The M3 is not well documented. The big thing to discover is the memory map to get at the IO. At least the firmware is open source to help provide a starting point and it includes some base addresses for peripherals in the system.

https://e2e.ti.com/support/arm/sitara_arm/f/791/t/284956

http://arago-project.org/git/projects/?p=am33x-cm3.git;a=sum...

http://www.ti.com/lit/ug/spruh73o/spruh73o.pdf page 1172