How can you reduce the clock frequency of an Arduino Mega?

Question

I'm trying to debug a problem and need to reduce the speed of the clock on my Arduino Mega from 16MHz to 8MHz.

I can't find any simple way of doing so, so I wanted to know if there were any gurus here who knew if this was possible, as well as how to do it.

WITHOUT PRESCALER:

long int runTime;
void setup()
{
 Serial.begin(9600); //Setting the data transfer rate 
}
void loop()
{ 
 delay(1000);
 runTime = millis();
 Serial.print("Runetime: ");
 Serial.println(runTime);
 delay(100);
 exit(0);
}
OUTPUT: 999

WITH PRESCALER:

long int runTime;
void setup()
{
 Serial.begin(19200); //Setting the data transfer rate for when CLK Freq. is Halfed
 CLKPR = _BV(CLKPCE); // enable change of the clock prescaler
 CLKPR = _BV(CLKPS0); // divide frequency by 2
}
void loop()
{ 
 delay(1000);
 runTime = millis();
 Serial.print("Runetime: ");
 Serial.println(runTime);
 delay(100);
 exit(0);
}
OUTPUT: 999

• Edgar Bonet: In your test code, you should put the changing of the clock prescaler between noInterrupts() and interrupts(). Otherwise there is a small possibility of this failing. Also, your test only proves the clocks of delay() and millis() have changed consistently. Which is to be expected since it's the very same clock.
• Isabel Alphonse: So how can I test if the clock is in fact running at 8MHz instead of 16MHz?
• Edgar Bonet: The only way to test a clock is to compare it with another clock. This other clock could be your own sense of time (that's how the test in my answer works), or the UART at the other end of the serial connection (that's how you own program, with 19200 bps nominal and 9600 bps actual baud rate, is actually a valid test). High end solution: hook a GPS with a 1PPS output to your Arduino. Low end solution: program the watchdog, which has its own independent clock, to generate periodic interrupts.

Answer 1

You can set the clock prescaler for that:

void setup() {
 noInterrupts();
 CLKPR = _BV(CLKPCE); // enable change of the clock prescaler
 CLKPR = _BV(CLKPS0); // divide frequency by 2
 interrupts();
}

This is explained in section 10.12 and 10.13 of the ATmega2560 datasheet.

Of course, changing the clock frequency will mess with the time-related function (millis(), delay() and co.) and the baud rate of the serial port.

Edit: Here is a small program to demonstrate the slowing of the clock:

//#define SLOW_CLOCK
void setup()
{
#ifdef SLOW_CLOCK
 noInterrupts();
 CLKPR = _BV(CLKPCE); // enable change of the clock prescaler
 CLKPR = _BV(CLKPS0); // divide frequency by 2
 interrupts();
#endif
 pinMode(LED_BUILTIN, OUTPUT);
}
void loop()
{
 digitalWrite(LED_BUILTIN, HIGH);
 delay(500);
 digitalWrite(LED_BUILTIN, LOW);
 delay(500);
}

This makes the LED flash at 1 Hz. If you uncomment the line #define SLOW_CLOCK, it instead flashes at 0.5 Hz.

• Isabel Alphonse: So when I added that I got a bunch of random symbols (i.e. †f憞f) and I'm not sure why. Are the setting different for an Arduino Mega?
• Gerben: Note that delay(1000); will now take 2 seconds, instead of 1.
• Gerben: You should also half the baud rate of your serial console, or double the baudrate in Serial.begin().
• Isabel Alphonse: I wrote some test code and there didn't seem to be any change. Is this what @EdgarBonet was talking about? Am I not supposed to see a difference in the output of millis()? is so then is there another way for me to test if it's working.
• Edgar Bonet: @IsabelAlphonse: What test did you do? The wacky symbols on the serial port kind of prove that the baud rate was unintentionally changed.