Tenn. Code Ann. § 48-18-107: Resignation of directors

(a) A director may resign at any time by delivering a written resignation to the board of directors, or its chair, or to the secretary of the corporation.

(b) A resignation is effective when the resignation is delivered unless the resignation specifies a later effective date or an effective date determined upon the happening of an event or events. A resignation that is conditioned upon failing to receive a specified vote for election as a director may provide that it is irrevocable.