在我用jk触发器，组成计数器（用schematic原理图构建）时总出现下列错误，清高手指点。

# ** Warning: (vlib-34) Library already exists at "work".
# vsim -lib work -t 1ps testjk
# ** Warning: (vsim-3479) Time unit 'fs' is less than the simulator resolution (1ps).
# ** Error: (vsim-3601) Iteration limit reached at time 50 ns.
# ** Note: (vsim-3602) Delays were truncated during elaboration of the design.
# Executing ONERROR command at macro ./testjk.ado line 13