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Raspberry Pi Zero W

DietPi is selected for this device due to its lightweight footprint, which is ideal for the limited resources (single-core CPU, 512MB RAM) of the Zero W.

⚙ Config

OS: DietPi

Hostname: pi00

IP: 192.168.2.190

MAC: b8:27:eb:1c:98:df

📡 Pi Zero W NTP Node

This document details the physical build, bill of materials, and hardware-level kernel configurations for the Stratum 1 NTP server node. For the software configuration (including chrony and thermal compensation), see chrony.

📦 Bill of Materials (BOM)

  • SBC: Raspberry Pi Zero W (v1)
  • Network: USB-to-Ethernet Adapter (Micro-USB OTG)
  • Time Receiver: Waveshare MAX-M8Q GNSS HAT
  • Antenna: Active GPS Antenna with SMA connector
  • Storage: High-endurance microSD card (for constant log writes)
  • Power: 5V 2.5A Micro-USB Power Supply

🛠 Assembly & Layout

Because the Pi Zero W lacks an onboard ethernet port, the USB-to-Ethernet adapter is required to eliminate Wi-Fi jitter, bringing network delivery accuracy down to the 50–200 µs range.

The active GPS antenna requires a clear view of the sky. Route the SMA cable from the MAX-M8Q HAT to a window sill. Avoid placing the antenna near heavy electromagnetic interference (e.g., directly on top of the UniFi Cloud Gateway or Proxmox nodes).

🔌 GPIO Pinout Mapping

The MAX-M8Q HAT interfaces with the Pi Zero W via the 40-pin GPIO header using two distinct communication channels:

Signal Pi GPIO Pin Purpose
TXD GPIO 14 (Pin 8) NMEA serial data (UART transmit)
RXD GPIO 15 (Pin 10) NMEA serial data (UART receive)
PPS GPIO 4 (Pin 7) Pulse Per Second hardware interrupt
3V⅗V Pins 1, 2, 4 Power delivery to the HAT
GND Pins 6, 9, 14 Ground

⚙ Base OS & Kernel Prep

Before chrony can read the hardware signals, the base OS must be configured to expose the UART serial interface and register the PPS GPIO pin.

⚡ 1. Enable Hardware Interfaces

Edit /boot/config.txt to enable the serial port, disable Bluetooth (which shares the primary UART on Pi Zero W), and load the PPS device tree overlay.

/boot/config.txt

# Disable Bluetooth to free up the primary UART for the GPS HAT
dtoverlay=disable-bt

# Enable the primary serial port
enable_uart=1

# Register GPIO 4 for the PPS hardware interrupt
dtoverlay=pps-gpio,gpiopin=4

🔇 2. Disable Serial Console

By default, the Pi tries to spawn a login console on the serial port. This conflicts with the NMEA data stream.

Edit /boot/cmdline.txt and carefully remove the console=serial0,115200 parameter. Leave the rest of the line intact.

✅ 3. Verify Hardware Detection

After rebooting the node, verify that the kernel recognizes the PPS device and the serial port is receiving data.

Note

The PPS device is commonly registered at /dev/pps0, but depending on kernel initialization order and loaded overlays, it can be assigned to a different path such as /dev/pps1. Use ls -l /dev/pps* to identify the correct device path on your system.

Verify PPS device creation

ls -l /dev/pps*

Test for the raw electrical pulse (Press Ctrl+C to stop)

sudo ppstest /dev/pps0

Replace /dev/pps0 with /dev/pps1 (or your detected path) if assigned differently.

Verify NMEA serial data stream

cat /dev/serial0

If ppstest outputs an assert timestamp exactly once per second, the hardware is successfully locked and ready for the chrony service configuration.

🔗 References