the second byte received from spi
  3. a 0 byte
    4-100. 97 random bytes