Timer/Counter questions

I have a couple questions I hope someone can answer about Timers. I have tried researching, but due to my inexperience I lack the terminology to answer some of my problems so please bear with me in this post.

I am trying to use timers to call an interrupt every hour or so to wake my Arduino Mega 2560 out of power save mode (or perhaps even power down mode). Using timer1 I have managed to create the code to do this, but as timer1 is only 16 bits it will only last 4 seconds, not anywhere near what I need. Thus leading to my question.

First, is it possible to link multiple timers with each other (internally). What I mean by this is that when timer1 overflows could I have timer2 or timer3 increment by 1? And, if that is the case can I instead of using a sleep function just manipulate the register so I sleep the cpu and all other modules while leaving just those two timers running until it interrupts?

If I can't link them internally how about externally? Would I have to have the external counter increment the internal one or would it be the other way around? If it still was slightly short of the time required could I 'terrace' these counters so that every time one overflowed it would increment the next one? Would a real-time clock work? If either of these (external counters/real-timer clock) would work do you have any recommendations about which brand?

Finally if you have any additional suggestions I would be glad to hear them. Thank you for your time.

Don't worry about the fact that it wakes every 4 seconds for a few microseconds. It will still be asleep for the vast majority of the time.

...R

At the same time, the Atmega16U2, 5V regulator, 3.3V regulator, power LED, and autopower switching comparator are all still powered , so the savings might not be as much as you expect.

Can always go for a minimal 2560 design if you don't need a lot of that, powered from 3xAA or 4xNiMH battery:
http://www.crossroadsfencing.com/BobuinoRev17/



Or a 2560V processor if you can run at a lower speed/voltage:
ATmega2560V:
0 - 2MHz @ 1.8V - 5.5V, 0 - 8MHz @ 2.7V - 5.5V