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WiFi Time Source for GPS Clocks The Raspberry Pi Pico W can be used as a substitute for GPS modules in existing time keeping designs, for when you can’t get a reliable GPS signal. It gets the time from an internet NTP server via WiFi and is accurate to a fraction of a second. Project by Tim Blythman S ince GPS modules have been affordable for the hobbyist, we have used them as accurate time sources. While GPS (and other similar satellite systems) has revolutionised navigation and mapping, it also provides easy global access to highly accurate time sources. Each GPS satellite is equipped with two atomic clocks and they transmit a very accurate time signal every second. We have used that signal for many projects to date, including the recent, very popular GPS Analog Clock Driver from September 2022 (siliconchip.au/ Series/391). While GPS was the first GNSS (global navigation satellite system), there are now several more, including the Russian GLONASS, European Galileo and Chinese Beidou systems. The Indian Regional Navigation Satellite System (IRNSS) and Japanese Quasi-Zenith Satellite System (QZSS) are designed to improve positioning on a national scale, with the QZSS also benefiting Australia as the satellites’ orbits bring them over us. While they use subtly differing technologies (even GPS has evolved over its 50-year existence), a common external interface has been established. In fact, the VK2828U7G5LF GPS module that we use for many projects can receive signals from GPS, GLONASS and Galileo satellites. For the purposes of this article, we’ll use GPS as an encompassing term for all the different navigation satellite systems. However, note that some of these systems are not truly global, as the satellites do not usually provide coverage at high latitudes (close to the poles). Previous GPS Time Source In the April 2018 issue, we published the Clayton’s GPS Time Source (siliconchip.au/Article/11039). As the name suggests, it doesn’t use any GPS What projects does it work with? New GPS-Synchronised Analog Clock, September 2022; siliconchip.au/Article/15466 GPS-Synchronised Analog Clock, February 2017; siliconchip.au/Article/10527 High-Visibility 6-Digit LED GPS Clock, December 2015 – January 2016; siliconchip. au/Series/294 6-Digit Retro Nixie Clock Mk2, February – March 2005; siliconchip.au/Series/282 6-Digit GPS-Locked Clock, May – June 2009; siliconchip.com.au/Series/37 60 Silicon Chip Australia's electronics magazine technology, but rather it can be used as a source of GPS-like time signals when an actual GPS signal is unavailable. It’s often recommended as a replacement for a GPS module in clock projects. The motivation for this concept was driven by many clocks being used indoors, where very weak GPS signals are hard to receive. On the other hand, WiFi signals are usually available indoors. The actual hardware of the 2018 unit is simply a D1 Mini WiFi ESP8266 microcontroller module. The module is programmed with firmware to connect to a WiFi network and update an internal clock from the internet using NTP (network time protocol). This time is then used to generate ‘sentences’ to communicate that time. A 1PPS signal is also generated, although this signal will not have the precision of an actual GPS module. Pico W update This project is an update of the original Clayton’s GPS but using a Raspberry Pi Pico W instead of a D1 Mini. While we could have refactored the same code for the Pico W GPS, there are several reasons why we did not. We have had many suggestions for improvements over the last five years, siliconchip.com.au WiFi Time Source Features Delivers NMEA 0183 data simulating a GPS time source Adjustable baud rate 3.3V logic levels work with 3.3V and 5V systems Synthesised 1PPS signal Gets the time from NTP servers via WiFi Generates estimated latitude and longitude based on IP address Can also output fixed dummy coordinates Can scan for up to eight WiFi networks (SSIDs) Configurable via virtual USB serial port, independent of data stream Uses a compact & low-cost Raspberry Pi Pico W module Integrated buck/boost converter runs efficiently from 1.8-5.5V Crystal oscillator offers better than 30ppm accuracy between updates Draws 50mA, or up to 100mA during WiFi transmissions (3.0V supply) so it made sense to incorporate them where possible. We’ve chosen to use the C SDK as we found it gave us better access to low-level functions and programs run more quickly. Some of the new features were possible (and much easier to implement) due to aspects of the C SDK and its software libraries. There is no doubt that the Pico W is very well priced, making it an attractive option when the module is all or most of the hardware required. Indeed, it is cheaper than the GPS module it can replace. But particular features of its RP2040 microcontroller helped us to create the WiFi Time Source. For example, it can implement a virtual USB serial port, meaning that the configuration menu is separate from the NMEA data stream (National Marine Electronics Association). Due to the nature of the serial port on the D1 Mini, these were shared on the Clayton’s GPS, so using the configuration menu interrupted the data stream. The Pico W also implements a virtual USB drive for flash memory programming. Some people had difficulty uploading to the flash memory to the D1 Mini for various reasons. For example, it requires either a dedicated siliconchip.com.au programming application or the Arduino IDE for programming. On the other hand, the Pico W can be flashed by just about any computer with a USB port. The process is as simple as copying the file to the virtual USB drive. The RP2040 processor on the Pico W has two cores, so one can be dedicated to sending out the NMEA data and not be blocked by activity on the other core, which handles the configuration and WiFi connections. The Pico W also has an onboard switchmode regulator that’s more efficient than the linear regulator found on the D1 Mini. Some readers reported problems powering the D1 Mini, so it is a welcome upgrade. It not only reduces the current requirement at higher supply voltages but also enables operation from supplies as low as 1.8V. Like the earlier time source, the WiFi Time Source emits three NMEA sentences: “RMC” (recommended minimum data for GPS), “GGA” (Fix information) and “GSA” (satellite data). Most of our GPS clock designs only use the RMC sentence, with some also using GGA. So this data is entirely adequate for driving those clocks. NMEA sentences Practically all GPS modules deliver data generally in accordance with the NMEA 0183 standard. The standard actually specifies 4800 baud serial data using a balanced signal complying with the RS-422 electrical standard. The newer NMEA 2000 standard uses a CAN bus network at 250kiB per second. The full contents of this standard are not publicly available, so the simpler NMEA 0183 is still widely used, as it is well understood. Most receivers nowadays use single-­ ended logic level signals (typically 3.3V) with baud rates of 9600 or even higher. Many modules also offer a 1PPS (pulse per second) signal that is synchronised to the satellite atomic clocks. The serial data consists of lines of ASCII characters called sentences. For our purposes, each sentence is marked at the start by a “$” character, followed by two characters that identify the ‘talker’. This is typically “GP” for GPS systems, although we have seen some modules that use “GN” where data from multiple satellite systems are combined. Australia's electronics magazine The next three characters identify the type of message, followed by sentence-­specific data and a checksum code to provide a degree of protection against corrupted data. The most common sentences that encode the time also contain location data, so the WiFi Time Source can produce dummy location data or even use an IP address geolocation data service to generate an approximate location. In any case, it’s a good idea to generate such data in case the receiving device expects there to be valid data in this location, even if it is not used. This approximation will never be good enough for navigation purposes. Still, it is usually sufficient to determine a timezone, which is ideal for those clocks that use GPS location data for this purpose. For example, the High Visibility 6-Digit LED GPS Clock from December 2015 and January 2016 (siliconchip. au/Series/294) uses location data to set the time zone and daylight savings rules automatically. With most of our GPS projects using the GPS signal for clock timekeeping, the WiFi Time Source is well-suited for use with indoor clocks, where they may not have a view of the sky and thus to a GPS signal, but can easily be connected to a WiFi network. Hardware The WiFi Time Source hardware is minimal. The dashed box in Fig.1 shows the pinout of the Pico W after it has been programmed. The remainder of Fig.1 shows the full map of all the pins with their features. As you can see, we’ve kept all the useful pins at one end. It would have been nice to be able to shorten the board by cutting off unnecessary section. Unfortunately, the entire board is needed and it can’t be made much smaller, especially as the WiFi antenna is at the end opposite the USB connector. The power pins are fixed on the right-hand side, near the USB connector. These are pin 40 (VBUS), pin 39 (VSYS) and pin 38 (GND). There are actually several GND pins (see Fig.1), but pins 3 and 38 are closest to the other important pins. Pin 37 (3V3_EN) is an input to the regulator on the Pico W; this is kept high by a 100kW resistor but can be pulled low to shut down the regulator and thus power off the Pico W. June 2023  61 Pins used for the WiFi Time Source Fig.1: the pins on the Pico W that can be used for the WiFi Time Source are shown in the dashed red box. Pin 1 (GP0) is the closest UART TX pin to the USB end of its PCB and is also near the relevant power pins. You probably won’t need all the connections shown here for most clock projects (see Figs.3-6); three or four connections are often sufficient. Pin 1: serial NMEA data; pin 2: 1PPS signal; pin 3: ground; pin 36: 3.3V; pin 37: 3.3V enable (active high); pin 38: ground; pin 39: 1.8V to 5.5V in; pin 40: USB supply. Pin 1 (GP0) is the source of the generated NMEA serial data, which idles at a 3.3V logic high level. The Pico W’s hardware UART (universal asynchronous receiver/transmitter) peripherals are only available on specific pins. This pin was chosen as it is the UART TX pin closest to the USB connector and the power pins. We selected the adjacent pin 2 (GP1) for the 1PPS output; it could have been any of the remaining GPIO pins. We’ve shown the 3.3V output only because it might be handy if you need a regulated 3.3V supply for your project. The regulator on the Pico W can deliver up to 2A, although some of that is used by the Pico W. Fig.2 shows the power circuitry of the Pico W and will help you decide how to connect the WiFi Time Source in your circuit. Most people will simply need to connect a supply between the VSYS and GND pins. But note that there is a diode between VUSB and VSYS, so if a USB cable is connected, it might feed into VSYS, particularly if VSYS is less than the 5V from USB. Unless you can be sure you won’t connect anything to VSYS while power is applied to VUSB (for example, via 62 Silicon Chip the USB socket), the safest option will be to connect the incoming supply to VSYS via a schottky diode, which will prevent current from passing from VBUS into your supply. Given that most people will use the USB port to program, configure and test the Pico W, the easiest solution is to disconnect the USB cable before connecting to the target circuit. In that case, direct connections to the Pico W pins will be fine. Later on, we’ll also show you how to connect the WiFi Time Source to some of our recent clocks. Software development The Raspberry Pi C SDK is still evolving, especially the parts of it that deal with the WiFi features of the Pico W. But it is well documented, and interest is sufficient that the online community is also very helpful. So, we ran into some minor difficulties during development, but we managed to work around them. We used version 1.5.0 of the SDK; versions before 1.4.0 did not support the Pico W and later versions might differ. As we noted, the Pico W has two processor cores. One of these (the second core) is programmed to do nothing Australia's electronics magazine more than generate the NMEA data and 1PPS pulses. This is crucial as we found that the D1 Mini (as used in the 2018 time source) would occasionally block (be busy and not be able to run other parts of its program) during WiFi operations. By setting up one core to do the critical activity, the WiFi Time Source can continue to operate, even in the extreme event that one processor core crashes entirely. This core can even reset the Time Source under some conditions. When a reset happens, some data is stored in RAM to preserve the current time across the reset. This is possible as RAM remains powered during the soft reset process. We saw very occasional crashes (and reset recovery) when the Time Source had been active for long periods, but this should not be an issue for operation with the recent GPS clocks, as the Time Source should only be powered long enough to set the time, after which it is powered off. This second processor core looks at the current time and calculates what the time will be when the next second rolls over. It then prepares all its data to suit this next second. As soon as the siliconchip.com.au Fig.2: the power supply circuit of the Pico W, shown here in case you wish to adapt the WiFi Time Source to a different application. For example, consider adding a diode feeding into VSYS to prevent VBUS power from feeding into your power supply if a USB cable is connected. second rolls over, the data is sent, and the 1PPS signal is pulsed. This means that the NMEA data and 1PPS pulses are delivered with minimal jitter. Providing the output as the second rolls over means that the fractional data can be ignored, simplifying the code slightly, both for us and potentially for any device receiving that data. The other core has the vital role of periodically getting an accurate value for the time and collecting the other data that is needed. One of these is a ‘validity’ flag, equivalent to the GPS ‘satellite lock’ that should always be checked to ensure valid data is being received. The Pico W implements an internal 64-bit counter with microsecond resolution. This counts up from zero when the processor starts or is reset. The documentation jokes that (in the vein of the Y2K or Millennium Bug) this will eventually cause a year 5851444 bug. Such bugs typically occur when a counter rolls over beyond its maximum value. While we are not too concerned about this particular counter, we need to be aware of a few other such bugs. We have a separate small article starting on page 70 that explains these ‘gotchas’. The main role of the software running on the first core is to fetch an accurate timestamp from the NTP servers. This timestamp is compared with the current value of the 64-bit counter, and an offset is used to calculate the actual siliconchip.com.au time (at any time) by simply adding the current value of the 64-bit counter. The RP2040 processor in the Pico W has an internal real-time clock peripheral, but this only has a resolution of one second, so we can’t really use this to keep time accurately. However, we set it and use it in places where it is accurate enough, such as reporting time in human-readable form on the configuration interface. The first core also provides a virtual USB serial port that is used to print an interactive menu with the help of a serial terminal program. This can be seen in Screen 1; we’ll look more closely at the options later. The menu allows up to eight SSIDs (WiFi networks) to be set. The software will automatically cycle through these networks until it successfully connects to one. It will attempt to reconnect if the connection is lost. Since many applications of the Time Source depend on it only being turned on briefly (to save battery power), the initial behaviour is to perform a network scan to ensure that the first attempted connection is to an available network. The virtual serial port also produces status information, mainly concerning the WiFi status and time since the last NTP update. One of the menu Time is 22:43:01 on 14/02/2023. NTP OK. Last updated 0 minutes ago. WiFi Status: Connected with IP: 192.168.130.140 Menu: 1 : Scan networks 2 : Show saved 3n : Delete SSID (n from saved list) 4n : Set SSID (n from scan list) 5 : Manual SSID 6n : Set Password (n from saved list) 7 : Test saved 8 : Save to flash 9 : Set Country Code (currently XX) A : Set IPAPI URL (ip-api.com/line?fields=lat,lon) B : Set Latitude (−27.467899 = 27°28’4”S) C : Set Longitude (153.032501 = 153°1’57”E) D : Set baudrate (9600 baud) E : Set Talker (currently GP) F : Set NTP validity timeout (200 min) G : Set NTP server (pool.ntp.org [139.99.222.72]) H : Set default year (2022) I : Turn debug on (currently off) J : Reboot Clayton’s Pico W GPS Time Source Screen 1: many options are available to configure the WiFi Time Source. At a minimum, you will probably need to use commands 1, 4, 7, 8 and 9 to set the country code and connect to your WiFi networks to operate it with our GPS clocks. Other commands could come in handy depending on your application. Australia's electronics magazine June 2023  63 Table 1 – WiFi Time Source configuration commands Comm. Function Notes 1 Scan networks and display a list in order of decreasing RSSI Channel and authentication are also listed. The number shown in column n is used for Command 4. 2 Show the current network list The list is active but may not reflect the contents in flash memory unless a save has been completed. 3n Delete item n from the list shown by Command 2 4n Add network n from Command 1 Also prompts for a password. If all slots are full, an error is printed and you will need to use Command 3 to free a slot. 5 Enter a network name manually 6n Enter the password Shouldn’t need to be used unless you need to for a network, using change a password. n from the list shown by Command 2 7 Test networks in the list Scans through the list and attempts to connect to each network in turn. This can take a while and success is only reported if an IP address is obtained. 8 Save all settings to flash memory It’s a good idea to reboot after this to ensure that all settings are reloaded correctly. 9 Set two-letter country code Only loaded on boot, so reboot after setting this and using Command 8 to save. A Set IP to lat/lon API URL This should return two lines of text with decimal latitude on one line and longitude on the next. Set URL to blank to disable. B Set default latitude Enter in the decimal format. C Set default longitude Enter in the decimal format. D Set NMEA baud rate The default is 9600, but any rate between 300 and 921600 can be used. E Set Talker code The default is “GP”, but it can be any two characters. “GP” works for all our clocks. F Set NTP validity timeout in minutes The longest period for which the time can be considered valid without a (typically hourly) NTP update, from 60min to 50000min (about a month). G Set NTP server URL The default is “pool.ntp.org”, which automatically redirects to a geographically nearby server. Others can be used, such as “time.nist.gov”. The IP address may not be correct until a network is connected. H Set default year The year used at boot when no other time data is available, from 1970 to 4095. See the separate article on the Y2K38 bug for why this is important. I Toggle debugging Can be used to check and debug the NMEA NMEA data output to data. This setting is saved in case you need USB serial port this data to always be available on the USB serial port. J Reboot Pico W 64 Silicon Chip It’s recommended to reboot after saving settings to ensure that all settings are reloaded at boot time. If you hold the BOOTSEL button while rebooting, you can use electronicsmode. magazine this methodAustralia's to enter bootloader options allows the NMEA data to be dumped to the virtual serial port for easy debugging. The first core is also responsible for controlling the Pico W’s inbuilt LED, which is used to flash a status indication. The LED is switched on solid when power is applied, indicating that the Time Source is powering up correctly. It can also flash once, twice or three times per second. One flash means it is connected to a WiFi network, while two flashes indicate that the time is considered to be correct. Three flashes occur when both those conditions are true. In general, the time is correct if an NTP update has been received in the last few hours, although this limit can be adjusted. The crystal oscillator used on the Pico W has a 30ppm tolerance, meaning it could drift by up to one second every eight hours. In practice, we saw NTP adjustments up to 200ms, so we’re confident that the time will be accurate within half a second with the default settings. Programming the Pico W It makes sense first to program the Pico W and check that it is working as expected. Hold the BOOTSEL button on the Pico W and plug it into your computer. A USB drive named “RPI-RP2” should appear. Copy the NEW_CLAYTONS_1.UF2 file to it; after a second or so, the LED should come on. You can then use a serial monitor program to access the menu. We use TeraTerm on Windows, while minicom can be used on Linux systems. Open the Pico W’s virtual serial port to access the interactive menu. Ensure that your terminal program uses CR or CR+LF as its line ending. Since it is a virtual serial port, the baud rate is unimportant, and any baud rate setting should work. Basic setup All commands should be followed by Enter. The Pico W implements country codes to ensure that the correct (legal) WiFi channels are used for communication. The default “XX” setting is a subset that is safe worldwide but does not allow the use of some WiFi channels. So it should work but might not be optimal. siliconchip.com.au It’s a good idea to set this to your country. Use command 9 (followed by Enter) and enter a two-letter country code (AU, NZ, US, UK etc), then save the settings with command 8 and reboot the Pico W with the J command. Editor’s note: the codes should be in the alpha-2 format, see: https://w. wiki/4kP Reconnect to the Pico W if necessary; TeraTerm usually does this automatically. Now use menu option 1 to run a WiFi scan; this should complete within a second or so. The networks are listed in order of RSSI (signal strength), so you should find your SSID near the top. Note that commands listed with an n suffix take a second numeric argument. For example, if your network appears first (next to number 0), enter command 40. You will then be prompted for the password for this network; type it in and press Enter. You can enter multiple networks without rescanning. If your network doesn’t appear, use command 5 to enter the name manually, and you will be prompted for the password too. Command 6 on its own is used to change or set a password if, for example, you have entered it incorrectly. Then try command 7 to test the saved networks. You should see a message saying “Connected with IP”, followed by an IP address for each SSID. If not, try again. If you get an “Auth failed (password?)” message, the password entered may not be correct; you can use command 6 to re-enter it. The serial port will print updates around every 15 seconds if nothing has been entered on the serial port; this is to prevent updates from interfering with your configuration process. If all is well, use command 8 to save the settings to flash and reboot again to ensure that the settings are loaded. This is necessary as some parameters can only be set once, and the easiest way around this is to reboot the device. This should be the minimal amount needed to set up the WiFi Time Source to work with most of our clocks. A detailed list of commands, along with their use and purpose, is shown in Table 1. Screen 2 shows the typical responses to the more common and complex commands. Most other commands require a simple response and will report a message if there is a problem. siliconchip.com.au Screen 3 shows the typical progression at startup, although events may not occur in this order. You might also see a much larger NTP adjustment; that is normal. You can toggle the printing of GPS sentences over the USB serial port by using the I command. Screen 4 shows this; naturally, your data might be different. If you have a PC program that can process GPS data, you can use it to verify the data. Connecting it to a clock The WiFi Time Source could feasibly connect to just about any system that expects logic level NMEA 0183 data; however, its lack of accurate speed and location data means it is not the best choice in some cases. We don’t recommend using it as the source for any of our GPS-based frequency references; the 1PPS signal provided by this time source is not intended to have the necessary precision. And since it only ever gives a speed of 0 knots, it won’t be much use in the GPS Finesaver (siliconchip.au/ Article/11673). Screen 2 (right): this edited screen dump shows the output of some of the more complex commands. Note that these commands have been issued in the order shown, to add and then remove an SSID. Commands 3 and 4 require a second parameter which is a number printed by commands 2 and 1 (respectively) issued prior. ----------------------------------Command 1 ----------------------------------1 Scanning Scan complete Scanned network list: n SSID RSSI Chan Auth 0 AndroidAP4AA0 −44 1 PASS 1 APV Admin Only −65 3 PASS 2 APHV Conference −66 3 PASS 3 TPW4G_ZeB426 −82 11 PASS 4 WiFi-5E5EE1 −84 8 PASS 5 NTGR_4E0C −93 11 PASS ----------------------------------Command 43 ----------------------------------43 2 TPW4G_ZeB426 Added OK Enter password. PASSWORD password saved. ----------------------------------Command 2 ----------------------------------2 Saved network list: 0 AndroidAP4AA0 1 Tim 2 TPW4G_ZeB426 ----------------------------------Command 32 ----------------------------------32 SSID deleted. Saved network list: 0 AndroidAP44A0 1 Tim ----------------------------------Command 7 ----------------------------------7 Testing networks. 0 AndroidAP4AA0 >connected with IP:192.168.208.140 1 Tim >SSID not found 2 Networks tested, 1 OK Time is 04:01:30 on 13/02/2023. NO NTP. Connect failed Connecting to 0 AndroidAP4AA0 Skip IPAPI fetch, no WiFi. **** NTP adjustment: 11953 **** Connected with IP: 192.168.130.138 Time is 04:01:45 on 13/02/2023. NTP OK. IPAPI start. Headers of 170 bytes report 18 bytes of Received 18 bytes. HTTP finished:200 OK Lat/Lon=−27.467899,153.032501 Time is 04:02:00 on 13/02/2023. NTP OK. Time is 04:02:15 on 13/02/2023. NTP OK. Time is 04:02:30 on 13/02/2023. NTP OK. Time is 04:02:45 on 13/02/2023. NTP OK. Time is 04:03:00 on 13/02/2023. NTP OK. Time is 04:03:15 on 13/02/2023. NTP OK. Time is 04:03:30 on 13/02/2023. NTP OK. Time is 04:03:45 on 13/02/2023. NTP OK. Time is 04:04:01 on 13/02/2023. NTP OK. Last updated 0 minutes ago. content. Last Last Last Last Last Last Last Last Last updated updated updated updated updated updated updated updated updated 0 0 0 1 1 1 1 2 2 minutes minutes minutes minutes minutes minutes minutes minutes minutes ago. ago. ago. ago. ago. ago. ago. ago. ago. Screen 3: the last few lines on this screen (using the TeraTerm serial terminal program) show that the WiFi Time Source has connected to WiFi and updated its time from NTP servers. The previous lines are typical of what might be seen on a normal startup. Australia's electronics magazine June 2023  65 $GPRMC,050215.000,A,2728.0004,S,15301.0057,E,0.00,000.00,130223,,,*3F $GPGGA,050215.000,2728.0004,S,15301.0057,E,1,04,1.0,0.0,M,0.0,M,,*78 $GPGSA,A,3,,,,,,,,,,,,,1.00,1.00,1.00,*2F $GPRMC,050216.000,A,2728.0004,S,15301.0057,E,0.00,000.00,130223,,,*3C $GPGGA,050216.000,2728.0004,S,15301.0057,E,1,04,1.0,0.0,M,0.0,M,,*7B $GPGSA,A,3,,,,,,,,,,,,,1.00,1.00,1.00,*2F $GPRMC,050217.000,A,2728.0004,S,15301.0057,E,0.00,000.00,130223,,,*3D $GPGGA,050217.000,2728.0004,S,15301.0057,E,1,04,1.0,0.0,M,0.0,M,,*7A $GPGSA,A,3,,,,,,,,,,,,,1.00,1.00,1.00,*2F $GPRMC,050218.000,A,2728.0004,S,15301.0057,E,0.00,000.00,130223,,,*32 $GPGGA,050218.000,2728.0004,S,15301.0057,E,1,04,1.0,0.0,M,0.0,M,,*75 $GPGSA,A,3,,,,,,,,,,,,,1.00,1.00,1.00,*2F $GPRMC,050219.000,A,2728.0004,S,15301.0057,E,0.00,000.00,130223,,,*33 $GPGGA,050219.000,2728.0004,S,15301.0057,E,1,04,1.0,0.0,M,0.0,M,,*74 $GPGSA,A,3,,,,,,,,,,,,,1.00,1.00,1.00,*2F $GPRMC,050220.000,A,2728.0004,S,15301.0057,E,0.00,000.00,130223,,,*39 $GPGGA,050220.000,2728.0004,S,15301.0057,E,1,04,1.0,0.0,M,0.0,M,,*7E $GPGSA,A,3,,,,,,,,,,,,,1.00,1.00,1.00,*2F $GPRMC,050221.000,A,2728.0004,S,15301.0057,E,0.00,000.00,130223,,,*38 $GPGGA,050221.000,2728.0004,S,15301.0057,E,1,04,1.0,0.0,M,0.0,M,,*7F $GPGSA,A,3,,,,,,,,,,,,,1.00,1.00,1.00,*2F $GPRMC,050222.000,A,2728.0004,S,15301.0057,E,0.00,000.00,130223,,,*3B $GPGGA,050222.000,2728.0004,S,15301.0057,E,1,04,1.0,0.0,M,0.0,M,,*7C $GPGSA,A,3,,,,,,,,,,,,,1.00,1.00,1.00,*2F Screen 4: the I command sends GPS sentences to the virtual serial terminal so that you can confirm the data being produced. This setting can be saved to flash, so the GPS data continues to be sent to the USB virtual serial port even after it reboots. It’s not going to be much use as a navigational aid either, ruling out the Advanced GPS Computer from 2021 (siliconchip.au/Series/366), so we’ll assume you are using the WiFi Time Source with one of our GPS clocks. We have instructions below on using the Time Source with some GPS clock projects we have published over the last ten years. Table 2 also summarises how this Time Source can replace some common GPS modules. Note that these connections may not be optimal, especially for clocks that run on batteries. You might want to experiment with alternative configurations. The suggested wiring for the recent battery-powered clocks is different to Table 2 for that reason. The problem is that the WiFi Time Source has a higher current demand than most GPS modules, and the circuitry sometimes cannot provide enough current to drive it. New GPS-Synchronised Analog Clock – September 2022 The most recent GPS-synchronised clock was published in September 2022 (siliconchip.au/Series/391) and was followed by an update describing how to connect the original Clayton’s GPS Time Source in the November 2022 issue. Like many of our recent GPS projects, it uses the VK2828U7G5LF GPS module. In fact, we recommend this module as a replacement for all the previous GPS modules we have used in clock projects. The VK2828U7G5LF has six connections, but you only need four for the Time Source. The connections are all fairly straightforward, although they don’t all connect to the GPS module header – see Fig.3. The black and blue wires are connected to the obvious points on the GPS module header. The red wire feeds power directly from the battery to the Pico W’s VSYS pin; one of the pins of the ICSP header is ideal for this purpose. Note that we’ve used a header pin for this connection, so power can be disconnected when we connect to the USB socket for programming. This prevents 5V from the USB cable being fed into the battery. With just these three wires, the Pico W would run continuously. So the green wire connects the 3V3_EN pin to pin 7 of IC1 on the Clock PCB. This pin is usually used to control the Clock’s boost regulator. This connects underneath the PCB, as shown in the photo, since it is easier to connect to the corresponding pad. Fig.3: connecting to the New GPS-Synchronised Analog Clock using the 3V3_EN pin on the Pico W makes the most efficient use of the Pico W’s onboard boost regulator, bypassing the Clock’s own boost regulator (the Pico W is shown larger than life size in Figs.3-6 for clarity). In this case, you could omit IC3, L1 and the two 10μF capacitors. 66 Silicon Chip Australia's electronics magazine siliconchip.com.au The WiFi Time Source wired to the New GPS-Synchronised Analog Clock from 2022. To save battery power, the boost regulator on the clock PCB is bypassed; in fact, those onboard components could be left off entirely. The photo at upper left shows the green wire connecting directly to pin 7 of IC1 on the reverse of the PCB. This scheme bypasses the boost regulator on the New GPS-Synchronised Analog Clock, which is possible as the Pico W has its own buck/boost regulator. That also means that if you are building the Clock board from scratch, you can leave off the boost regulator IC and its associated components. With this arrangement, the Pico W will power up even when the battery is down to 2V, the lower limit of the Clock. By that stage, there wasn’t enough voltage to power the blue LED on the Clock, but everything else worked as expected. The photos show the Time Source connected via short lengths of wire and then mounted on the ICs using a pad of double-sided tape. Note how the Pico W’s WiFi antenna is clear of the PCB below. The WiFi Time Source typically takes about 25 seconds to ‘get a fix’, often faster and occasionally longer if it does not connect to the WiFi network immediately. This should be the same with most Clocks that use the Time Source. After powering on the Clock with the Time Source connected, the Clock would flash its LED once or twice, after which the Time Source’s LED would come on and start flashing at the same rate as the Clock LED. After a few more seconds, the LED on the Time Source would turn off, showing that the Clock has obtained the correct time and powered down the Time Source. Generally, the Clock LED should also turn off after half an hour at most (and the clock should start ticking), so if it continues flashing for longer than that, you should investigate. In general, we found that if the data displayed on the USB serial terminal appeared correct, the Time Source would work correctly when connected to the Clock. GPS-synchronised Analog Clock Driver – February 2017 The GPS-synchronised Analog Clock Driver from February 2017 (siliconchip.au/Article/10527) also recommended the VK2828U7G5LF GPS module. Note that we have not tested this arrangement or any of the following arrangements with clocks before 2022. Here we propose a variation that will avoid a small amount of inefficiency by also bypassing the Clock Driver’s boost regulator. Since the Pico W can work from voltages down to 1.8V at VSYS, we take 3V directly from the input of the boost regulator, as shown in Fig.4. GPS clocks from 2015 All the earlier GPS clocks we published used external power supplies, so they should not have any problems due to the limitations of a battery supply. Figs.5 & 6 show how to connect the WiFi Time Source to the 6-Digit Retro Nixie Clock Mk.2 and High Visibility 6-Digit LED GPS Clock, respectively. Note that both use the same header pinout for connections to their respective GPS modules, corresponding to the connections shown in Table 2. For efficiency reasons, the GPS power supply voltage link for these Fig.4: how to connect the Time Source to the GPS-synchronised Analog Clock Driver from 2017. This also bypasses the Clock’s onboard regulator to power the Pico W. Note that we have not tested this configuration. siliconchip.com.au Australia's electronics magazine June 2023  67 Fig.5: connections to the 2015 Nixie Clock. LK1 (which chooses between a 3.3V and 5V supply for the connected module) should be set to the 5V position. Still, this design is not powered by a battery, so efficiency is less critical. Fig.6: the High Visibility 6-Digit LED GPS Clock uses the same header pinout as the Nixie Clock, so the wiring is much the same, as is the choice to set LK1 to the 5V position. projects (LK1 for both projects) should be set to the 5V position, since the Pico W will happily work with 5V at its VSYS input. If you have any problems after connecting the Time Source to one of the other clocks, it is most likely a power problem. Check that the 3V3_OUT pin is near 3.3V. If not, the circuit may not be able to supply enough current for the Pico W. Conclusion The Pico W board provides helpful features in roles like this, such as its integrated buck-boost power supply, dedicated USB peripheral allowing a separate configuration console and good software support. The WiFi Time Source is a natural progression of the original Clayton’s GPS Time Source from 2018 and is 68 Silicon Chip similarly simple and well-priced. The Pico W variant adds extra features, particularly the ability to connect automatically to one of several WiFi networks. At the time of writing, Bluetooth support is in its early (beta) stages, so we will investigate if it is possible to add a Bluetooth interface for configuration. This would be very handy for updating settings as it would remove the need to connect a USB cable. SC Table 2 – Time Source pin mapping compared to GPS modules Pico W VK2828 EM408 Pin 1 GP0 (NMEA data) TxD(4, blue) Tx(4) Pin 2 GP1 (1PPS) 1PPS (6, white) Not connected Pin 3/38 GND GND (2, black) GND(2) Pin 39 VSYS VCC (5, red) V+(5) Pin 40 VBUS Not connected Not connected Not needed EN (1, yellow) EN (1) Not needed RxD (3, green) RX(3) Australia's electronics magazine Notes Not needed for most applications Or another source of 1.8V to 5.5V siliconchip.com.au