I thought it might be interesting to put all the different revisions of “TimeHat” hardware on to a single page, so this is that!

Before I ever made a “Hat” with a GPS and an RTC - I plugged my first DS3231 module onto the Pi's header, and modified a cheap USB GNSS dongle to tap the PPS signal off the LED. This worked surprisingly well, if sat in a window, and was the first step in making me realize that I wanted to make an all-in-one Hat with GNSS receiver and RTC. I called this one “TimePi”.

Early December, 2020

Once I knew I wanted to make something in the Hat form factor, I decided to start with what was already on the market. The Adafruit “Ultimate GPS Hat” was relatively inexpensive, and readily available, I liked that it had a proto-board space for the RTC module, as well as a u.FL connector for adding an external antenna. I found a Pi4 case that looked easily modified, so I expanded it a bit with some standoffs to make room for the Hat, and added a u.FL to SMA pigtail. The Adafruit hat used an MTK3339 GNSS receiver.

December 29, 2020

At this point, I had started talking to friends about what I was working on, and I quickly learned that there was a lot of interest among them in an inexpensive Pi “Time”Hat, so I found an even cheaper standalone MTK3339 module, and some inexpensive Raspberry Pi protoboard hats. I thought the project was going to stop here.

January 7, 2021

Something I realized about the MTK3339 GNSS modules is that… they didn't perform very well. They don't have particularly sensitive receivers, and their PPS wasn't very clean. I was surprised to see that the first “TimePi” (with the USB ublox7 dongle) was regularly outperforming them. I did a bit of research and found that other folks had similar complaints about MediaTek GNSS receivers, so I decided to look elsewhere. I found some nice looking u-blox MAX-M8Q modules that didn't cost that much more than the MTK3339, so I ordered a couple.

This one is the same as above, on the Pi protoboard with the same DS3231 RTC module.

January 20, 2021

I didn't really care for wiring up the Pi protoboards by hand, so I decided that if I was going to make a batch of these, I might as well get PCBs made up. I still planned to use the pre-assembled RTC and GNSS modules, though. Once I saw how inexpensive getting these PCBs made was, as well as how much the prices on the modules and other components fell in bulk - I decided to go a bit further afield.

On February 2nd, 2021, I posted to the Time-Nuts mailing list: https://www.mail-archive.com/[email protected]/msg09397.html

The response was overwhelming - there was tremendous interest! Ultimately I made and sold around 250 of this model. To my knowledge, none have stopped working.

I wrote a bit about the assembly process millerjs.org - More TimeHats!

Feb 2, 2021

I used one v0.1 board and one v0.2 board to make “FYGM interfaces” for connecting to the Nokia FYGM boxes with the LEA-M8T. I drew up a few board designs that included an HDMI port (which the FYGM used to connect to its system) but ultimately scrapped it. Both of these units are alive and well, though.

I made one in March 2021, and the v0.2-based one in early 2022.

In June 2021, I decided to design a Hat using the LEA-M8T. I had a lot of fun experimenting with these modules in the FYGM, so made a Hat with them. I only made two of these. The OLED screen was kind of a shot in the dark - it was a fun idea, but ultimately, kind of silly - these aren't really devices you want a screen on. At the time, there wasn't an affordable source of LEA-M8Ts, and I didn't think that the retail price was worth it, so these never really went anywhere.

June 8, 2021

Right away, I made a revised board. I realized that no one was interested in using the small proto-area (it was just too small) and I also had made a bone-headed mistake of having the RTC module cover up one of the standoff screws. Not the end of the world, but something I wanted to correct. I also liked the idea of having a barrel jack for power, I specifically placed it on the right side of the Hat so that ethernet and power would come out of the same side.

I also had a couple of DS3231 RTC and MAX-M8Q GNSS boards that were physically damaged. The GNSS modules were still functional, so I decided to make up a very small board with the the GNSS and RTC directly soldered.

I ultimately made maybe 10 of these v1 minis, and very few of the v0.2 - at this point I knew that if I wanted to make another big batch of them, I would go with a fully-assembled approach.

For some reason I have only this photo of the v0.2 assembled(bottom right), the mini, v0.1, and v2 are also shown. This particular v0.1 was my personal “tester” for a very long time - I lifted the PPS pad on the GNSS board, thus the blue wire. You can also see the RTC board clipped for access to the standoff screw. Whoops.

May 27, 2021 - TimeHat v0.2

June 22, 2021 - TimeHat mini v1

A couple of folks expressed interest in a TimeHat with PoE capabilities. I found these nice little PoE-48v to 5v modules on Adafruit, and made about ten of these. They were also my first foray into buying new u-blox modules (MAX-8Q) and hand soldering all the SMT parts. I didn't have any problem doing these in small numbers, but definitely didn't want to do more than maybe two dozen.

April 6, 2022

Towards the end of 2022, I started experimenting with another GNSS module - the ATGM336H-5N11. This part is effectively a drop-in replacement (I'm not sure if it would be considered a clone?) of the u-blox MAX-M8Q, at a fraction of the cost. What I found most attractive, though, was that JLCPCB - who I used for PCB manufacture - had them available for their automated assembly process. I quickly drew up and updated TimeHat mini board and ordered a few.

January 4, 2023

In mid-2022, an individual on the Time-Nuts mailing list reached out with an inquiry about a having a one-off LEA-M8T based TimeHat made. The specifically wanted TP2 and EXTINT0 broken out to SMA connectors.

January 26, 2023

I decided I wanted to re-spin the ATGM336H version of the mini with the battery mounted on the top (the v1 and v2 had the battery on the bottom which made for a smaller board, but could result in clearance issues) and also the RTC swapped out for the Micro Crystal RV-3032-C2. The idea being that all the parts but the header and SMA connector could be populated by the PCB assembler, and I could more easily handle larger orders of these. The reduced BOM and labor costs mean that even with the added expense of assembly, this unit can still be sold for less than the cost of the original TimeHat v0.1.

I also added a bridgeable jumper to the bottom to connect the backup battery to the GNSS and RTC, or just the RTC. I decided to go with just the RTC by default, because the ATGM336H has flash storage (unlike the MAX-M8s) for configuration and ephemerides, so it has less need for battery back up. A CR1220 will keep the rv-3032-c2 ticking for a very long time.

March 16, 2024

Not TimeHats, but some other things I made along the way that are similar and may be interesting.

If you ever embark on a similar project, make test fixtures! This one was very useful for helping me quickly testing the GNSS and RTC modules.

This isn't really a TimeHat, more just.. a collection of parts, but I still use it today (this Pi in particular is my PiHole). One of the most readily available sources of LEA-M8T are these WD22URGC modules that largely seem to be coming from decommissioned Huawei networking and cellular equipment.

I did some testing with units in boxes made of rigid insulation foam boards to measure the impacts of temperature on clock stability. I'm not sure where the results of this testing ended up, but I did find that it made a measurable improvement - though not anywhere as significant as replacing the Pi's main oscillator.