Tutorial 10: EEPROM
Saving data such that it is preserved after a power cycle or an update to the code may be important to your project. In this tutorial I show you how to access the Arduino's EEPROM, allowing you to save data just like it were an SD card!
Of course the amount of data you can save to the EEPROM is quite small: the Arduino UNO only allows for 1kB of data storage and the Arduino MEGA2560 only allows 4kB. However if all you're trying to achieve is hold a last known value of a sensor, input, or some other parameter, the EEPROM provides more than enough space!