I wrote some code to stress test my Arduino. The code below will make your arduino calculate prime numbers.

void setup() { Serial.begin(9600); } long i = 2; // Start at 2 void loop() { char output[50]; int prime = is_prime(i); // Check if the number we're on is prime char* status; if (prime) { status = "yes"; } else { status = "no"; } if (prime == 1) { Serial.print("Checking "); Serial.print(i); Serial.print(" for prime "); Serial.print(status); Serial.print(" in "); Serial.print(millis() / 1000.0f); Serial.println(" seconds"); } i++; } int is_prime(int num) { // Only have to check for divisible for the sqrt(number) long upper = sqrt(num); // Check if the number is evenly divisible (start at 2 going up) for (long cnum = 2; cnum <= upper; cnum++) { int mod = num % cnum; // Remainder if (mod == 0) { return 0; } // If the remained is 0 it's evenly divisible } return 1; // If you get this far it's prime }