I will not need a like a calendar time ... but just to have the millis between last 2 pulses.
but just to have the millis between last 2 pulses.
Is there nowhere near an electric meter that you can plug a USB charger in?
Why? Don't you just want to count the number of pulses over 5 minutes? If you want to know the number of watts, who cares when the pulse occurred, to the nearest millisecond?
If that's what you truly want to do, look into msp430: their crystal oscillator is "instant on". on that.
how much ampere an external RTC can use ?