Emulation of 'System OFF' mode in ARM nRF51 SoCs

[Chris Laplante]

Hi all,

[I also posted this to qemu-devel but should have CC'd here too]
 
I am working on improving nRF51 emulation. Specifically I want to implement the 
special "System OFF" mode. System OFF is a power saving mode. In this mode, the 
system can only be woken up by a reset or a handful of peripherals (most 
notably, GPIO via high/low level detection on configured pins). System reset is 
triggered upon wakeup.
 
I've been digging into the QEMU mailing list and source code and have come to 
the conclusion that deep sleep and low power modes are not implemented. There 
seems to be support for turning off ARM CPU cores, e.g. as used by imx6_src.c. 
But that doesn't apply here because I only have one CPU.
 
So ultimately what I think I will try to implement is what the nRF51 reference 
manual calls "Emulated System OFF mode". From the reference manual:

If the device is in debug interface mode, System OFF will be emulated to secure
that all required resources needed for debugging are available during System 
OFF...
Since the CPU is kept on in emulated System OFF mode, it is recommended
to add an infinite loop directly after entering System OFF, to prevent the CPU 
from
executing code that normally should not be executed.
 
Does anyone have any guidance on how to implement this? I don't particularly 
care about fidelity. As long as a GPIO level trigger can break the CPU out of 
the infinite loop (which the reference manual tells users to add) and jump into 
the reset vector, it will be good enough for my use. I don't really care about 
masking out other interrupt sources, for example.

Thanks,
Chris