Entering sleep mode on arm cortex m4

Question

I'm trying to put a cortex m4 processor to sleep for a little less than a second. I want to be able to tell it to sleep, then a second later, or when a button is pressed, pick up right where I left off. I've looked in the reference manual and VLPS mode looks like it would fit my needs. I don't know how to begin to enter that mode or how to program the NVIC.

More Info: I am doing this in C, on the bare metal.

Answer 1

You can download and inspect the code that implements this demo. Although the demo is for an RTOS the code used to place the CPU into a sleep mode is the same whether an RTOS is being used or the application is running on bare metal.

There are generic things you can do to place a Cortex-M3 core into a low power state (see the WFI instruction). To get extreme low power then you have to do chip specific things as well. The above linked code performs some chip specific pre-sleep processing (turn of peripherals, set the chips own sleep mode, etc.) before calling WFI, then does some chip specific things when it returns from the WFI instruction.

Answer 2

You don't need a RTOS in order to wake up from sleep a Cortex M4, what you need is to use and interrupt (ISR) you should refer to the manufacturer manual, you may wake up with a timer(ISR) or a button(GPIO) depending of the sleep-hibernation modes of your particular chip. Here is a ARM document more in depth about it.

http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0553a/BABGGICD.html