PRU PPS: a nanosecond-precision GPS Stratum 1 clock on the BeagleBone Black

Writeup: BeagleBone PRU PPS timestamping for precise GPS time

Turn a BeagleBone Black and a GPS module with a PPS output into a Stratum 1 NTP server whose time is disciplined to nanoseconds, by capturing the GPS pulse-per-second edge in PRU hardware instead of through a Linux GPIO interrupt. The PRU-ICSS latches every PPS edge with the 200 MHz IEP timer, a small real-time daemon feeds it to chrony over shared memory, and the result is a self-hosted GPS reference clock with none of the interrupt-latency jitter that limits the usual pps-gpio approach.

It is also independently cross-validated. Measured over hardware-timestamped PTP against a separate Intel i210 plus u-blox NEO-M9N grandmaster, this clock agrees with a second, fully independent GPS reference to within a couple of microseconds. See CROSS-VALIDATION.md for the methodology and how to reproduce it.

Performance Results

By entirely bypassing the Linux GPIO IRQ pathway and capturing the pulse edge using the 200 MHz PRU Industrial Ethernet Peripheral (IEP) hardware timer, this implementation eliminates traditional interrupt latency jitter.

• Edge capture: pulse-to-pulse latch jitter of ~40 ns RMS. The PRU services rpmsg only in the shadow of a pulse, so nothing interrupts the edge-detect loop.
• IEP-to-wall transfer: the daemon fits a least-squares line through a window of timer/wall sample pairs centered on each pulse, with residuals of ~8 ns RMS, instead of projecting through a single sandwich read (the old method's 100 to 800 ns).
• Sawtooth correction: the receiver's per-pulse quantization error (UBX-TIM-TP qErr, up to ±10 ns on an M8N) is matched to its pulse by GPS week/tow and subtracted, so the largest remaining deterministic error term is gone.
• System Clock Offset: Once the Chrony servo converges, chronyc tracking reports system time offsets in the low nanosecond domain (often under ±10 ns).
• Jitter Profile: chronyc sourcestats standard deviation sits in single-digit nanoseconds, orders of magnitude better than standard pps-gpio, which routinely shows 5 to 20 µs of offset dispersion.

Detailed tracking data and logs can be found in the Measurements & Validation document.

Cross-validation against a PTP grandmaster

Precision is not the same as accuracy, so the clock is checked against a second, independent GPS reference. Using the BeagleBone's idle CPTS Ethernet timestamp clock as a measuring instrument (disciplined to this box's GPS, while ptp4l runs free-running and measures only), the system is compared over hardware-timestamped PTP to an Intel i210 plus u-blox NEO-M9N grandmaster, whose own GPS PPS is hardware-captured on an i210 SDP pin (extts) and disciplined into the NIC PHC by ts2phc. Neither clock is disciplined to the other; each keeps its own GPS.

The two independent GPS clocks agree to within about 1 to 2 µs (median). Since the grandmaster is SDP hardware-timestamped and nanosecond-class, that floor is set by the AM335x cpsw timestamping, not by the clocks, which is precisely why this project drives the real clock from the PRU and not from cpsw. Full methodology and a reproduction recipe are in CROSS-VALIDATION.md.

Documentation

1. Architecture & Overview: What is a PRU, why use it for PPS, and the system architecture
2. Hardware Requirements: Supported hardware, GPS modules, and prerequisites
3. Initial Setup & Overlays: uEnv.txt configuration and device tree overlays
4. PRU Firmware: Compiling and installing the PRU microcontroller firmware
5. Userspace Daemon & Chrony: Building the shared memory daemon and configuring Chrony
6. Verification & Troubleshooting: Checking system behavior, remoteproc state, and common issues

Deep Dives

7. Timestamp Model: What exactly is captured, which edge, what defines t=0
8. Clock Domains & Synchronization: IEP to wall correlation, memory mechanism, ordering guarantees
9. Measurements & Validation: Expected performance, real-world tracking/statistics, and diagnostic guidance

Source Code

• Firmware (firmware/): PRU0 C source, resource table, INTC map, linker command
• Daemon (daemon/): Userspace SHM bridge, RT priority tuning, and systemd services
• Overlays (overlays/): Device tree overlays for pinmux and rpmsg
• Tools (tools/): Log parsing and statistics helpers
• Measurement (measurement/): phc2sys service and agreement script for the PTP cross-validation (CROSS-VALIDATION.md)