PPS over USB

Goal

For keeping your system clock on time, you can't beat the Pulse Per Second (PPS) signals available from $10 GPS modules. They typically have better than 100 nanosecond accuracy. GPIO PPS is one of the best ways to get that timing information into your system, but the hardware needed to use a GPIO PPS is not available on every platform. USB is available on almost every server, but by itself adds unpredictable latency to PPS timing information. This project aims to measure and compensate for USB latency to make it a decent PPS timing source for servers that can't use GPIO PPS sources.

Diagram of USB PPS

Notes

  • The same PPS signal is connected to both the GPIO PPS and USB PPS
  • GPIO PPS does not compensate for PPS to IRQ latency (expected values ~10us)

PPS to IRQ data

This is the top part in the diagram: the time between when the USB device gets the PPS and when its interrupt handler is run.

PPS to IRQ time

This ended up being around 1.7us-1.8us

USB latency

This is the middle part of the diagram: the time between when the USB device buffers the PPS message and when the USB host picks it up on the next poll.

The USB Device's control interface bInterval is set to 1 (=1000us). This tells the USB host to poll the device's status every USB Frame. The majority of USB latency comes from this polling.

USB latency pattern

The USB latency varies from 26us-1033us. I'll go more into the triangle shapped pattern later.

Measurement message

The measurement message is a simple text message sent as regular UART data.

Putting all this data together

After the host gets the measurement message, it has all the info it needs to calculate the difference between its time and the GPS's time. This difference is also called the offset. Below is a graph comparing the GPIO PPS and the USB PPS time sources. In an ideal world, these would both be flat lines at 0. Things are not ideal however. The USB PPS has a ~125us static offset which is possibly from the USB hub buffering the data. Static offsets are easy to deal with, so I removed it for this graph. The static offset was the same between two different USB hosts I tested with: a Raspberry Pi 2 and an Intel machine.

GPIO PPS vs USB PPS

The USB PPS time source stayed within +/-5us, so it's good for long term stability. The USB PPS source has a lot steeper and longer changes compared to the GPIO PPS, which is bad for short term stability.

Graphing this data as a histogram of offsets, I would expect it to look somewhat like a Gaussian distribution.

histogram

The GPIO PPS is much closer to this than the USB PPS. This is another way to measure short term stability.

Extra: USB latency pattern

Looking closer at the USB latency, you can see the PPS drifting relative to the host schedule of polling the USB device for its status. The system clock error was 2.215ppm during this time period, and this drift matches that error exactly. This probably means USB on this system shares the same clock as the system clock. This hardware is a Raspberry Pi 2, and I suspect it won't be true for other platforms. I also suspect the stability of the polling schedule can't be relied upon, as the host controls it and can reschedule it whenever it wants.

PPS drifting

Subtracting the 2.215ppm drift gives the remaining jitter (root mean squared error=2.278).

RMSE

Extra: a breakdown of the pieces needed

Extra: related posts

UART/USB bridge timing, Old related USB PPS, Older related project from 2013