Hi Leemin,

It's a great idea to have the micro do as much as possible but sometimes it's necessary to have external hardware do some work to free up the micro to do other things more important. Add a gated pulsing circuit that you only need the micro to turn on and off. This can be a single chip circuit like using a CD4093 or even a CD4060. A CD4060 lets you choose flash rates juct by selecting the appropriate output. These are simple circuits and only require a few discretes to finish the job. Let the micro do the more serious stuff.

BobK