add eeprom library explicitly

digistump-avr/libraries/EEPROM/examples/eeprom_update/eeprom_update.ino

@@ -0,0 +1,69 @@
+/***
+   EEPROM Update method
+  
+   Stores values read from analog input 0 into the EEPROM.
+   These values will stay in the EEPROM when the board is
+   turned off and may be retrieved later by another sketch.
+  
+   If a value has not changed in the EEPROM, it is not overwritten
+   which would reduce the life span of the EEPROM unnecessarily.
+   
+   Released using MIT licence.
+ ***/
+
+#include <EEPROM.h>
+
+/** the current address in the EEPROM (i.e. which byte we're going to write to next) **/
+int address = 0;
+
+void setup(){ /** EMpty setup **/ }
+
+void loop()
+{
+  /***
+    need to divide by 4 because analog inputs range from
+    0 to 1023 and each byte of the EEPROM can only hold a
+    value from 0 to 255.
+  ***/
+  int val = analogRead(0) / 4;
+
+  /***
+    Update the particular EEPROM cell.
+    these values will remain there when the board is
+    turned off.
+  ***/
+  EEPROM.update(address, val);
+  
+  /***
+    The function EEPROM.update(address, val) is equivalent to the following:
+  
+    if( EEPROM.read(address) != val ){
+      EEPROM.write(address, val);
+    }
+  ***/
+  
+
+  /***
+    Advance to the next address, when at the end restart at the beginning.    
+    
+    Larger AVR processors have larger EEPROM sizes, E.g:
+    - Arduno Duemilanove: 512b EEPROM storage.
+    - Arduino Uno:        1kb EEPROM storage.
+    - Arduino Mega:       4kb EEPROM storage.
+    
+    Rather than hard-coding the length, you should use the pre-provided length function.
+    This will make your code portable to all AVR processors.    
+  ***/
+  address = address + 1;
+  if(address == EEPROM.length())
+    address = 0;
+    
+  /***
+    As the EEPROM sizes are powers of two, wrapping (preventing overflow) of an 
+    EEPROM address is also doable by a bitwise and of the length - 1.
+    
+    ++address &= EEPROM.length() - 1;
+  ***/
+
+  delay(100);
+}