The Casio G-Shock and a MEMS TCXO

Microelectromechanical Systems

Project Start

I have enjoyed these Casio watches since the late 80's. They are super tough, relatively cheap and still the best bang for the buck, I would argue. When you want something tougher than a smart watch with a battery that you don't have to worry about daily, this is the watch. The G-Shocks still have all the timers, alarms, and stopwatch functions you need, with a package you still don't mind mowing the lawn with. These watches have a very prestigious hard-use pedigree with a government assigned NSN number. Many of these watches end up on the wrist of Navy SEALS, and other such elite people. Many pictures of G-shocks can be seen on the arm of astronauts on the ISS as well.

They can take more heat than a human can stand and more cold as well. Yes, cold. LCDs have come a long way; just put your Casio in the freezer and you'll see it's perfectly readable, to the second in the cold. The magnetism that plagued the mechanical watch is also no longer a factor.

Improving on Perfection..?

The quartz crystal that the G-Shocks come with does a great job at a great price. They even put a potentiometer on the board to adjust the rate. This is a feature not found on most quartz watches. With Quartz comes caveats inherent to the technology. One of the great advantages of quartz crystals when they were first put into watches, was their great temperature stability as compared to a mechanical timing system, as well as their low power consumption as compared to an Accutron. However, times have changed! MEMS technology makes it possible to lower the power consumption by a great deal, lower the size, and greatly increase the thermal stability; all at a price that is very competitive.

MEMS oscillators aren't perfect. Applied-Science's youtube channel will teach you how to defeat these modules/chips with nothing more than a helium balloon! Further, as I have so recently learned, they require extremely tight board population techniques.

First, I had to make a breakout board just to get to the MEMS TCXO's pins, they are SMALL! I also sprang for the super expensive, very small ball, no-clean Chip-Quick solder paste just to make sure the surface tension of the solder had every excuse not to short pads when I hit it with the re-flow gun.

These things are small!! This board has one populated out of 3 slots!! (Below)

Every time I put one of the SI chips (MEMS) on a board, I guess I overcooked it. The datasheet shows that after re-flow, the "jitter" (my word for this purpose) bandwidth increases, and the frequency lowers. This was definitely the case for me, but I was getting WAY more jitter, no matter the sample speed, and WAY lower frequencies.

In every board I populated I could get nowhere near the datasheet's 5ppm spec, even at sample rates of 300ms; in fact they recommend a 1 kilocycle sample rate. I was getting a bandwidth of a couple of Hertz and a sustained rate of a couple of hertz down from the specified 32.789Khz.

I'm sure there is a home-gamer out there that has the equipment to populate their boards in such a way as to not kill these chips, but it's not me! I even tried just heating the board from the bottom, which worked, but I still couldn't get the chip to a precision and accuracy specified of the datasheets.

Tools of the Trade

Out with the Old

So after populating the board (Chip-Quick, X-acto, Curse words), I cut the circuit out of the 4-circuit board. I then removed the stock crystal and super-glued the board where the crystal was.

In with the New

There is already a pull-down resistor built into the bubble-chip on the Casio board so all you really need to do is connect the output to where the crystal input is. The power comes from either side of the decoupling capacitor. The cap is huge compared to the other components on the board, presumably to smooth out the power during the high-frequency switching of the "Illuminator" thermoluminescence feature.

There were a few places I needed to cut into on the board pallet. It was a filled material, stiff and easy to cut and melt with an iron.

So how do I like my setup? It's Awesome! First, I'm running around with the only G-Shock in the world with a MEMS oscillator. Second, the battery life is better, so now it's not-so-shabby five year battery, is five years plus. After install, it's very precisely loses about a minute and a half every month. I know not great, but it's neat, one of a kind, and I had fun.

Here it is! With the Crystal out and the MEMS installed!!

What else is out there

Enter the DS3231

It did make me start wondering what else is out there though. Even though this would make for a relatively large watch (way too big for a G-Shock), the Dallas DS3231 chip, IS AMAZING! This thing is better than it's datasheet-claimed 3.5ppm. It will not only hold that bandwidth and the 32.786Khz frequency at room temp, but in my freezer, with a heat gun on it, and at 3.3volts or 5! This thing is great! Another characteristic of Quartz crystals is that they "age." Meaning, as the crystal gets older it mechanically oscillates in a different way, just as any other mechanical system would wear-in. A neat feature of this chip is that you program the actual date and time into the IC. The chip knows the date it was manufactured and compensates automatically for the aging of the crystal, that's neat! There are of course other features like alarms, sleep timers, etc.

All of these devices have a 50% duty cycle, and all are supposed to be trimmed to 32,768 cycles per second. The Dallas chip is ON THE MONEY! The HP 533B is taking a sample every 100ms.

Other Mods..

I had a broken Luminox to cannibalize, (8 dollar Rhonda movement in the new ones, wow! CHEAP for a 250 dollar watch in 2018). So I used the nice rubber dive strap, and the luminous Tritium, and phosphor sealed tubes to illuminate the face hands-free. (or at least tell me where my watch is)

The Files



Dallas DS3231

The computer used to build this was a pi with a billet case!!!

McPi Billet RaspberryPi Case

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