I am making a small, mobile device that depends on batteries. It will only require a high CPU frequency occasionally and is at a low frequency the rest of the time. The default frequency is 700MHz. Would my battery last 10 times longer if I set the frequency to 70MHz?
No, You will reduce the consumption, but not exactly factor 10 (it can be less or even more). There are at least two things I'm aware of which consume independently of the frequency of a component:
- Leakage: Current lost because of "inefficiency" of components; this is where the heat is coming from. And leakage is logarithmic. http://en.wikipedia.org/wiki/Leakage_(electronics)
- Periphery: Some parts of a clock-driven device (e.g a CPU) can be driven with a clock derived from the main clock thus different and not necessarily directly proportional. For example: the memory (RAM) will not be clocked 10 times less because you divided the clock of the CPU by ten. Another example is the host-interface.
There are a lot of more details to check to get the full picture of this problem.