Reset an Arduino Uno in code

Question

Is it possible to reset an Arduino (i.e., to reboot it) from code (i.e from the sketch itself)? I know that is possible with a special circuit but is there a chance to make it just with code?

Below is my code and the comment //reset is where I want to force a reset.

#include <TrueRandom.h>

int i;
int randSeed;
long randNumber;

void setup(){
  Serial.begin(9600);
  Serial.println("20 pseudo Zufallszahlen:");
  for (i=1;i<=20;i++) Serial.print(random(10));
  Serial.println();
  Serial.println();
  //randomSeed(TrueRandom.random());
  randSeed = analogRead (A0);
  randomSeed(randSeed);
  Serial.print("Der 'seed' Wert: ");
  Serial.println(randSeed);
  Serial.println();
  Serial.println("20 Zufallszahlen mit analogem 'seed' Wert:");
  for (i=1;i<=20;i++) Serial.print(random(10));
  Serial.println();
  Serial.println("---------------------------");
  Serial.println();
  delay(500);
  //reset  
}

void loop() {
}

I want to reset the micro-controller at the end of the setup function to show the effect of random numbers with and without a seed.

Answer 1

There three ways to accomplish this. (last is my favorite)

1) Jumper an unused IO to the RESET pin. Leave it as INPUT for normal run, As it is externally pulled high. And when desired to reset set it as LOW and Output. (bang its rebooting).

setup() {
  ...
  pinMode(PINtoRESET, INPUT);    // Just to be clear, as default is INPUT. Not really needed.
  digitalWrite(PINtoRESET, LOW); // Prime it, but does not actually set output. 
  ...                            // Does disable 10K pull Up, but who cares.

then when desired...

...
  pinMode(PINtoRESET, OUTPUT);   // lights out. Assuming it is jumper-ed correctly.
  while(1);                      // never gets here.

2) Jump to beginning of the code.

void(* resetFunc) (void) = 0;  // declare reset fuction at address 0
...
resetFunc(); //call reset

But be careful, this does not perform a true reset, in that all the registers ARE NOT DEFAULTED. Rather they and the IO are left as is. Where somethings from the bootloader and then the heap will be initialized. And reset are not!

3) Use the watchdog. The SoftReset library makes it easy. Although it is not difficult to implement directly. Shown below..

#include <avr/wdt.h>
...
setup() {
  ...
  MCUSR = 0;  // clear out any flags of prior resets.
  ...

then when desired...

...
wdt_enable(WDTO_15MS); // turn on the WatchDog and don't stroke it.
for(;;) { 
  // do nothing and wait for the eventual...
} 
...

Answer 2

In case you have the original Arduino bootloader which you want to execute as a part of the reset, you can do a SW reset by jumping to the bootloader reset address (0x7800 on ATmega328p boards)

void reset() { asm volatile ("jmp 0x7800"); }

The watchdog reset approach will not work because of a bug in the bootloader. Here's a note from ATmega328P Datasheet page 45:

Note: If the watchdog is accidentally enabled, for example by a runaway pointer or brown-out condition, the device will be reset and the watchdog timer will stay enabled. If the code is not set up to handle the watchdog, this might lead to an eternal loop of time-out resets. To avoid this situation, the application software should always clear the watchdog system reset flag (WDRF) and the WDE control bit in the initialization routine, even if the watchdog is not in use.

This is exactly what will happen after a watchdog reset on a system with Arduino bootloader.