In reply to tejasakulu:
Without knowing the details of the error, I am guessing that the
always @() has not been trigerred even once. Remember the always @()
will be evaluated only when dll_speed_mode goes through the first
transition. Till then the real values MIN and MAX_FREQ will be 0.
After the first transition to 2’b00, then the MIN and MAX_FREQ should be
100 and 180.