The last source for generating ping frames is an
external trigger. One of up to 32 different triggers can be selected. See
Section 25.2.6 for the list of input sources.
CAUTION: Ping frames can be triggered by
both an external trigger source and the internal ping timer. If
TX_PING_CTRL.EXT_TRIG_EN is set to 1, the external trigger source takes
precedence and the ping timer is ignored.