I never thought that I could be stopped by a datasheet (a Freescale microcontroller).
The device was always on, but poorly managed, since stand by current was demanding on battery life.
Aside from some circuit modifications, the meat of the job was in managing the stop function of the micro, where peripherals and core are stopped, allowing for a very low quiescent current.
For the purposes of this project two stop modes were available: wake on keyboard / wake on stop mode timer.
Due to original requests from customer, both modes were useful. I was thinking about:

• Put the micro in stop/wake on keyboard, lowest consumption.

• Wake from keyboard

• Perform requested operation

• Enable stop/wake from timer (every 25ms) for 3s, with higher consumption

• If no more operation are requested, then go back to stop/wake on keyboard, lowest consumption

This two handling modes allow for a rapid response from operator requests and lower consumption on the other hand.
BUT.......
If you go to MC68HC908QY4A datasheet, it is written that once you select one of the two modes after reset, you are not allowed to switch to the other stop mode.....
Thus I had to implement the following cycle :

• Put the micro in stop/wake on keyboard, lowest consumption

• Wake from keyboard

• Perform requested operation

• Wait for 3s, with very high consumption

• If no more operation are requested, then go back to stop/wake on keyboard, lowest consumption

Pity, isn't it ?