In reply to Celine Chen:
Thanks for your help, I add these ns_task , ps_task and us_task in the tb_top , and call these task in my testcase , I get the ps ns and us delay cocrrectly.
But there is still a question I want to ask , I put “timescale 1ps/1ps” in the top of the testcase just for test ,and call the $printtimescale(path), the sim_log shows me that the timescale is still “timescale 1ns/1p”,could you tell me why ?