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Build a Pi Time Server With GPS & Kernel PPS

Updated August 2023 - I followed the guide on this page with a new install of Raspberry Pi OS Bullseye and it worked.

By Rob Robinette K9OJ

Not too long ago only high-dollar commercial GPS units supplied a highly accurate pulse-per-second (PPS) time signal but now it's a common feature in inexpensive GPS chips. We can easily pair a $35 raspberry pi with a $10 GPS card with PPS to create a highly accurate network time server with complete independence from the internet. You can then build a network time display to show this uber-accurate time anywhere there is a WiFi signal.

I created this webpage because the GPS/PPS time server info I found online was incomplete or out of date. Hopefully the info here will help get you up and running quickly and easily.

Contents

Pi Time Server

    Pi Pin-Out

    Set Client Computer to Use Pi Time Server

    GPS NMEA Offset Tuning

    ntppool.org Tuning

    Testing and Troubleshooting

    Troubleshoot the Serial Port

    Add a Time Server to the ntp.org Pool

    Give the Time Server a Static Address

    Use Chrony Instead of ntp

        GPS and PPS Tuning With Chrony

Arduino Network Time Display

Raspberry Pi Network Time Display

Pi Time Server

The Raspberry Pi OS since the Stretch release has kernel PPS (Pulse Per-Second) support built in. I did not have to compile the kernel, ntp or cgps to get kernel PPS working with a Raspberry Pi. I simply installed the time server software using apt install. Kernel PPS is more accurate with less latency than standard PPS and orders of magnitude more accurate than bare GPS so if you're going to build a time server you really should use kernel PPS as described below.

I used a $12 u-blox NEO-6M GPS card with built-in patch antenna to supply GPS and PPS time signals to a raspberry Pi 1, Pi 2 and Pi 3B+ running ntp and gpsd. I recommend you source your GPS card from reputable dealers like sparkfun.com and digikey.com because there are many Chinese made NEO clones available on aliexpress.com and ebay.com that do not perform nearly as well as a real u-blox NEO GPS.

PPS signal + GPIO18 input + kernel processed PPS handling = a very low latency and accurate clock source.

With only 5 push-on jumpers and no soldering to connect a GPS to a Raspberry Pi it's easy to build a time server that can feed all the computers on your network with extremely accurate time synchronization even with no internet connection. This little time server is ideal for Amateur Radio Operators using digital modes that require accurate time such as FT8. It's also a nice fit for isolated or remote locations such as repeater or telecommunication sites that require an accurate time source.

Raspberry Pi 3B+ with u-blox NEO-6M GPS and antenna. This GPS board didn't have a PPS output pin so I soldered the brown wire to a surface mount resistor with a direct connection to the NEO 6M PPS pin 3 (the black ball on the NEO 6M chip designates chip pin 1). 2.54mm female-to-female jumper wires were used to connect the GPS and Pi. VCC=3.3 volts, RX=receive data from pi, TX=transmit GPS data to the pi, GND=ground, PPS=pulse per second timing signal. I used 3M double-sided tape to attach the GPS and antenna.

This is the GPS board I recommend. It's a u-blox NEO-6M GPS with PPS, battery backup and built-in antenna with optional external antenna connector sold on ebay for $12 delivered. This performs the best of the four GPS boards I've tried and the battery backup gives you a very quick initial fix. It has a PPS output pin and a micro-USB port. Note the output pins are pre-soldered and ready for push-on jumpers (see below). The big white square on the circuit board is the built-in antenna but if you want to use an external antenna you can solder on the included SMA Female connector shown in place in the above pic. The NEO GPS chip, micro-USB port and flashing green "position acquired" LED are on the other side of the circuit board.

Sparkfun NEO-M9N GPS with SMA antenna connector, USB and UART interfaces, PPS and breakout connections can receive all nations' GNSS (Global Navigation Satellite System) satellites. This version with an SMA antenna connector requires a separate GPS antenna but will perform much better than the version with the built-in chip antenna.

I now use a $70 NEO-M9N GPS with an SMA antenna connector (see photo above) for my primary time server but I had to disconnect the unused GPS TX wire to get the serial port to work. After that it worked really well in my attic. Unlike the NEO-6M that can only receive US GPS, it can also receive the Satellite Based Augmentation System, European Galileo, Russian GLONASS and Chinese BeiDou satellites. This is the multi-band GPS antenna I recommend. You'll get significantly better satellite reception compared to the same NEO-M9N GPS with a built-in chip antenna. There is also a version of the NEO-M9N with a U.FL antenna connector.

4 inch (10cm) long 2.54mm female-to-female jumper wires (40 pieces) available here from ebay seller miniduino for $3 delivered. Five of these jumpers are all that's required to connect the GPS + PPS to a Pi.

I use both, a Pi 1B and a couple Pi 3B's as GPS time servers and all run fine at very low cpu usage. Any version of the Pi will work, including the Pi Zero. Ntp and gpsd generate very little cpu load so you don't have to dedicate a Pi to just time serving. If you have a Pi already running you can literally stick a GPS card to the case and it'll get the job done. One of my Pi 3B+'s runs rtl_sdr in the attic. An SDR (software defined radio) USB dongle with a 6 foot vertical dipole antenna is connected to the Pi and allows remote use of SDR software from my office on the first floor. Even with rtl_sdr streaming digital radio signals and simultaneously serving time the cpu load never exceeds 20%.

Be aware that other single board computers such as the Beaglebone, Odroid and Orange have different GPIO pinouts and may not support kernel PPS using ntp, chrony and gpsd like the Raspberry Pies. I highly recommend you stick with a true Raspberry Pi for your server.

Connect the Pi & GPS

If your GPS has a USB port you can connect it to your pi using a USB cable and a single jumper for the PPS. Connect the GPS PPS output to the pi's pin 12 (see diagram below).

To connect your GPS using a serial port see photo above and connect your GPS to your Pi. The pin numbers are the physical pin location. Don't confuse them with the GPIO numbers. We will use GPIO18 located on physical pin 12 to receive the PPS time signal:

GPS Pin.......Raspberry Pi Pin

GPS VCC → 3.3V Power Pin 1 (this powers the GPS)
GPS GND → Ground Pin 6 (power and signal ground reference)
GPS RX   → GPIO14 / TXD Transmit Data Pin 8 (optional Pi→GPS communication)
GPS TX   → GPIO15 / RXD Receive Data Pin 10 (GPS time and position data)
GPS PPS → GPIO18 / PPS Input Pin 12 (PPS pulse-per-second signal)

Note how the GPS transmit data pin is connected to the Pi's receive data pin.

Pi Pinout

Pin 1 is at top left, Pin 2 is at top right, odd numbers on left, even on right. The pinout of pins 1 to 12 are the same for all Raspberry Pi models.

I used 3M double-sided tape to secure the GPS card and antenna to the top of the Pi case. You can also drill holes in the pi case and install PCB standoffs.

I recommend you connect your time server to your network using an ethernet port to reduce network latency and jitter.

When describing GPS output below I will use "GPS" for the GPS NMEA position and time data stream and "PPS" for the pulse-per-second signal.

Pi 1, Pi Zero and Pi Zero W CPU Limitations

For low horsepower Raspberry Pies like the Pi 1, Pi Zero and Pi Zero W I recommend you set the Pi to boot to the console and not the desktop to keep the cpu load down. I discovered my Pi 1 was running a cpu load of 1.35 with only the timeserver running because I had it set to boot to the desktop even though it was a headless system that I occasionally used VNC to access the desktop. Even with no display attached the damn screensaver was running full time and using up the cpu! Use the command uptime to see the cpu load. A load of 0.15 means the cpu is running at 15% of capacity. My Pi 1 runs at around 0.08 when booted to the console and running as a time server. The Pi Zeroes are more powerful than the Pi 1 so they should run an even lower cpu load.

To boot a Pi to the console run sudo raspi-config from the command line and select Boot Options and select one of the boot to console options.

If you need to access a remote desktop through VNC you must ssh into the Pi first and run vncserver :1 from the command line, then use VNC to access the :1 desktop. When done, exit VNC, open an ssh terminal and issue sudo killall vncserver to end VNC and save some cpu load.

Does the GPS Need to Be Near a Window?

I was worried about having to place my time server near a window to ensure GPS satellite reception but I was pleasantly surprised to find I could get a good 3D fix and serve time from deep inside my home. I permanently placed the time server in a windowless attic with some ham radio gear and it has no problem keeping a solid 3D fix. Note that for initial GPS position I had to place all my GPSs outside (powered by USB power brick) for about 15 minutes. Once that initial fix was made the GPSs performed fine inside the house and in the attic.

Get Your Raspberry Pi Up to Date

I recommend you run this upgrade sequence before installing the time server software.

        sudo apt update

        sudo apt full-upgrade  (this can take a while)

        sudo rpi-update

        sudo reboot

You should set a static IP address for your time server so your client computers have a fixed address to get time updates.

Ensure the serial port is available to the GPS

You need to edit these two files to ensure the GPIO serial port is not in use (ignore this if you connect your GPS using USB):

sudo nano /boot/cmdline.txt

Delete this part of the first line: console=serial0,115200 and change any mention of ttyS0 or ttyAMA0 to tty1

Save your edits with ctrl-o <return> then exit with ctrl-x

sudo nano /boot/config.txt

Change any mention of ttyS0 or ttyAMA0 to tty1 and add the following lines to the end of the config.txt file:

enable_uart=1  # Enable the hardware serial port

dtoverlay=pps-gpio,gpiopin=18  # Listen for the GPS PPS signal on gpio18

init_uart_baud=38400  # Set the serial port baud to 38400. The NEO GPS chips can run at up to 115200 baud. The NEO M9N GPS can track so many satellites that it puts out a huge amount of data so it's default is 38400 baud. Some older GPSs use only 9600 or even 4800 baud.

The following three steps are for the Pi3B+ newer model pi's including the Pi4:

dtoverlay=disable-bt  #  if you want to disable Bluetooth, choose this OR the next line to keep Bluetooth but move it off the main serial port.

dtoverlay=miniuart-bt  #  you must include this line to move Bluetooth to another serial port because Bluetooth uses the serial port the GPS needs.

dtoverlay=disable-wifi  #  only if you want to disable WiFi which may help the GPS reception.

Save your edits with ctrl-o <return> then exit with ctrl-x

Add pps-gpio to /etc/modules File

sudo nano /etc/modules

pps-gpio  #  Add this line to the file

Save your edits with ctrl-o <return> then exit with ctrl-x

Reboot: sudo reboot

To make sure the GPS is sending data over the serial port run this to show GPS NMEA output straight from the serial port: sudo cat /dev/serial0

NOTE: If you don't see any NMEA messages streaming out try sudo cat /dev/serial1  [the pi 1 & 2 used ttyAMA0 and ttyS0 for the serial port name]

We should see GPS NMEA messages streaming out. Data will flow even before the GPS gets a position fix. Hit <ctrl>-c to exit.

If you don't see the GPS data then troubleshoot the GPS-to-serial interface connection. Don't proceed until you can see the GPS data.

Software Package Installation

Install the time server software:

sudo apt install ntp gpsd gpsd-clients pps-tools

Configure gpsd

sudo nano /etc/default/gpsd  # and make the following changes:

START_DAEMON="true"
USBAUTO="false"  # Change this to "false" even if you're using a USB connected GPS
DEVICES="/dev/serial0  /dev/pps0"  # OR DEVICES="/dev/ttyACM0  /dev/pps0" if using a USB connected GPS
GPSD_OPTIONS="-n"

gpsd will now auto start with the GPS & PPS configured.

Configure ntp

I wanted to add a time server to the ntppool.org time server pool and to do that you must be running ntp (not chrony) so the following ntp configuration uses the ATOM kernel PPS driver for the seconds beat and the NMEA GPS driver for the date and time.

I commented out all the time sources in the ntp.conf file and added these settings to the end of the file:

sudo nano /etc/ntp.conf

pool us.pool.ntp.org iburst noselect

broadcast 192.168.1.255

# GPS NMEA driver

     server 127.127.28.0 minpoll 4 maxpoll 4 prefer

     fudge 127.127.28.0 time1 0.09 refid GPS stratum 0

     # kernel PPS driver

     server 127.127.22.0 minpoll 4 maxpoll 4

     fudge 127.127.22.0 flag3 1 time1 0.005 refid kPPS

127.127.28.0 is the NMEA driver

127.127.22.0 is the ATOM kernel PPS driver

"time1" is offset in seconds but keep in mind that offset reported by ntpq -p is in milliseconds.

I recommend you start with "time1" settings of 0.09 for GPS and 0.005 for PPS. After extensive tuning these values are spot-on for my NEO GPS M9N cards and the Pi3.

With these settings ntp will serve time update requests from clients and also broadcast time updates.

Verify gpsd and ntp Are Working

Reboot with sudo reboot and give the GPS a few minutes to gain a 3D fix. You can verify a 3D fix by using the cgps command. If cgps shows data flowing at the bottom of the display but no position data then the GPS doesn't have a fix yet. Most GPS boards have an LED that will flash once per second when it has a fix. Two of my GPS boards have a red flashing LED and one has a green flashing LED on the GPS chip side of the board signifying at least a 2D position and time fix.

It can take up to 30 minutes for a new GPS to get its first fix. I had to take all of my NEO-6 and NEO-7 Pi time servers outside to get their first GPS fix. Don't begin troubleshooting until the GPS board shows a 3D fix. I used a USB power brick to power the pi outside for it's initial fix. You don't need a network connection--when the GPS PPS led flashes once per second it has a good fix.

cgps  # Show GPS fix info

Note: Use ctrl-s to pause the GPS data stream and ctrl-q to resume

lqqqqqqqqqqqqqqqqqqqqqqqqqqqq
x Time: 2018-09-18T20:04:37.000Z
x Latitude: 36.147821 N
x Longitude: 82.574643 W
x Altitude: 429.6 m
x Speed: 0.0 kph
x Heading: 0.0 deg (true)
x Climb: 0.0 m/min
x Status: 3D FIX (5 secs) <---Shows the GPS has a good navigation and time fix

If gibberish shows up on screen try setting the serial port baud rate to 4800 or 38400. sudo nano /boot/config.txt and change: init_uart_baud=4800 and reboot.

The xxxGNSS two letter designators shown in cgps tell what satellite constellation is being received: GP: US GPS, SB: Satellite Based Augmentation System (Wide Area Differential GPS), GA: Europe Galileo, GL: Russia GLONASS, BD: China BeiDou, IR: India IRNSS, QZ: Japan QZSS, IM: Japan IMES

If you can't get a 3D FIX after an hour outside see the Troubleshooting section.

Once you have a 3D FIX run the ntpq -p command and you should see something similar to this:

remote.................refid.....................st...t...when...poll...reach..delay...offset....jitter
==============================================================================
us.pool.ntp.org.....POOL.................16...p....-.........64......0......0.000...0.000...0.004
192.168.1.255.....BCST..................16...B...-..........64......0......0.000...0.000...0.004
*SHM(0)..............GPS.....................0....l....14........16...377.....0.000...0.375...0.758
oPPS(0)..............kPPS...................0....l....13........16...377......0.000...0.002...0.004
+1.time.dbsinet...146.186.222.14....2...u....64........64....37....51.421....0.987...6.939
-clockb.ntpjs.or...193.190.230.65.....2...u....63........64....37....40.544...-0.635...8.630
-lithium.constan...216.218.254.202..2...u....63........64....37....51.586...-1.499...5.759
+chl.la.................127.67.113.92.......2...u....63........64....37....87.467....2.086...6.950
-mirror1.sjc02.s...162.213.2.253......2...u....62........64.....7.....84.516...-6.266..11.765
-lga1.m-d.net.......132.163.96.1........2...u....61........64.....7.....51.145....5.246....8.928

Offset is in milliseconds so a PPS offset of 0.002ms = 0.000002 seconds (2 millionths of a second!). See this for more info on ntpq output description.

We're looking for the GPS offset to stay below +/-200ms (0.2 seconds) and we need an asterisk in front of the GPS source which signifies ntp is using it as the primary time source. We also want to see an o in front of the PPS line which means ntp is using kernel PPS for the seconds beat. A positive offset indicates the local clock is fast. A negative offset indicates the the local clock is slow.

If there is nothing in front of a time source it means the time source has noselect in its ntp.conf line or it is unreachable. If after an hour the GPS line does not have a "*" in front of it see the troubleshooting section.

NOTE: If the GPS and PPS signals are not within 200 milliseconds (0.2 second) of one another the PPS signal will be ignored. See GPS Tuning to get the time difference under 200ms.

If Gpsd Will Not Auto Start at Boot

This seems to be a problem for the Pi 3B+ and this is how I got gpsd to run at boot:

We'll copy a file and then edit the copy. By editing this copy the change should persist with gpsd updates:

sudo cp /lib/systemd/system/gpsd.service  /etc/systemd/system/gpsd.service

sudo nano /etc/systemd/system/gpsd.service

Change to:

[Install]

WantedBy=multi-user.target

Also=gpsd.socket

Then at the command line:

sudo systemctl enable gpsd

sudo systemctl daemon-reload

sudo systemctl restart gpsd

sudo systemctl unmask gpsd

sudo reboot

Verify gpsd started automatically by running ntpq -p a few minutes after the reboot and getting an "o" in front of the PPS line. Don't run cgps because that will cause gpsd to start.

Set Client Computers To Use Your Time Server

To set a Windows computer to time sync from your timeserver press the Windows "Start" button and type “Control Panel”, then click Control Panel. Click the search bar in the top-right of Control Panel and type “Date and Time.” Click "Set the time and date." Select the “Internet Time” tab and press “Change setting.” Enter your time server address (my time server is 192.168.1.79) ensure the "Synchronize with an Internet time server" box is checked and click the "Update now" button. You should get a "The clock was successfully synchronized . . ." message. Click the "OK" boxes.

To set most Linux computers to time sync from your timeserver you need to install ntp with sudo update and sudo apt install ntp. Edit the /etc/ntp.conf file and add your timeserver's address:

sudo nano /etc/ntp.conf

server 192.168.1.79 prefer  #  Enter your time server's IP address here

The prefer directive lets ntp know you prefer that server over other time sources.

Restart ntp: sudo systemctl restart ntp or just reboot the computer.

Pi 1B Time Server

Even the lowly Pi 1B works well as a GPS + PPS time server.

GPS & PPS Offset Tuning

After we get the GPS running with good GPS NMEA data we may need to adjust the GPS time source to match up with the PPS signal so the PPS signal will be recognized and used by ntp. Once we get the GPS offset nailed down we can then tweak the PPS offset for maximum accuracy.

NOTE: If the GPS and PPS signals are not within 200 milliseconds (0.2 second) of one another the PPS signal will be ignored. The ntpq -p command will show nothing from the PPS.

I didn't know about this limitation and it caused untold head banging while I tried to get the timeserver to use the PPS signal. We might need to adjust the GPS ntp.conf time1 (offset) value to get the GPS time within the required 200 milliseconds.

offset

This option can be used to compensate for a constant error. The specified offset (in seconds) is applied to all samples produced by the reference clock. The default is 0.0.

I recommend you start with these ntp.conf settings, they should get you close enough to get everything working:

sudo nano /etc/ntp.conf

#GPS

#PPS

time.richiemcintosh.com       23     12   47m   +31.039        0.548        -0.125  675us

ha81.smatwebdesign.com    26     11   49m   +31.754        0.471        -0.122  604us

up2.com                              13       7    840   +32.788        1.331        -0.127  293us

rrcs-97-78-159-178.se.bi>   29     17   52m   +32.595        1.646        -0.119 1987us

Offset is given in milliseconds.

Calculate average public server offset:

-0.125 +  -0.122 +  -0.127 +  -0.119 =  -0.493

-0.493 / 4 = -0.123 milliseconds public server average offset

Note: Don't use a public time server to calculate the average offset if it's way off compared to the others because it will throw off your average.

Now we subtract the public server average offset from the current GPS offset and use the result as the ntp.conf GPS offset.

     Offset is in milliseconds, 0.123ms = 0.000123 seconds

Current GPS offset is 0.125 seconds:  0.125  -  -0.000123 = +0.124877 is the new GPS offset

Add the Calculated Offset to ntp.conf

Edit sudo nano /etc/ntp.conf and remove the # from the PPS line. On the GPS line modify the GPS offset and we have our ntp.conf configuration:

pool us.pool.ntp.org iburst

broadcast 192.168.1.255

ha81.smatwebdesign.com    26     11   49m   +31.754        0.471        -122ms  604us

up2.com                              13       7    840   +32.788        1.331        -127ms  293us

rrcs-97-78-159-178.se.bi>   29     17   52m   +32.595        1.646        -119ms 1987us

Note how the public timeserver offsets are listed as milliseconds (ms) and the GPS offset is listed as nanoseconds (ns). You will also see microseconds (us or µs). Since the chrony.conf file uses seconds we have to do some conversions.

To convert ns to seconds we divide ns by 1,000,000,000 or simply shift the decimal point 9 positions to the left.

To convert us to seconds we shift the decimal point 6 positions to the left.

To convert ms to seconds we shift the decimal point 3 positions left.

Calculate average public server offset:

-125 +  -122 +  -127 +  -119 =  -493ms

-493ms / 4 = -123ms public server average offset

Note: Don't use a public time server to calculate the average offset if it's way off compared to the others because it will throw off your average.

Now we subtract the public server average offset from the current GPS offset and use the result as the chrony.conf GPS offset.

Current GPS offset is 0.2 seconds or 200 milliseconds:  200ms  -  -123ms = +323ms

Convert milliseconds to seconds: 323ms / 1,000 =  0.323 seconds (shift decimal 3 places left) = new GPS offset

Add the Calculated Offset to chrony.conf

Edit sudo nano /etc/chrony/chrony.conf and remove the # from the PPS line. On the GPS line replace prefer with noselect and modify the GPS offset and we have our chrony.conf configuration:

pool us.pool.ntp.org iburst

allow

broadcast 60 192.168.1.255

refclock SHM 0 refid GPS precision 1e-1 offset 0.323 noselect

refclock PPS /dev/pps0 refid PPS lock GPS precision 1e-9 offset 0.002 prefer

Notes: offset is in seconds. The GPS noselect ensures the computer will use the PPS (with GPS date, hour and minute) as its time source.

Restart chronyc and gpsd:

sudo killall -9 gpsd chronyd

sudo chronyd -r -s -m -f /etc/chrony/chrony.conf

sudo gpsd -n /dev/serial0 /dev/pps0

After a few minutes run chronyc sources

chronyc sources
210 Number of sources = 6
MS Name/IP address          Stratum Poll Reach LastRx   Last sample
===============================================================
#? GPS                                     0      4     377       14    +28ms [ +28ms] +/- 101ms
#* PPS                                      0      4     377       13   -601ns [-1531ns] +/- 185ns
^- ntp4.doctor.com                     2    10    377      860  -3632us [-3636us] +/- 54ms
^- static-72-87-88-202.prvd>      2    10    377      831  -3408us [-3412us] +/- 75ms
^- mta5.1strepublicwebsites>     2    10    377      722  -5816us [-5820us] +/- 59ms
^- ec2-35-171-237-77.comput>  2    10    377      839  -2381us [-2385us] +/- 98ms

We're looking for the GPS offset to stay below +/-200ms and we need an asterisk in front of the PPS source which signifies chrony is using it as the primary time source. A positive offset indicates the local PPS synced clock is higher. A negative offset indicates the other clock is higher.

Calculate the PPS Offset

This step usually isn't necessary unless you need the highest accuracy because I have found for GPS receivers mounted close to the time server like mine the PPS offset will only be 2 to 3 milliseconds.

We adjusted the GPS offset above to get the GPS & PPS times close. Now we'll adjust the PPS offset to get our time server's time close to the average public time server time.

After a few hours of run time run chronyc sourcestats and again calculate the average public time server offset. It will normally be a negative number around  -2000us (-2000 microseconds =  -2 milliseconds =  -0.002 seconds).

Convert the offset to seconds: -2000us / 1,000,000 = .002 seconds (just shift the decimal point 6 places left)

Subtract the average public server offset from the current PPS offset:

Current PPS offset 0.0  -  -0.002  =  +0.002 PPS offset

Edit chrony.conf:

sudo nano /etc/chrony/chrony.conf

refclock PPS /dev/pps0 refid PPS lock GPS precision 1e-9 offset 0.002 prefer

If the average public server offset is negative (it normally is) then the PPS offset needs to be positive.

Restart chronyc and gpsd:

sudo killall -9 gpsd chronyd

sudo chronyd -r -s -m -f /etc/chrony/chrony.conf

sudo gpsd -n /dev/serial0 /dev/pps0

After an hour or so again run chronyc sourcestats. The longer you let the server run the more accurate the result.

Calculate the average public time server offset again and verify it's closer to 0. You may want to repeat the procedure to get the average public server offset even closer to 0.

Note: When PPS is being used as the time server's time source (* in front of the PPS line in the chronyc sources output) offset math works like this:

To calculate a new GPS offset we add the chronyc sources GPS offset to the chrony.conf GPS offset.

To calculate a new PPS offset we subtract the chronyc sources average public time server offset from the chrony.conf PPS offset.

With these fine tuned offset values we'll get a quicker GPS lock and more accurate time server.

Build an Arduino Network Time Display

a.k.a. high precision WiFi Clock

We now have ultra-accurate network time so let's build a display that shows the current GPS kernel PPS time. I searched for a commercial network time display but the only ones I could find were uber-expensive but we can use a very inexpensive $3 Wemos D1 Mini ESP8266 microcontroller with built in WiFi and a $4 D1 Mini OLED Shield (.66" 64x48). Both are available on eBay and aliexpress. The network time display is updated every second. Of course you can use the following technique to build a display with a much larger display screen but this simple project will get you started. If you prefer to work with raspberry pi's then see this.

Network Time Display

Date and time with seconds displayed. Once programmed we just power it up with a Micro USB cable and it runs. Note the female jumper pin block soldered to the D1 Mini and the male pin block soldered to the OLED shield.

Start by assembling the D1 Mini and OLED shield like it is in the Network Time Display picture above using the included male and female jumper pin blocks. Stack the boards then solder the OLED shield pins first, then turn the assembly over and solder the D1 Mini pins. Soldering the jumpers with the two boards assembled will ensure the pins and jumpers align correctly.

The D1 Mini connects to your computer by USB and the free Arduino IDE programs and flashes it. Once programmed the network time display will power on, connect to your WiFi network, get the current time and display the date and time on the OLED screen.

Wemos D1 Mini Microcontroller

Solder female jumper pin blocks to the two rows of terminals. The OLED shield will plug into these two jumper blocks.

D1 Mini OLED Shield

After soldering on the male jumper blocks the OLED shield (hat) will sit on top of the D1 Mini and display our network time. Match up the "RST" pin at top right to ensure the correct orientation of the shield on the D1 Mini.

The nice thing about the D1 Mini is it's built in WiFi and its Arduino compatibility. We can program and flash it using the Arduino IDE (click the link to download). We will use this Arduino program (sketch) to run the network time display: Network Display at github. Download Adafruit_SSD1306.cpp, Adafruit_SSD1306.h and the D1 Mini 8266 ino program file into one folder as shown below:

The three files needed to compile the network time display program. "GPS_Time.ino" is the renamed "ESP8266_TN008g_NTP_Time_OLED_and_WIFI_Manager_UK_Webserver_02.i.ino" file.

Open Window's Device Manager and expand the Ports (COM & LPT) section. Plug the assembled D1 Mini + OLED into your computer using a Micro USB cable. You should see a new COM port show up in Device Manager. If it doesn't show up you may need to download the CH340 serial port driver. Unzip the file and run the installer.

Device Manager

My D1 Mini is the "USB-SERIAL CH340 (COM11) port. If you plug the D1 Mini into a different USB port the COM port will usually change.

Note the COM port and run the Arduino IDE.

Select "Tools/Port" and set the COM port.

Select File/Preferences and copy and paste this text into Additional Boards Manager URLs: http://arduino.esp8266.com/stable/package_esp8266com_index.json

Select "Tools/Board" and select "LOLIN(WEMOS) D1 R2 & mini".

Select "File/Open" and open the ESP8266_TN008g_NTP_Time_OLED_and_WIFI_Manager_UK_Webserver_02.i.ino file.

Set the Timezone and ntpServer constants and set the gmtOffset_sec and daylightOffset_sec to 0. For Universal Time Coordinated use "UTC0" as the Timezone.

We need to set the Timezone and ntpServer constants to our desired timezone and ntpServer address. Find your time zone here: Time Zone list. In the screen capture above I have USA Eastern time set and "192.168.1.79" is the IP address of my network GPS time server. If you don't have a local time server then use "0.north-america.pool.ntp.org" or the time server of your choice. To display UTC (Zulu time) use "UTC0" as the Timezone. Set the gmtOffset_sec and daylightOffset_set variables to 0.

Save the file and click the IDE right arrow (compile and send to Arduino) button and the program will compile and flash to the D1 Mini board.

The first time the board powers up it will create a WiFi access point called "ESP8266_AP". Connect to that access point using a computer with WiFi and open your web browser to address "192.168.4.1" and click on "Configure WiFi". Choose the WiFi network you want the board to use and enter the WiFi password. The D1 Mini will then join that WiFi network, get the time from your network time server and then display the current date and time as shown in the Network Time Display photo above. The time is updated by the network time server every second. Once programmed the D1 Mini requires only USB power and access to the WiFi access point specified in the program. To change to another WiFi access point simply edit the program and re-flash the D1 Mini.

Although the OLED shield display used here is very small we can use this technique to use a larger, more readable OLED display.

D1 Mini Pinout

Raspberry Pi Network Time Display

a.k.a. high precision WiFi Clock

A raspberry pi with a 3.5 inch LCD display and case makes a very nice network time display.

Start by getting the pi up and running and booting to the desktop. Then physically install the LCD display and install the display driver so the LCD display will show the desktop. We then download the clock program and a little program that will hide the mouse pointer. Finally, we set the clock program for auto start and full screen. Optionally, we can flip the display upside down to put the power connector on the top of the display.

Raspberry Pi3B + 3.5" LCD Display As Network Time Display

Modified date to allow larger font and rotated display to put the power connector on top. Use my  modified uhr.html for this display output format.

Note the power connector at the bottom of the display. We can rotate the display 180 degrees to put the connector at the top so the display can stand upright.

3.5" LCD Touch Screen Install Drivers

This worked for my 3.5" screen

From the terminal command line:

cd ~

sudo rm -rf LCD-show

git clone https://github.com/goodtft/LCD-show.git

chmod -R 755 LCD-show

cd LCD-show/

sudo ./LCD35-show

When you have your raspberry pi booting to the desktop and displaying it on the LCD proceed with the following steps:

Download the Clock Program

From the terminal command line:

https://github.com/carli2/raspberry-projects/tree/master/clock

This will download two files: jquery.js and uhr.html

    Use my modified uhr.html for "12/24/2019  20:14:22" large font display format as seen above in the first photo.

Hide the Mouse Pointer

From the terminal command line:

We will turn off the screensaver and screen blanker and run the clock program

Set the Time Server Client

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