Check duty cycle of a clock

 High time check:

@(posedge clk) (1, start_t = $realtime) |-> @(negedge clk) ($realtime - start_t == half_period);


Low time check:
@(negedge clk) (1, start_t = $realtime) |-> @(posedge clk) ($realtime - start_t == half_period);