[vidarh]

The C64 did this (pretty much). It had a rarely used realtime clock function in the CIA 6526 (Complex Interface Adapter) chips. You could power the C64 off just 5VDC for most purposes (or a voltage roughly in that region - I experimented with running mine off both a 4.5V and 9V batteries; it was surprisely robust against abuse from people like me who preferred physical experimentation to actually reading up on what'd be advisable to subject it to), but if you didn't provide an AC source on the right pin too, the realtime clock and the user port wouldn't work as they should.