Countdown timer using LC3 Assembly Langugae

I want to make a timer that will countdown from a number that a user will give such as 20 min. If it is easier I could use the timer as a set amount each time. I want the timer to countdown from 20:00 down to 00:00 (min:sec). I want users to be able to pause and start the time by pressing a s for start and a p for pause. If you have any questions let me know, any help will be greatly appreciated.