Update of code for the SPI testing

hdk/getting_started.md

@@ -36,8 +36,72 @@ The easiest way to program the HDK is via an Arduino. This allows you to use the
 
 ### Example program 
 
-```C
+You can obtain the original of this code from here: https://gist.github.com/chrismeyersfsu/3317769
 
+```C
+// Written by Nick Gammon
+// February 2011
+/**
+ * Send arbitrary number of bits at whatever clock rate (tested at 500 KHZ and 500 HZ).
+ * This script will capture the SPI bytes, when a '\n' is recieved it will then output
+ * the captured byte stream via the serial.
+ */
+
+#include <SPI.h>
+
+char 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 (c == '\n')
+      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.println (buf);
+    pos = 0;
+    process_it = false;
+    }  // end of flag set
+    
+}  // end of loop
 ```
 
 ### Programming the HDK