Android: Widget updating a clock with custom images as numbers

What way is most efficient for updating an widget that will be a clock(Timer/Alarm), but for a clock to really works, like a clock it would be needed to update at least twice in a minute (30 sec period or less).

But that can't be effective? Have looked at some of the "threads/topics" here at StackOverFlow but haven't found any information regarding effectiveness.

(Don't own an android device yet, so I can't really test battery drai开发者_高级运维ning and cpu)

For AppWidget the docs recommend using a service running in background.