suppose you have an SAR ADC connected to a 4 channel analogue multiplexer connected to 4 sensors. what would be the most power efficient method to keep an eye for the readings of these sensors (note that it is a safety critical system). obviously I can't just keep reading them over and over again. 

Your starting constraint is the sample frequency you must meet, with its response,

to keep whatever safe. A PID type approach can be an efficient way of handling a control

loop, and keeping power down.

Power consumption (things you can optimize) -

1) Due to measurement process

2) Due to analysis process

3) Due to response process

Non-linear computations can speed up control loops, under the right

circumstances.

Your question a tad broad, more detail of what you are trying to do would help.

Regards, Dana.

Using interrupt driven timing it will save power just to read the 4 sensor values every 250ms or what reaction-time your system requires. In the mean-time you may shut down the unused analog sections with the appropiate _Sleep() APIs and get them working again with _Wakeup() and / or reduce the clock frequency.. If the timing is not critical you may send the whole chip to one of the power-saving modes.

Bob

 the sensors used are force, pressure and temperature sensors. these are inserted in a medical catheter to ensure the device is running properly and shuts down the device in case a reading has exceeded the threshold level. hope this clarifies things.

I haven't determined the desired frequency because I am still waiting for the components.

personally, I like the interrupt method more than the PID. but in this case is it more efficient to use intterupts?

But when you are caring only about thresholds, maybe using 4 comparators checking for them is even simpler? Or do you need to do calculation with the values you retrieve from the ADC?

Not to over sell PID, Bob is right on ISR (timed) based approach, but PID

is predicitive. You can predict need for a shutdown under correct design

constraints.

Maybe this is like trying to squeeze a drop of blood out of a stone........

Regards, Dana.

 @ hli, well you actually gave me an idea. some sensors are needed to be read, others are not the same. so thank u.

@Dana, the PID algorithem enables precise control and predicting. but this is not needed on my project, sorry Dana but I have to go with Bob suggestion on this one.

thanks all