You have a valid point that the needed pins are not broken out on a Mega; I did not check that when I posted. However, each U(S)ART has an associated XCKn pin for clock and the datasheet specifically mentions synchronous operation.
You have a valid point that the needed pins are not broken out on a Mega; I did not check that when I posted. However, each U(S)ART has an associated XCKn pin for clock and the datasheet specifically mentions synchronous operation.