Merge pull request #491 from c1728p9/lp_ticker_info

Add synchronization delay section in lp ticker

Author: Amanda Butler

docs/reference/contributing/target/lp_ticker.md

@@ -36,6 +36,12 @@ Be careful around these common trouble areas when implementing this API:
 - The ticker keeps counting when it rolls over
 - The ticker interrupt fires when the compare value is set to 0 and and overflow occurs
 
+#### Handling synchronization delay
+
+Some low power tickers require multiple low power clock cycles for the compare value that `ticker_set_interrupt` sets to take effect. Further complicating this issue, a new compare value typically cannot be set until the first has taken effect. Because of this, when you make back-to-back calls to `ticker_set_interrupt` without a delay, the second call blocks and violates the above requirement that `ticker_set_interrupt` completes in 20us.
+
+To meet this timing requirement, targets that have this synchronization delay must set `LOWPOWERTIMER_DELAY_TICKS` to the number of low power clock cycles it takes for a call to `ticker_set_interrupt` to take effect. When the targets set this value, the timer code prevents `lp_ticker_set_interrupt` from being called twice within that number of clock cycles. It does this by using the microsecond time to schedule the write to happen at a future date.
+
 ### Dependencies
 
 Hardware low power ticker capabilities.