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Userspace Daemon & Chrony Configuration

Userspace Daemon: pru_pps_shm

How It Works

  1. Opens /dev/rpmsg_pru30: sends a setup byte to register its endpoint with the PRU
  2. /dev/mem mmap: maps PRU DRAM0 (0x4A300000) and IEP registers (0x4A32E000)
  3. Blocks on poll(): waits for the PRU to send an rpmsg notification on each PPS edge (2 s timeout)
  4. IEP-wall calibration: takes 10 back-to-back clock_gettime(CLOCK_REALTIME) + IEP read samples, keeps the tightest bracket to get the most accurate mapping between IEP ticks and wall time
  5. ns/tick estimation: uses consecutive IEP deltas (should be ~200,011,500 ticks per second at ~5 ns/tick) with a 0.9/0.1 IIR filter
  6. Projects PPS wall time: pps_wall_ns = cal_wall + (pps_iep - cal_iep) * ns_per_tick
  7. Writes NTP SHM: using mode-1 count handshake (odd while writing, even when done)

This blocking approach uses zero CPU between PPS pulses, compared to the ~20,000 wakeups/s a polling loop would require.

SHM Struct Layout

The NTP SHM struct uses 64-bit time_t (Y2038-safe on this Debian Trixie system even on 32-bit ARM):

Offset Field Notes
0 mode (int32) Set to 1
4 count (int32) Handshake: odd=writing, even=done
8 clockTimeStampSec (int64) Rounded UTC second the pulse represents
16 clockTimeStampUSec (int32) Always 0 (PPS is on-second)
24 receiveTimeStampSec (int64) Wall time when edge was detected
32 receiveTimeStampUSec (int32) Sub-second µs
36 leap (int32) 0
40 precision (int32) -29 (~2 ns)
56 clockTimeStampNSec (uint32) Always 0
60 receiveTimeStampNSec (uint32) Sub-second ns

Chrony uses clockTS - receiveTS as its raw offset sample, so receiveTimeStamp must be the best available wall-clock estimate of the actual edge time, without rounding.

Full Source & Build

gcc -O2 -Wall -o /usr/local/bin/pru_pps_shm pru_pps_shm.c -lrt -lm

Systemd Service

Create /etc/systemd/system/pru-pps-shm.service (or use the one provided: pru-pps-shm.service):

systemctl daemon-reload
systemctl enable --now pru-pps-shm

Note: remoteproc1 is PRU0 (4a334000.pru). remoteproc2 is PRU1. The ExecStartPre sleep gives the rpmsg channel time to initialize. When the daemon starts, it writes a setup byte to /dev/rpmsg_pru30 which tells the PRU the ARM endpoint addresses to use for notifications.

Chrony Configuration

Relevant section of /etc/chrony/chrony.conf:

sched_priority 80

# GPS NMEA via gpsd/SHM unit 0: used for coarse time and lock reference
refclock SHM 0 refid GPS precision 1e-1 delay 0.4 poll 2 trust noselect

# PRU PPS via SHM unit 2: primary disciplining source
refclock SHM 2 refid PPS precision 1e-9 poll 1 prefer trust lock GPS pps

makestep 0.5 -1
maxupdateskew 100.0
rtcsync
rtcdevice /dev/rtc0

lock GPS tells Chrony to only use the PPS refclock when the GPS SHM refclock (unit 0) is also valid. This is essential for correct UTC second assignment. pps enables the PPS refclock mode.

PHC Discipline (phc2sys)

Since the PRU driver captures the PPS sequence to discipline CLOCK_REALTIME via Chrony, the PTP hardware clock (e.g., the AM335x CPTS at /dev/ptp0) must be synchronized in software from the system clock.

This is accomplished using the standard phc2sys daemon from the linuxptp package:

# Sync the CPTS PHC (/dev/ptp0) from the system clock (CLOCK_REALTIME)
phc2sys -s CLOCK_REALTIME -c /dev/ptp0 -w

By running this as a systemd service along with ptp4l, your CPTS hardware clock will tightly track the PRU-disciplined system clock, enabling your device to act as an accurate PTP Grandmaster on your network.

Systemd Service Examples

You can deploy phc2sys and ptp4l using the following systemd units.

1. /etc/systemd/system/phc2sys.service

[Unit]
Description=Synchronize PTP hardware clock (PHC) to system clock
After=chronyd.service network.target ptp4l.service
Wants=ptp4l.service

[Service]
Type=simple
# -s: source clock (system clock)
# -c: destination clock (CPTS hardware clock)
# -w: wait for ptp4l to start
ExecStart=/usr/sbin/phc2sys -s CLOCK_REALTIME -c /dev/ptp0 -w
Restart=on-failure
RestartSec=5

[Install]
WantedBy=multi-user.target

2. /etc/systemd/system/ptp4l.service

[Unit]
Description=Precision Time Protocol (PTP) Grandmaster service
After=network.target

[Service]
Type=simple
# -i eth0: bind to your network interface
# -m: print messages to stdout/syslog
ExecStart=/usr/sbin/ptp4l -i eth0 -m
Restart=on-failure
RestartSec=5

[Install]
WantedBy=multi-user.target

Next: Verification & Troubleshooting | Previous: PRU Firmware