My girlfriend's Macbook has been suffering from a wake from sleep issue for around 12 months now. It appeared to go away and then last week with an upgrade to Leopard the issue came back even worse than before. The problem: Macbook automatically wakes from sleep with a log entry stating 'USB caused wake event (EHCI)'.

What follows are the steps I've taken to try to eliminate the problem and also what I hope is the permanent solution.

After scouring many articles, blog posts and forum topics I discovered that Apple recommend deleting some plist (preference) files to reset some of the power management functions to their defaults. The system automatically recreates these files on reboot. Another recommended approach was to reset the SMC and PMU on the Macbook. Neither of these approaches appeared to help at all. In case you want to try (it may solve your particular issue) the files to move to your desktop (or delete) are:

/Library/Preferences/SystemConfiguration/com.apple.PowerManagement.plist
and
/Library/Preferences/SystemConfiguration/com.apple.AutoWake.plist

Now there is a good chance that the com.apple.AutoWake.plist file is missing from your machine. This is not unusual as according to the developer document about the command pmset:

Files
All changes made through pmset are saved in a persistent preferences file
(per-system, not per-user) at
/Library/Preferences/SystemConfiguration/com.apple.PowerManagement.plist

Scheduled power on/off events are stored separately in
/Library/Preferences/SystemConfiguration/com.apple.AutoWake.plist

pmset modifies the same file that System Preferences Energy Saver modifies.
fies.

This made me realise that the missing AutoWake.plist file would be created if I made a scheduled sleep or wake event, which I did through the System Preferences Energy Saver panel. I then disabled the created wake event and double checked the newly created plist file, ensuring that it was empty of any settings. I then tested the sleep issue I was having and it seems to have cured it.

So what I conclude from my little trial and error experiment is that the system requires the com.apple.AutoWake.plist file to be present otherwise some sleep issues can occur. Strange but so far true.