Hi yahoo2016,
Sorry for not able to respond earlier as this is not a straightforward answer. Bottom line is the hardware support for PPS signal is only available in the chip after Jetson TX1. To achieve similar result, one can use the approach to input the same signal, say GPS PPS, to both devices to synchronize the clock. This is interrupt based and there is latency issue to resolve. For that, we suggest using TSC (TRM section 8.2) due to its nature of oscillator granularity. There is a ktime_get_ts() kernel call, however it depends on your latency needs.