Feature: Periodic alarms between 1 sec and 1 minute

[sslupsky]

The smallest period with the current version is 1 minute. It would be useful to be able to set periodic alarms that fire more frequently.

Perhaps something like MATCH_MS (match milliseconds) ? That would provide a 1 second period. I do not require anything fast than that.

It would be useful to be able to set the period to an arbitrary period between 1 and 59 seconds.

[sslupsky]

I would like to submit a PR for this issue so I would like to get some feedback. I see two ways to accomplish a better than 1 minute periodic interrupt.

1. Set a new ALARM timeout in the RTC hander.
2. Use the Periodic Events

The first method is the technique I presently use. It has the disadvantage that it requires a write to the alarm register. This write causes a SYNCBUSY and requires about 6ms to complete. This is a concern when trying to save power.

The second method involves the Periodic Events generated from the RTC Prescaler. I think the SYNCBUSY delay can be avoided.

Are there any other ways to accomplish this? Any other feedback?

[ravelab]

If SYNCBUSY delay can be avoided, that would be great.