The reference clock output mode can be accessed via one of the two software primitives: OBUFDS_GTE5 and OBUFDS_GTE5_ADV. The choice of the primitive depends on your application. Use OBUFDS_GTE5 when the CH*_RXRECCLKOUT[0/1] is always derived from the same channel. Use OBUFDS_GTE5_ADV if the channel providing CH*_RXRECCLKOUT[0/1] can change during runtime.