we always meet this phynomenon "this pin is already assigned when we pin the pins"

题目

we always meet this phynomenon "this pin is already assigned when we pin the pins"
however ,when we found a new project this error lost.
i want to know why this happend to me sometimes.
if you knew ,please tell me

答案

in the project there are many pins your defined,and you need to assign a pin number(pin of the CPLD or FPGA) to the pin you had defined.But you should not assign the same pin number to the different pin which you defined yourself.