I am making a timer (mm:ss) with a 4 74169 counters(required). So far I have the a timer that is able to count down from (59:59) as the values of the data pins are preset to the respective values. I am struggling to implement further logic which will allow a user to select a value such as (35:02) and for the counter to count down from that value (35:02) to (35:01) to (35:00) to (34:59) as the data inputs are set to a certain value and once the counter reaches 0 it will load the value that is on the pins. I am looking for a solution for this problem that lets me take the user's input, count down from that cycle to 0 and reload a value of say 9 for the 1's seconds column. I have provided a schematic of the circuit so far and any insight or suggestions would be greatly appreciated. Any sort of primitive logic is fair game as well as any off the shelf IC's.

 

Why is it required? Is that a constraint of the homework assignment?

 

Why is it required? Is that a constraint of the homework assignment?

Yes it is a required part of the assignment.