MySQL 'Update Timestamp' Column - Trigger

I'm adding a column tsu (timestamp update) of type DATETIME to a number of my tables.

I need to write BEFORE UPDATE triggers that will update the column to CURRENT_TIMESTAMP(), but I can't get it right. Tried:

DELIMITER $$
CREATE
    TRIGGER `cams`.`tsu_update_csi` BEFORE UPDATE
    ON `cams`.`csi`
    FOR EACH ROW BEGIN
        UPDATE csi SET tsu = CURRENT_TIMESTAMP WHERE csi_code = OLD.csi_code;
    END$$
DELIMITER ;

Can anyone point me in the right direction pls? MTIA