from man update

  set x 1000
              set done 0
              after 1000 set done 1
              while {!$done} {
                  # A very silly example!
                  set x [expr {log($x) ** 2.8}]

                  # Test to see if our time-limit has been hit.  This would
                  # also give a chance for serving network sockets and, if
                  # the Tk package is loaded, updating a user interface.
                  update
              }