Dallas Semiconductor, now owned by Maxim Integrated, is well known for making some excellent real-time clocks (RTCs). Take, for example, the DS1307: it’s simple, works with essentially any cheap 32,768 Hz watch crystal, is easily accessible over I2C, and is extremely power efficient (500nA current when running the oscillator on battery power).
As great as it is, the DS1307 has a major drawback: it relies on an external crystal and lacks any sort of temperature compensation. Thus, any change in temperature will cause the clock to drift. A 20ppm error in the frequency of the crystal adds up to about a minute of error per month. Not so great.
Fortunately, Maxim also offers the DS3231, which is advertised as an “Extremely Accurate I2C-Integrated RTC/TCXO/Crystal”. This chip has the 32kHz crystal integrated into the package itself and uses a built-in temperature sensor to periodically measure the temperature of the crystal and, by switching different internal capacitors in and out of the crystal circuit, can precisely adjust its frequency so it remains constant. It’s specified to keep time within 2ppm from 0°C to +40°C, and 3.5ppm from -40°C to +85°C, which means the clock would only drift 63 and 110 seconds per year, respectively. Very cool.
The one (very minor) downside is that it draws about twice the current, a bit less than 1 μA, than the DS1307. Still, a common 220mAh CR2032 battery could power the chip for at least a decade with no problem. Such a circuit would be mostly limited by the CR2032’s self-discharge rate anyway.
In my case, I wanted to use such RTCs on several of my Raspberry Pis that are not regularly (read: almost never) connected to the internet, and so cannot always get their time from NTP servers.
Some clever person designed a very simple board that fits on the Raspberry Pi’s pin headers for power, ground, and I2C and has the DS3231 chip, pull-up resistors for the I2C bus, and a decoupling capacitor. It even has pads for a backup battery (not included, but adding a battery holder and coin cell is straightforward). Chinese vendors on eBay sell the board for about $1.50, with free shipping. Perfect.
Here’s the board I’m using on one of my Pis, along with the backup battery and holder I added.
Considering that the DS3231 is not a cheap chip, costing ~$3.80 USD per chip in minimum quantities of 1000 from Digi-Key, it’s a bit surprising that complete board only costs $1.50 per board. Like Edward Mallon, I wondered if these were counterfeit chips that were pin and function compatible, QC rejects, or somehow otherwise illegitimate chips.
For science, I ordered a few extra boards and tested them over the last year, where “tested” means “set the time on the chips with a Pi that was NTP synchronized to a GPS timing receiver, disconnected them from the Pi, and left them on the shelf running on battery power for a year”. The chips would be in direct sunlight in the mornings, and the temperature in the room would range between about 15°C and 30°C throughout the year. Not extreme, but not precisely regulated either. I did not adjust the “aging register” in the chip to trim the oscillator before this test, and the register was set to its default value of “0”. After a year, the chip with the largest drift was only 16 seconds off, which is about 0.5 ppm. That’s well within spec, so I’m happy. If these chips were counterfeit, they were at least good counterfeits that worked as advertised.
However, I wanted to look closer so I sacrificed one of the chips for science. Thanks to my friend Jesse for reminding me that I can just snip off the legs of the chip rather than trying to de-solder it. That made things a lot easier.
Here’s the top of the package. It claims to be an SN model, which means it is specced for the full -40°C to +85°C temperature range. The date code says it was made in week 33 of 2011, as part of lot 917AC. The # mark means it’s RoHS compliant.
The laser markings seemed a bit dodgy and not like the normal high-quality laser markings I see on other Maxim chips. I contacted Maxim, explained the situation, and sent photos of the package and die (see below). After checking their records, they say the style of the markings, the date code, and lot number are all consistent with that particular lot made in 2011, which strongly suggests the chips are legitimate. They also reminded me that they do not warrant or guarantee any products purchased from unauthorized resellers. Good to know, and not unexpected.
I zoomed in with my USB microscope to examine the markings in more detail. It’s a bit hard to see in this close-up, but you should be able to see the digits “31”. Compare these markings to those on the Maxim MAX3232 chip I investigated earlier and you can see why I was a bit skeptical as to their legitimacy at first. Obviously, Maxim must have different types of laser marking equipment on their different production lines.
I normally would digest the epoxy packaging of the chip in acid at work, butI was at home that day and didn’t have access to the chemicals and safety equipment I have in the lab at work, plus I didn’t want to dissolve the integrated crystal and its metal can. Instead, I embrittled the packaging by heating it in the flame of a common Bic lighter for several seconds and then quenching it in a glass of cool water. I repeated this process several times.
Next, I sanded down the back of the ship (assuming that the interesting parts of the die would face upwards, which they were — if they hadn’t been on the top, I’d sacrifice another chip and sand the top down) with fine sandpaper until I hit metal.
It turns out I was a bit too vigorous in my sanding, and accidentally sanded through the crystal’s metal housing and broke one of the forks of the tuning fork oscillating element. Oops.
In the photos below, the notch on the chip is to the left, so pin 1 is to the top left. The main die is behind the large copper pad to the left. The fuzzy “hair” at the bottom are strands of the epoxy package that I didn’t clean up.
This was interesting, but even after Maxim said the packing and exterior markings looked legitimate, I was curious if the die itself was an actual Dallas/Maxim die or if it was a fake. Using tweezers and a fine, sharp knife I was able to crumble away more of the epoxy package and remove the die. Unfortunately, the bond wires were still embedded in the package and so broke off when I removed the die. I also slightly scratched part of the die and cracked off part of the top-right corner. Clearly, acid digestion is the way to go.
Here’s the first look at the die itself. I had washed it with isopropanol and both the chip and the microscope slide are a bit wet. The die measures ~3.6 x 2.3 mm, and the images below were taken with my USB microscope.
First, I wanted to check to see if the die was actually made by Maxim or if it was a fake. The die clearly says “DALLAS SEMICONDUCTOR”, as well as “©2004 (M) MAXIM”. Looks legit. That’s refreshing.
Here’s some more photos of the die.
In addition to my cheap USB microscope at home, I was later able to take the die into the lab at work and use the (very expensive) Zeiss microscope to take more pictures. I was also able to clean it more thoroughly using the ultrasonic cleaner so the images came out considerably better.
Alas, compatibility issues between the camera mounted on the microscope and my computer prevented me from using the camera to get high-quality photos at this time. I’ve ordered an adapter so I can get better photos,
but it will be several weeks. At that time I will either update this post or link to a new one. I plan on creating large composite images of the die at various levels of zoom, and with different optical filters. In the interim, here are a few photos I took using my smartphone aimed through the eyepiece of the lab microscope. They are nowhere near as clear or stunning in appearance as they are when viewed directly through the eyepiece or via the on-scope camera.
Addendum 2017-07-29: I’ve been able to get the camera on the microscope to cooperate and have gotten several high-quality photos. As the microscope has an extremely short depth of focus, particularly at high magnification, some images have been “focus stacked” by combining several images at different focus depths. Similarly, the large composite images are made from several individual images that may be focused slightly differently from each other. These processes may cause visual artifacts to be present.
In general, images with green and red colored layers use standard reflected microscopy with no filters, while images with blue and gold layers use reflected differential interference contrast (DIC).
That’s all the photos for now. I hope you found this as interesting as I did.