Pulse Per Second (PPS) is an out-of-band signal used in synchronized systems. 5T technology support PPS-in and PPS-out on selected devices.
PPS-out is a signal generated every time the PHC reaches 1- rounded second. If the device is working in real-time clock mode, the signal triggering is performed fully by the HW and is thus very precise.
![worddav0f651a158cb59cf9cdd2e88b1127aabc.png](https://docscontent.nvidia.com/dims4/default/7c585eb/2147483647/strip/true/crop/900x458+0+0/resize/900x458!/quality/90/?url=https%3A%2F%2Fk3-prod-nvidia-docs.s3.us-west-2.amazonaws.com%2Fbrightspot%2Fconfluence%2F00000188-48d0-d97f-a9f9-d8f0111a0000%2Fimages%2Fnetworking%2Fdownload%2Fattachments%2F89130379%2Fworddav0f651a158cb59cf9cdd2e88b1127aabc.png)
Testing Using PPS-out
The signal itself is a square wave. The following figure presents the result on the measurement equipment.
![worddavfa92f19932e49d780d7fdd55003fcf59.png](https://docscontent.nvidia.com/dims4/default/5fb4287/2147483647/strip/true/crop/900x666+0+0/resize/900x666!/quality/90/?url=https%3A%2F%2Fk3-prod-nvidia-docs.s3.us-west-2.amazonaws.com%2Fbrightspot%2Fconfluence%2F00000188-48d0-d97f-a9f9-d8f0111a0000%2Fimages%2Fnetworking%2Fdownload%2Fattachments%2F89130379%2Fworddavfa92f19932e49d780d7fdd55003fcf59.png)
PPS-out on a Scope
The green and yellow signals are generated by two different ConnectX-6 Dx devices and appear to correlate strongly with one another. If the user zooms-in and extends the timescale in the measurement equipment, they should be able to see the Time Alignment Error (TAE) between the devices:
![worddave45146a02e0d9301822a90cf3c144a3d.png](https://docscontent.nvidia.com/dims4/default/0e76c04/2147483647/strip/true/crop/900x650+0+0/resize/900x650!/quality/90/?url=https%3A%2F%2Fk3-prod-nvidia-docs.s3.us-west-2.amazonaws.com%2Fbrightspot%2Fconfluence%2F00000188-48d0-d97f-a9f9-d8f0111a0000%2Fimages%2Fnetworking%2Fdownload%2Fattachments%2F89130379%2Fworddave45146a02e0d9301822a90cf3c144a3d.png)
PPS-out Time Alignment Error (TAE) between two synchronized ConnectX-6 Dx devices
PPS-in is a signal that is pushed into the NIC from an external device. This allows the NIC to be synchronized to an external source capable of providing a PPS signal. Examples of such sources include other NIC cards, GNSS receivers or Timecards. Whenever the hardware observes a rising edge on the PPS-in input, it samples the value of the PHC. This value is then reported to software, which can, for example, implement a servo control loop to synchronize the PHC. (For PPS-In testing refer to PPS In events using testptp).
If the NIC is running in real-time clock mode, and the PPS-in is connected to the PPS-out of another PHC device, the TAE (Time Alignment Error) is very low as the following figure illustrates.
![worddav6f4244433e82d2524719a6e8606165a8.png](https://docscontent.nvidia.com/dims4/default/a942873/2147483647/strip/true/crop/900x130+0+0/resize/900x130!/quality/90/?url=https%3A%2F%2Fk3-prod-nvidia-docs.s3.us-west-2.amazonaws.com%2Fbrightspot%2Fconfluence%2F00000188-48d0-d97f-a9f9-d8f0111a0000%2Fimages%2Fnetworking%2Fdownload%2Fattachments%2F89130379%2Fworddav6f4244433e82d2524719a6e8606165a8.png)
PPS-in Events in Real-time Clock Mode
The following figure very clearly shows that the peak-to-peak time error is 20 nanoseconds, with a constant time error of approximately 33.7 nanoseconds; this assumes that the sampling represents a steady state,
As the following diagram emphasizes, this use case allows using PPS-in as the measurement (instead of the PPS-out) It is now theoretically unlimited by the number of devices that can be tested.
![worddav718c24a607fa382c7485262e30f991f1.png](https://docscontent.nvidia.com/dims4/default/a8d2140/2147483647/strip/true/crop/900x456+0+0/resize/900x456!/quality/90/?url=https%3A%2F%2Fk3-prod-nvidia-docs.s3.us-west-2.amazonaws.com%2Fbrightspot%2Fconfluence%2F00000188-48d0-d97f-a9f9-d8f0111a0000%2Fimages%2Fnetworking%2Fdownload%2Fattachments%2F89130379%2Fworddav718c24a607fa382c7485262e30f991f1.png)
PPS-in for Testing