On the 40D (which is vxworks) what looks like sleeptask (at FFD44314 based on the string "SleepTask : call from interrupt handler") appears to be called as the second to last instruction in a function (starting at FF812CB0) that refers to strings like "[HotPlug] USB Connect ->->->-> O", "[Sw Detect] JUMP SW ON", "[Sw Detect] Detect Secret sw." (ooh secret switch ?).
The last instruction is branch back to near the start of the function, i.e. the main loop.
Maybe some of this code caries over the later DSLRs.
Note, when search for strings, use "search for sequence of bytes", not just searching the strings window. This will find strings that aren't correctly found by ida.