Your offense meets the exception from your posting. You don't need a waiver. Waivers are needed only when you don't meet the exception. But if you are going to the consulate for a visa, have a copy of TN law on the maximum sentence. Let the consul know about your meeting the exception.............Sometimes some consuls can be biased due to the record and deny your visa........it rarely happens.Legally you should not be denied based on that since you qualify under the exception.......they can however come up with a different reason to deny you.