The last source for generating ping frames is an external trigger. One of up to 32 different triggers can be selected. See Section 7.5.8.2.4 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_ALT1_.EXT_TRIG_EN is set to 1, the external trigger source takes precedence and the ping timer is ignored.