Battery backup for a clock.

Forum 2005-2010 (read only) Hardware Interfacing
1

I tried to use the search, but couldn't find anything that would give me a definitive answers.
I'm currently using the arduino as a clock and running an array of LEDs for the display.
What I'm needing is battery backup for the board, and some a way to shut the LEDs off if the main power is lost.

1st) I understand that if I use the external power plug for the main power, and add the batteries with a modified USB cable to the USB plug, it would work.
Is this correct ?

2nd) What voltage should the batteries be ?
As we are talking about short power losses, I'm thinking about coincells.
Would 3V keep the arduino running, and if not, is 6V to the USB too much ?

3rd) Now, in the event that the power is cut for a longer time, is there a command in code I could use to shut the LEDs off to save power ?

Also, I understand that this piece of code should make the arduino run at 2MHz, thus saving power ?

  TCCR2A = 0;
  TCCR2B = 0<<CS22 | 1<<CS21 | 0<<CS20;
  TIMSK2 = 1<<TOIE2;
  TCNT2=0;
2

The code you posted only changes the frequency of one of the three hardware timers, it does not save any power.

The easiest way to do what you want is to use an I2C Real Time Clock to maintain time if power is lost. In addition to the one that Richard linked, most suppliers of Arduino components sell an I2C RTC module you could use.