Arduino-to-Arduino SPI unexpected characters

Question

I've been trying to follow the posts on Nick Gammon's page which are admittedly a bit old, but seem to be everyone's recommendation for trying out Arduino to Arduino SPI. I'm able to get communications working, but there's a small issue I can't seem to pinpoint. Every once in a while there are three characters that show up in my output that are completely unexpected. Where are these characters coming from?

Here is what they are:

Hello, world!
Hello, wo&⸮BHello, world!
Hello, world!
Hello, world!
Hello, world!

In the second row, you'll see &⸮B in place of what should be rld!\n.

I have two Name-brand Arduino UNOs both being powered via USB to my computer, and they each have pins 13,12,11, and 10 directly wired to each other. Here's the code I'm using:

The unexpected characters are the same every time, and seemingly show up as UTF-8...the decimal values for those characters amounts to 38, 226, 66, 10. I'm not sure if that has any significance.

Master:

#include <SPI.h>
void setup (void)
{
 Serial.begin(115200);
 digitalWrite(SS, HIGH);
 SPI.begin ();
 SPI.setClockDivider(SPI_CLOCK_DIV8);
}
void loop (void)
{
 char c;
 // enable Slave Select
 digitalWrite(SS, LOW); // SS is pin 10
 // send test string
 const char p [14] = "Hello, world!\n";
 for (int i = 0; i < 14; i++) {
 SPI.transfer(p[i]); 
 Serial.print(p[i]);
 }
 // disable Slave Select
 digitalWrite(SS, HIGH);
 delay (2000);
}

Slave:

#include <SPI.h>
volatile char buf [100];
volatile byte pos;
volatile boolean process_it;
void setup (void)
{
 Serial.begin (115200);
 // have to send on master in, *slave out*
 pinMode(MISO, OUTPUT);
 // turn on SPI in slave mode
 SPCR |= _BV(SPE);
 
 // turn on interrupts
 SPCR |= _BV(SPIE);
 // get ready for an interrupt 
 pos = 0; // buffer empty
 process_it = false;
 // now turn on interrupts
 //SPI.attachInterrupt();
}
// SPI interrupt routine
ISR (SPI_STC_vect)
{
 byte c = SPDR; // grab byte from SPI Data Register
 if (pos < sizeof buf)
 {
 if (c == '\n') {
 buf [pos++] = '0円';
 process_it = true;
 }
 else{
 buf [pos++] = c;
 
 }
 
 }
}
void loop (void)
{
 if (process_it){
 char * bufp = buf;
 Serial.println (bufp);
 pos = 0;
 process_it = false;
 } 
}

• gre_gor: String "Hello, world!\n" is 15 bytes long not 14.
• TheEmpireNeverEnded: @gre_gor, I know. I was testing if that affected it. Previously also tested 15 and 16 as well to see if there was any change. No effect.

Answer 1

Despite the fact that both units were being powered from the same USB Hub, there was a difference in voltage on the GNDs of each Arduino. Jumping the GND pins between the two got rid of the stray characters and strange behavior. D'oh.