I have moved to Attiny1604 for which I have downloaded megaTinyCore and I am able to compile the example projects.
However, to port my Atmega328 based application to the new megaTinyCore is proving to be quite a challenge. Can somebody please point me to APIs for watchdog, sleep, interrupt, uart and few tutorials, if available for the new megaTinyCore?
megaTinyCore provides a Serial object with the normal functions - so UART should be absolutely standard unless you're taking it over for special configuration - in which case (as always) your resource is the datasheet.
I have long planned to write a library for sleep and power management, but I need to get ATTinyCore in order first because I've been neglecting that core for the past 6 months. There is basic sleep info in extras. But I don't really even have much experience with it... because I haven't written that library yet. The main selling point of it will be the ability to seemlessly keep time using the RTC while in standby sleep pulling ~1uA, which is now quite practical.
Watchdog can only be used for reset on the new parts (and it also doesn't do that obnoxious thing where it would start up in the shortest timeout after a watchdog reset), if you want periodic interrupts, you want the RTC/PIT (there's some guidance on that in second link - it's much more powerful than the WDT ever was, especially in standby sleep (as opposed to power down - standby has extremely low power consumption, but lets you leave peripherals enabled if you want to; it enables some crazy functionality
"can you be more specific?"
Like - a while back I solicited suggestions for docs and didn't hear anything, and your request above is so general that it's not actionable.