network:gps []

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1. Choosing the Right Raspberry Pi

For a PTP (Precision Time Protocol) server, the Ethernet controller is the most important component.

Best Choice: Raspberry Pi 4 or Raspberry Pi 5. These models have Ethernet controllers (BCM54213PE) that support Hardware Timestamping, which is essential for PTP to achieve sub-microsecond accuracy.
Avoid: Pi 3 or Zero. These use USB-to-Ethernet bridges or lack hardware timestamping support, which forces you to use “Software PTP,” significantly degrading accuracy.

2. Hardware Connections

You must use a Logic Level Shifter (or a simple resistor voltage divider) to drop the QLG2’s 5V signals down to 3.3V.

QLG2 Pin	Connection to Raspberry Pi	Note
GND	GND (e.g., Pin 6)	Common ground is required.
TX	RXD (GPIO 15 / Pin 10)	Must be shifted to 3.3V.
1 PPS	GPIO 18 (Pin 12)	Must be shifted to 3.3V.
5V	5V (Pin 2 or 4)	To power the QLG2.

Set the QLG2 to 2.8V PPS voltage level: * UPPER: 5V logic level (DEFAULT) * LOWER: 2.8V logic level SER voltage level (GNSS module serial data output, which is the TXD pin on the 4-pin header at the right of the board): * UPPER: 5V logic level (DEFAULT) * LOWER: 2.8V logic level

3. Software Configuration

Step A: Enable Serial and PPS

Edit your boot configuration file: sudo nano /boot/firmware/config.txt (or /boot/config.txt on older OS versions).

Add these lines:

# Enable the hardware UART
enable_uart=1
# Define the PPS pin
dtoverlay=pps-gpio,gpiopin=18

Then, disable the serial console (so the OS doesn’t try to use the GPS data as a login terminal):

sudo raspi-config → Interface Options → Serial Port → No (Login Shell) / Yes (Hardware enabled).

Step B: Install Time Tools

sudo apt update
sudo apt install pps-tools chrony linuxptp

PPS Check: Run sudo ppstest /dev/pps0. You should see “source fetch sampled” messages once per second.
GPS Check: Run cat /dev/ttyS0 (or ttyAMA0) to see the NMEA sentences ($GPRMC, etc.).

Step C: Configure Chrony (The Grandmaster Clock)

PTP usually gets its time from the System Clock, which should be disciplined by GPS/PPS via chrony.

Edit /etc/chrony/chrony.conf:

refclock SHM 0 offset 0.1 delay 0.2 refid GPS
refclock PPS /dev/pps0 lock GPS refid PPS

Step D: Configure PTP (linuxptp)

PTP on Linux uses ptp4l to manage the hardware clock and phc2sys to sync the System Clock (disciplined by your GPS) to the Ethernet controller’s PTP Hardware Clock (PHC).

<HTML><ol style=“list-style-type: decimal;”></HTML> <HTML><li></HTML><HTML></HTML>Sync System Clock to PHC:<HTML></HTML> <HTML></HTML>sudo phc2sys -s CLOCK_REALTIME -c eth0 -w &<HTML></HTML><HTML></li></HTML> <HTML><li></HTML><HTML></HTML>Run PTP4L:<HTML></HTML> <HTML></HTML>sudo ptp4l -i eth0 -m -S (The -S is for software-assisted hardware timestamping).<HTML></HTML><HTML></li></HTML><HTML></ol></HTML>

To turn your Raspberry Pi 4 into a PTP Grandmaster, you need a configuration that tells ptp4l two things: “I am the boss” and “Never listen to anyone else’s clock.”

Since the Pi 4 supports hardware timestamping, we will leverage that for maximum precision.

The ptp4l.conf File

Create or edit the file (usually located at /etc/linuxptp/ptp4l.conf):

Ini, TOML

[global]
# --- Hardware Settings ---
# Use hardware timestamping (Pi 4 supports this on eth0)
time_stamping          hardware
# Two-step is standard for the Pi's Ethernet controller
twoStepFlag            1

# --- Grandmaster Role ---
# Priority 1: Lower values win the Best Master Clock Algorithm (BMCA)
# Default is 128. Setting it to 127 or lower makes it likely to be Master.
priority1              127
priority2              128
domainNumber           0
# We are NOT a slave-only device
slaveOnly              0
# We ARE capable of being a Grandmaster
gmCapable              1

# --- Network Transport ---
network_transport      UDPv4
delay_mechanism        E2E
# How often to send Sync messages (2^0 = 1 second)
logAnnounceInterval    1
logSyncInterval        0

# --- Clock Discipline ---
# Use the Pi's Proportional-Integral servo
clockServo             pi
# Point to the local PTP Hardware Clock (PHC)
# ptp4l will find this automatically based on the interface

How to Run It

To start the service using this configuration on your Ethernet interface:

Bash

sudo ptp4l -f /etc/linuxptp/ptp4l.conf -i eth0 -m

The “Secret Sauce”: phc2sys

Running ptp4l alone only manages the clock inside your Ethernet chip. It doesn’t know about the GPS time yet. You must use phc2sys to “push” the high-accuracy time from your System Clock (which Chrony is fixing with the QLG2) into the Ethernet Hardware Clock (PHC).

Run this in a separate terminal or as a service: