rtel wrote on Wednesday, April 15, 2009:
Using an RTOS can save current because you can design the system to be completely event driven - removing any CPU cycles that would otherwise be wasted by polling. When an event is not being processed the idle task will run and immediately put the processor into a power down state.
Using an RTOS can cost current because it needs a periodic tick to work. Lots of aps have a periodic interrupt anyway so if that is the case then it makes no difference.
As richard_damon says, setting the tick to the slowest possible is key. We have in the past played around with slowing the clock down before placing the processor into sleep mode too. To do this you have to increment more than one tick each time the tick interrupt occurs. For example, if you half the clock speed then increment the tick twice on each tick interrupt (while the clock remains slower). If you quarter the clock speed then increment the tick four times on each tick interrupt (while the clock remains slower). Etc. Once an event occurs and there is processing to do set the clock back to its full speed, assuming the processing needs to be done quickly that is.