I have an MSP430 waking up every millisecond from a timer interrupt (to do a tiny amount of work, ie increment a counter and check a couple of input pins). It is set up to run at 25MHz. What should I expect as the total power consumption? How would one calculate that from values in the spec sheet?
(The specs for this exact part say 195uA/MHz running from flash and 2.1uA in LPM3, plus "fast wakeup" I think 6us from LPM3)
The instruction time is 1/25M = 40ns. Assuming that your interrupt routine is around 20 instructions (it's probably less from the sounds of it), then the routine will last 0.8us.
Assume that the active mode current is drawn for 6us after the ISR has finished, at which point it instantly changes to 2.1uA (for simplification).
In 1ms, the routine will be firing for 6.8us (in active mode) and LPM3 will be on for 993.2us.
If the values you quoted are accurate, then assuming your micro has a 3V supply, 99.32% of the time it will consume 3 x 2.1u = 6.3uW.
For the rest of the time (0.68%), it will consume around 25 x 195u x 3 = 14.6mW.
Since you have the power consumption and the relative times, you can work out the average power.
