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How to use Internet Time with GPS clocks The “Clayton’s” GPS Time Source* We’ve produced a number of GPS-based clocks over the last few years but they can be problematic if you can’t get a GPS signal – deep inside a building, for example. But how about this for a clever alternative: program a cheap WiFi Module to act as a time reference, kept accuracy via the internet! It pretends to be a GPS unit, so any GPS clock can use it! By Tim Blythman C The beauty of this system is that it presents this time lock projects that depend on a GPS source as the reading as if it’s coming from a GPS module, so you don’t time reference can be relied on as being highly achave to make any changes to the clock hardware or softcurate – after all, the time code in the GPS signal is ware. You could build any of our GPS clocks and with just derived from an atomic clock. a small amount of extra effort, get them to run off Network The most recent GPS-synchronised clocks we’ve pubTime Protocol (NTP) time via your existing WiFi network. lished are the Analog Clock Driver (February 2017; It should even work with commercial, pre-built GPS clocks. siliconchip.com.au/Article/10527 and the High Visibility In fact, this concept can be used with any device that 6-Digit LED GPS Clock (December 2015 & January 2016; uses a GPS module to get a time signal. siliconchip.com.au/Series/294). It doesn’t need to be a clock. But keep in And in the case of the High Visibility mind that it may not be accurate enough 6-Digit LED GPS Clock, it requires no to use as part of a time reference system. manual adjustments for daylight saving or time zones as it can determine these How it works adjustments based on location data from As already noted, we’re using an that same GPS module. ESP8266 module which is available preUnfortunately, all GPS-based clocks assembled, already having an onboard are subject to one caveat: they won’t WiFi transceiver. We program it using the work well without having a clear Arduino IDE. IDE stands for Integrated “view” of the sky, so that they can pick Development Environment, which means up the signals from multiple GPS satthat it lets you write code for an Arduino, ellites. While GPS signals can usually then compile it and upload it to the board. be picked up indoors, whether you will And note this point; while we are usget them at a given location depends on ing the Arduino IDE, we are not using an the construction of the building – and Arduino Uno or related Arduino board! even the weather. The software we have developed But many readers will already have fetches the current time and date from a WiFi network at home and that gives The NTP WiFi module we chose, the an NTP server via the internet, then it an alternative source of time that’s al- ESP8266 WeMos D1 Mini Module uses this timestamp in combination with most as accurate (far more accurate than (available from Jaycar, Cat XC-3802). its internal clock to generate a stream of you’re ever likely to need!). So why not GPS-compatible (NMEA encoded) data, take advantage of it? including time signals. This project takes a cheap and readily available ESP8266 It is possible to estimate your location based on your WiFi/ARM processor module and uses it to fetch accuinternet IP address, so our software does just that, transrate time readings from the internet and pass it through lating the IP into a latitude/longitude pair using an online to the clock. *The time source you have when you don’t have a GPS time source! 58 Silicon Chip Celebrating 30 Years siliconchip.com.au service. So if you are using this module with the High Visibility 6-Digit LED GPS Clock, it can use these coordinates to calculate the correct time zone and daylight saving rules, and it will display the correct local time. While we’ve found that the estimated position can sometimes be a few dozen kilometres out, in 99.99% of cases, this will still be in the same time zone, so the time displayed will still be correct. But if you are using a proxy or VPN, this information may be inaccurate, as the IP service may return coordinates based on your proxy location. But for most home WiFi setups, this should not be a problem. NMEA GPS data NMEA stands for National Marine Electronics Association; NMEA 0183 is a specification for communication between marine electronics systems, including GPS receivers. Pretty much all GPS receivers produce serial data in this format, which is why we’ve designed our unit to use this same standard. We’ve described the NMEA 0183 standard in detail in the past and we will give some examples below. But for now, you just need to understand that the data is sent over an RS-232 (or similar) serial link, in ASCII text format, with each distinct line being considered a “sentence”. Each sentence is prefixed with a code which indicates what data is contained within. So units receiving this data can skip any sentences with codes that they don’t understand. Each sentence contains a series of values separated by commas and terminated with a checksum, so that data which is corrupted during transmission can be ignored. Since the information from a GPS receiver is simply serial ASCII data, it’s easy for a microcontroller to mimic. A GPS receiver will usually transmit around 3-10 sentences, sent once per second. But for our clock projects, only two are important. These are the RMC (minimum recommended GPS) and GSA (fix validity and active satellites) sentences. RMC contains the GMT/UTC time, date, latitude, longitude, speed, course and magnetic variation data. GSA indicates whether the unit has obtained a GPS fix and a partial list of the satellite Ids used to obtain that fix. See Fig.1 for an example. This was taken from a real GPS receiver. Without an actual GPS receiver, the latitude, longitude and satellite status will have to be fabricated or estimated; the only information which absolutely needs to be correct is the time and date. We’ve also written the software to produce a “dummy” status sentence to show some information about the status of the WiFi connection. Since this is a sentence type that the clocks are not programmed to interpret, they will ignore it. But you could monitor the output of the serial port using a PC as a debugging aid. What is a Clayton’s GPS Time Source? Some readers (particularly younger ones) may not understand the reference to “Clayton’s” – arguably, one of the most famous tag lines to come out of Australian advertising (OK, that and “NOT HAPPY, JAN!”). Back in the 1970s and 80s, there was a non-alcoholic drink called “Clayton’s” which was advertised as “the drink you have when you’re not having a drink”. They, of course, meant alcohol. Very quickly, the phrase entered the Aussie vernacular; it meant the (fill in blank) you had when you’re not having (or possessing, or owning, or using etc etc etc!) a (fill in blank). Our “Clayton’s” GPS time source is therefore the time source you have when you don’t have access to GPS – whether that’s because you don’t have one (!) or because it doesn’t have the clear view of the (northern in Australia) sky and so cannot receive the valid GPS signal. Network Time Protocol NTP is one of the oldest internet protocols still in use. It’s used by pretty much every computer and smartphone, to keep their clocks accurate. NTP, designed to be simple and fast so that the overhead of checking and adjusting the time is minimised, is well suited to implementation on a small device like an ESP8266 module. It’s also designed to compensate for networks delays. Ultimately, NTP time comes from a source which typically has a caesium atomic clock. From there, the time is distributed to other nearby servers. Our unit will most likely be getting its data via a path that is three or more levels (or “strata”) removed from the atomic clock. In other words, we’re synchronising our time to servers which synchronise their time to other servers which synchronise their time to other servers which have atomic clocks attached. Whew! But you can still expect the resulting time to be accurate within about 10ms. Given that we are transmitting this data $GPRMC,013115.000,A,3345.6276,S,15116.8171,E,0.00,157. 35,140218,,,A*76 $GPGSA,A,3,05,31,25,29,02,,,,,,,,2.22,2.02,0.92*02 Fig.1: an example of typical RMC and GSA sentences from a real GPS receiver. The RMC sentence provides basic fix data such as latitude, longitude, speed, heading, and most importantly, date and time. In this case, the UTC time is 1:31:15am and the date is 14/02/2018. siliconchip.com.au Celebrating 30 Years Here’s an alternative ESP8266 module, the ESP-01 (available from the Silicon Chip Online Store, Cat SC3982). It’s smaller but it’s not quite as easy to program in situ. We have an article describing this module in detail on page 76 of this issue. April 2018  59 Fe at ur es & sp ec ifi ca tio ns : to a clock which will be only results and use that as the corbe displaying to the nearest rect time. second, that should be accu- * Uses low-cost ESP8266 WiFi module Keeping track of time rate enough. Comparable in size to a GPS module To determine the time us- * The NTP-based GPS Time ly required ing NTP, the unit sends out * Little or no assemb Source uses its internal oscilincluding a number of packets over * Generates standard GPS NMEA data lator and timer to keep track dity vali al sign the internet to NTP servers of time in the short-term, so time, date, position and and uses its internal clock * Also produces 1PPS pulses (D1 version only) it does not need to constantly to keep track of when each re-query the servers to detert rne inte from e tim NTP tically fetches packet is sent and when a re- * Automa mine the time. servers ply is received. The response This oscillator’s frequency tion, for clocks with also includes the time (ac- * Provides approximate loca may not be exactly right and location-based time zone support cording to the server) when it could vary with temperaour query was received and * Adjustable baud rate ture and other factors but when the response was sent. since we synchronise it fret * Configured via serial por The packets are sent using the quently using NTP, it should ing ugg deb for my status sentences, low-overhead User Datagram * Produces dum never drift very far. Protocol (UDP). In fact, by looking at how * Power supply: ~70mA at 3.0-5.5V Using this information, we it’s drifting each time we get can determine the round-trip a time update via NTP, we time, ie, the time it takes for our query to get to the server can account for and cancel out some of this drift. plus the time it takes to get the response. Normally, the That’s important since we use this oscillator to deterroute taken by both packets will be similar and so the demine the one-second intervals on which to send the NMEA lay will be similar. data and this gives the clock its seconds “tick”. If that was By subtracting the time that the server spent processing inconsistent or worse, glitched (eg, giving two pulses in our request from the round trip time and dividing by two, short order), you would probably notice. we can get a pretty good idea of how long the response By default, we perform an NTP update at hourly intertook to get to us. We can then add that delay time to the vals. The oscillator in the ESP8266 micro is typically acaccurate time we receive from the server and that should curate to within about ±0.001%. That means that, uncorbe close to the exact time when the response was received. rected, it will drift by up to 42ms each hour. That’s hardly Since we query a number of servers, if a majority of the noticeable and the corrected drift is likely to be well within times determined from the responses are within a few mil10ms. You aren’t going to notice a 10ms “jump” when the liseconds of each other, we can be fairly sure that we have time is updated from NTP. a good determination of the time and we can average those One thing we haven’t mentioned yet is that there are ac- Fig.2: circuit diagram of the original WeMos D1 mini, which has now been cloned and is widely available. It’s based on an ESP8266 WiFi module with onboard processor but contains extra circuitry to make it easier to program and use. 60 Silicon Chip Celebrating 30 Years siliconchip.com.au Fig.3: there are several different versions of the D1 Mini board but they are all suitable for this project and have the same pin-out, as shown here. The one we used (a common clone) looks like this but there may be slight variation in the components on the board. Fig.4: the only part mounted on the the top side of the D1 Mini module is the ESP8266 sub-module, which contains the IC itself plus a few passive components inside a metal shield can. This is the side normally visible when the module is plugged into another board. tually two different hardware units which you can use for this project. One is smaller and cheaper (ESP-01) but only has eight pins. The slightly larger module (D1 Mini) has more pins and this allows us to also provide a 1PPS (one pulse per second) output, for clocks which require it. See Figs.2, 3 & 4 for details of the D1 Mini. If available, the 1PPS output is generated from the same oscillator and is driven high briefly at the same time that we start to transmit the NMEA data. We’ll get back to discussing the two different hardware modules later. NTP always gives UTC or Universal Coordinated Time. This is the modern, more accurate version of Greenwich Mean Time, which differs from GPS time by (currently) 18 seconds due to the fact that the GPS satellites are not adjusted for leap seconds when they occur. However, the GPS data stream does include information about how many leap seconds have occurred, so this can be corrected for. Most GPS receivers are able to use this information and so give the correct UTC time, but some receivers don’t apply the leap second change at the right time. This means that using NTP may actually be more accurate than some GPS receivers. some status information. While the small PCB antenna on this module doesn’t have a long range, it’s expected that it will be used indoors (where GPS isn’t available) and probably not too far from a router. ESP8266 module The small size of the ESP8266 module means that it can function as a direct drop-in replacement for many GPS modules. There are quite a few different ESP8266-based modules available. Our preference is for the WeMos D1 Mini, which includes an onboard USB/serial converter to simplify programming, as well as a 5V regulator, allowing it to be used with both 5V and 3.3V power supplies. Other modules like the slightly smaller ESP-01 can also be used, but you will need a USB-serial converter to program it and pull-up resistors are also required to get it to operate in the correct mode. We’ll show you how to use either module for this project. The features of the module that we are using are the WiFi connectivity, serial port and also the onboard LED to report siliconchip.com.au Circuit diagram The circuit diagram for the ESP8266 WeMos D1 Mini module (available from Jaycar, Cat XC-3802) is shown in Fig.2 and its pinout is shown in Figs.3 & 4. Since this is a pre-built module which does pretty much everything we need it to do, there’s no additional circuitry required. We simply feed power into the GND and 3V3 pins and the emulated GPS serial data appears at the TX pin. If your clock supplies 5V to the GPS module then you can feed this into the 5V pin instead. The onboard regulator then derives the 3.3V supply which powers the module. Regardless, the serial data output will have a 3.3V swing but this is true of most GPS modules (even if they run off 5V) so the clock should not have any problem with this (5V micros can normally accept 3.3V logic levels at their digital inputs). You can also use the smaller ESP-01 Module (available from Altronics, Cat Z6360) but it cannot easily be programmed as-is. You will need a breadboard, some jumper wires and a few resistors so that you can program it. You will also need to add some components to the board before mounting it in the clock, so that it will operate normally. The ESP-01 also does not include a 3.3V regulator, so the host circuit will have to supply it with 3.3V or thereabouts (3.0-3.6V is acceptable). Programming it Regardless of which module you’re using, you need to install the Arduino IDE and the ESP8266 processor addon so that you can upload the code to it. If you haven’t already done this for a previous project, use the following steps – and if you do already have this software installed, check to make sure you have the latest version. First, install the most recent version of the Arduino IDE onto your PC, if you don’t already have it. This can be Celebrating 30 Years April 2018  61 Fig.5: this is how the ESP-01 module is connected for programming. Note the two loose wire “ends”, which are used to put the module into programming mode. downloaded for free from www.arduino.cc/en/Main/Software Next, install the ESP8266 board files. This is also a free download but it’s quite large and will take a while. To do this, open up preferences in the Arduino IDE and under “Arduino Board Manager URLs”, enter: http://arduino. esp8266.com/stable/package esp8266com index.json (as shown in Fig.6). Hit OK, then go to Tools  Boards  Board Manager, type in “esp8266” in the search box, click on the entry which appears below and then click on the “Install” button (see Fig.7). This will result in around 160MB of compilers and associated files being downloaded and installed on your computer. There are two main build options, so we’ll run through the easier option first. This is using the D1 Mini module. Go to Tools  Board menu and select the “WeMos D1 R2 & mini” entry. There are no additional libraries to install, as the basic WiFi feature libraries are installed with the ESP8266 processor add-on. Using a micro-USB cable, plug the D1 Mini module into your PC. Check the ports under Tools  Ports to see that the driver is installed and select the port. If it is not installed, the driver can be downloaded from https://wiki. wemos.cc/downloads Open the .ino sketch file (downloaded in a ZIP from the SILICON CHIP website) and select Sketch  Upload. If everything completes successfully, you can jump ahead to the Setup section. Using the ESP-01 module The ESP-01 module is less than half the size of the D1 Mini, which means it can’t fit a lot of the nice features of the larger board (such as the onboard USB/serial converter). Still, it isn’t too difficult to build a rig for programming this tiny module. You will need a separate USB/serial converter with a 3.3V supply output, as the ESP-01 will not like 5V! We have a suitable device in our SILICON CHIP Online Shop (Cat SC3437). See Fig.5 for the connection diagram. Connect one male/female jumper lead to each of the ESP01’s pins except for GPIO2 and run the other end to one edge of the breadboard. We used red for VCC, black for GND, orange for TX, yellow for RX, green for RST, blue for GPIO0 and mauve for CH_PD (“Power down”). The mauve lead for CH_PD can connect to the same row as VCC, as CH_PD needs to be pulled up to Vxx for the module to do anything. Connect another four male/female jumper leads to the http://arduino.esp8266.com/stable/package_esp8266com_index.json Fig.6: before you can install the ESP8266 Board file, you need to tell the Arduino IDE where to find it. You do that in the Preferences dialog, as shown here. 62 Silicon Chip Fig.7: this shows how you install the ESP8266 “core” files in the Arduino Board Manager. That lets you compile and upload code to ESP8266-based boards, including the ESP-01 and WeMos D1 Mini. Celebrating 30 Years siliconchip.com.au USB-serial converter and then connect these to Our ESP-01 the ESP-01 via the breadboard as follows: red programming rig, lead to 3.3V, black to GND, orange (from TX corresponding to on the ESP-01) to RXD on the USB/serial conFig.5. It looks a bit verter, and similarly, yellow (from RX on the complicated but ESP-01) to TXD on the USB/serial converter. really there isn’t much to it apart The ESP-01 module needs some pull-up refrom a number of sistors for correct operation, so connect a 10kΩ jumper leads. resistor from the VCC row to the RST row (between the green and red jumper leads), and another 10kΩ resistor from the VCC row to the GPIO0 row (between red and blue jumper leads). See Fig.5 for details. Finally, using male-to-male jumpers, add flying leads to the RST and GPIO0 rows (green and blue). These are now our reset and programming jumpers and we plug them into the GND row to activate them. If you prefer, you could even fit some tactile pushbuttons to the breadboard if you are going to be using this setup more than once. To reset the ESP-01, touch the green lead to GND. To enter UART upload (programming) mode, hold the blue wire against GND, then briefly touch the green wire to GND, then release the blue again. Usually, the blue LED on the module will blink on wire. You can even touch the green wire to the blue wire and off in “run” mode, but will only briefly flicker once (while it is against GND) to ensure everything happens in in programming mode, so if the LED is blinking, you may not be in programming mode. the right sequence. We’ve found that uploading to the ESP8266-based deAnother trick you could use if you want to make the setup more permanent is to (carefully!) glue the sockets that vices is sometimes less reliable than other devices, so it are plugged into the ESP-01 together to form a single large may simply be a case of trying a few times. socket which can be removed as one piece from the ESP01. If you are using a cyanoacrylate type adhesive (Super Set-up With the sketch completely uploaded to the device, Glue), gently separate the sockets to allow the glue to penetrate, apply a small amount away from the ESP-01, then open the Serial Monitor at 9600 baud (you can do this via the Tools menu or, in Windows, the key combination firmly squeeze them together until the glue takes. Now, having built our programming rig, we can upload CTRL+SHIFT+M). The program will transmit its current baud rate at 9600 baud before running, so if you see a differthe code to the ESP-01. Click Sketch  Upload in the IDE and while the sketch is ent number or garbage output in the Serial Monitor, check the displayed baud rate and use that instead. compiling, touch GND to the blue wire, then green and then Within a few seconds, there should be a stream of data release green and release blue. The ESP-01 will stay in programming mode until it is reset or a sketch is successfully on the screen, similar to Fig.8. The lines beginning with “$GPRMC” and “$GPGSA” are our emulated GPS (NMEA) uploaded, in which case it will run the uploaded sketch. Any errors will appear in the bottom pane of the Ardui- data, while the “$ESP82” lines are debugging informano IDE window. If you get errors like “error: espcomm_up- tion so that we can follow what our NTP-based GPS Time load_mem failed”, this is because the computer cannot send Source is doing. These two groups of three lines show the data from a data to the ESP-01. In this case, try the blue/green sequence Fig.8: sample output from the completed NTP/GPS Adaptor unit. The GPRMC, GPGGA and GPGSA sentences mimic those produced by a GPS receiver (hence the GP prefixes) while the ESP82 sentence contains our debugging data. You can see that the unit acquired a WiFI IP address (192.168.43.252) between the first and second instances. siliconchip.com.au Fig.9: the NTP GPS Source set-up menu. Celebrating 30 Years April 2018  63 Parts list – NTP Time Source 1 D1 Mini ESP8266 module [eg, Jaycar XC3802] or 1 ESP-01 module [eg, Cat SC3982 & Altronics Z6360] To program the ESP-01, add: 1 USB/serial converter [eg, SILICON CHIP Online Shop Cat SC3543] 1 small breadboard 2 10kΩ 0.25W resistors (1% or 5%) 2 male-to-male jumper wires (to suit breadboard) 11 male-to-female jumper wires (to suit breadboard) configured module that has already connected to a WiFi network. You can see that it updates its time, latitude and longitude between the first and second group of sentences (it really is that quick!). You will probably see something that looks like the first group repeated, as your unit will not be connecting to a WiFi network just yet. To enter the configuration menu, type “~” and press enter on the Serial Monitor. You will need to have the serial monitor set up to produce carriage return (CR) or carriage return/line feed (CR/LF) at the end of each line, as the menu looks for CR on some commands; this happens by default. The menu will appear, as shown in Fig.9. You will need to configure options three and four to suit your local WiFi network by pressing “3” and Enter, followed by your WiFi network’s SSID name and then press Enter. Then type “4” and Enter, followed by the password and Enter. As you might imagine, the password is saved in a nonsecure fashion and can easily be viewed by anyone who has access to the module, so be careful who you give it to. The NTP server and dummy coordinates should not need to be changed but this can be done in a similar fashion if necessary. The dummy coordinates correspond to Sydney, so should be a good default if you are using the High Visibility 6-Digit LED GPS Clock in Victoria, NSW or ACT. If you are using the GPS-synchronised Analog Clock Driver, these don’t matter, as the time zone is set in the PIC on the driver PCB. In any case, the unit should get a reasonably accurate latitude and longitude from the IP web service. It’s only in the case that this fails that the defaults are used. Finally, press “9” and then Enter to save, then press the reset button on the side of the D1 Mini to load the new defaults. You should see a valid IP address appear after the second comma of the “$ESP82” sentence if the WiFi connects successfully. See Fig.8 for more detail on this. Checking the “$GPRMC”, “$GPGGA” and “$GPGSA” sentences should reveal valid data, including the current (UTC) time and date after “$GPRMC”. If everything seems to be working here, we can connect it up to our clock. The D1 Mini also has an onboard LED to help set-up and troubleshooting. While it is looking for a WiFi network at start-up, the LED is on solidly – so if the LED lights up and never goes out, the unit is not connecting to your WiFi network. After this, the LED will blink every time data is transmitted, which should be once per second. 64 Silicon Chip Configuring the ESP-01 As well as needing the right combination of pull-ups and pull-downs to be programmed, the ESP-01 will also need to be configured to run correctly before connecting it to a clock. Fortunately, this is easily done by adding solder bridges to some of the pins to connect them to the correct voltage levels. Soldering the pins simply prevents the ESP-01 from entering programming mode, so it can still be configured via the setup menu on the serial port if necessary. Carefully run a bridge of solder between the 3.3V pin (the top left pin when looking at the top of the board, pins at the top), CH_PD, RST, GPIO2 and GPIO0 (the four centre pins). This effectively forces the ESP-01 into run mode every time it is powered up. We found it easiest to bridge out the four centre pins, then tilt the module to allow the bead of solder to reach the 3.3V pin. See the photo below for how the ESP-01 should look after it has been bridged. Alternatively, you could solder a couple of short lengths of hookup wire to join the pins. To connect it to the clock, plug jumper leads into 3.3V (top left), GND (bottom right) and TX (above GND). From now on, the ESP-01 can be treated like the D1 Mini and these are the only three connections you need. Note that with the ESP-01 module, the blue LED is connected to the TX pin, so it will flicker to indicate data is being transmitted but will not solidly stay on while the module is attempting to connect to WiFi as with the WeMos board. Connecting to the GPS-synchronised Analog Clock Driver There are only three connections needed to work with the GPS-synchronised Analog Clock Driver from February 2017: power (3.3V), ground and the serial data. Remove the battery from the Clock Driver, ensure that JP1 is set to the 3V position, then wire the unit up to the GPS module header. See the top photo opposite for details. This means connecting the 3.3V pin on the D1 Mini to the VCC pin on the clock, Ground (“G”) to GND and TX to TX. We used jumper socket lead off-cuts to wire up our D1 Celebrating 30 Years Bridging the pins of the ESP-01 with solder forces it into “run” mode and prevents it stalling in UART upload mode. With a bit of care and a solder sucker, the solder can be removed and the ESP-01 can be reprogrammed if necessary. The pins that are bridged are VCC, CH_PD, GPIO0, GPIO2 and RESET. The copper tracks at the bottom of the PCB are the onboard antenna. siliconchip.com.au Mini, so it’s easier to remove in future if necessary (for example, if we need to configure it to a different WiFi network). You’ll see in the photo that we’ve actually connected 3.3V to the “EN” pad on the PCB, because it’s directly connected to VCC on the reverse of the clock PCB and it means that the wires don’t need to cross. Set the hands of the clock to 12, and re-insert the cells. You should see the STARTUP LED on the Clock Driver flash once, then twice as it powers up the D1 Mini. If the D1 Mini’s LED does not light, check the wiring connections. The LED should go out again in a few seconds as it connects to WiFi, and provided the Internet connection is good and the NTP servers are online, the STARTUP LED on the Clock Driver will flash four times and it will start doublestepping towards the correct time. Connecting to the High Visibility 6-Digit LED GPS Clock The set-up for the High Visibility 6-Digit LED GPS Clock, from the December 2015 and January 2016 issues, is similar to that for the GPS-synchronised Analog Clock Driver. Because our prototype unit did not have a 1PPS output (and it is not required by the clock), we did not connect it. If you need it, it’s on pin D2 of the Mini. Ensure that LK1 is set to the 3.3V position and connect +V to 3.3V on the D1 Mini, GND to G and TX to TX, as shown in the photo below. Power on the clock, and after a few seconds, you should see the GPS display indicating that the clock is awaiting a valid GPS signal. Once valid data has been received by the clock, it will go to the normal clock display. Since this clock uses latitude and longitude to set the time zone, incorrect data here may lead to an incorrect time being displayed. If you find the time is incorrect (especially if it is out by a whole number of hours), the NTP-based GPS Time Source may not be providing correct latitude and longitude data, in which case the dummy values may need to be changed. The NTP-based GPS Time Source updates its time from the NTP servers every hour. We’ve tried to program it so that it won’t “jump” when that happens; in fact, you probably won’t notice it. But for the first few hours, as the drift compensation will not be operational yet, you may notice an occasional slight glitch in the time. Only three wires are needed to connect the D1 Mini to the GPS-synchronised Analog Clock Driver (Feb 17) and although slightly larger than the GPS Module, the D1 Mini is a great fit for the PCB. the US government, and it was a server we tried while testing the NTP-based GPS Time Source. In practice, because the servers are so far away, the round-trip time for our data was too long to maintain accuracy, so a closer server was chosen at pool.ntp.org This server is actually a large number of servers around the world and the internet’s DNS system tries to point you to a nearby server. The server can be changed via the configuration menu by pressing “5” and enter, then entering the URL or IP address of the new server. SC Using it with other clocks and NTP servers Any device that uses GPS data as its clock source should be able to use this project. With NTP being used so widely, the time provided is quite accurate and the three connections for power, ground and data are usually easy to find. The default NTP server used in our sketch is actually provided by volunteers who donate their server time to make NTP widely available. See http://www. pool.ntp.org/en/ for more information. As always, servers on the internet may come and go, and there are alternatives. For example, nist.time.gov is provided by siliconchip.com.au Using a row of header pins makes it much easier to connect our completed unit to the clock. In this case, we only need three pins. Celebrating 30 Years April 2018  65