My DS3231 test setup

I wanted to test several DS3231 (M and non-M variants) boards for drift, so I mounted eleven of them (including one known-genuine DS3231M, the leftmost one on the front row, with a green bodge wire) to a breadboard, connected a regulated power supply (AMS1117) at 3.3V to both power rails, and made sure they all worked.

Eleven DS3231 (including 3 M variants) on a breadboard for testing.

Yup, they all work. The boards have either orange or red LEDs, so they emit a pleasing glow at night that prevents me from crashing into things in my office at home.

Why use 3.3V? One, it makes interfacing with the 3.3V I2C pins on a Raspberry Pi easy since I don’t need a level-shifter, and two, it minimizes drift in the event that I need to disconnect the power and have the clocks run on their coin cell backups. The CR2032 batteries have a nominal voltage of 3.0V, but all currently measure 3.3V (they’re brand-new, Energizer-brand cells from Digi-Key). The DS3231 datasheet says the drift can change by up to 1ppm/volt, so I want to minimize the voltage difference between the normal power supply and the coin cells.

To ease the comparison of drift, I want to ensure all the clocks start counting at the same moment. I could set them all one at a time, but this is complicated because (a) I don’t have an I2C multiplexer chip, and (b) setting them sequentially means they’re not all set at the same moment. It probably wouldn’t matter much in the long run, but it would make me happy to set them at the same time.

The DS3231 modules all have the I2C address of 0x68, and it cannot be changed. Normally, you cannot have multiple chips with the same address on the same I2C bus, as they’ll talk all over each other and the resulting signals will be garbage.

Fortunately, we don’t need the DS3231s to talk; they need only listen to the master and make the appropriate ACK/NAK signals as needed. They should all send the same ACK/NAK signals at the same time so, in theory, there shouldn’t be a problem.

Next, we need to worry about bus current. Each module has a 4.7k ohm pull-up resistor for the I2C bus. With eleven modules, that means the effective pull-up resistance is ~430 ohms. At 3.3V working voltage, a device would need to sink nearly 8mA to correctly signal a logic low. The Raspberry Pi I have can sink 16mA per GPIO pin, so that’s fine. The DS3231 datasheet says the IOL is 3mA, though I spoke with an engineer at Maxim Semiconductor and they said the absolute maximum current the process used on the chips is 10mA. 8mA is close to that limit, but the current would hopefully be spread across many devices and would only be for a few microseconds in total, so it should be fine.

I was satisfied I wasn’t going to blow anything up (and if I did, replacements are cheap), so I connected all eleven modules in parallel to the same I2C bus and commanded them to set their date and time to an arbitrary date in the past. If this was successful, I could send a command to read the time and, if all the modules had the same time, it would come through without an error. If things didn’t work, garbage would come in and I’d have to check them individually for the correct time. One read to all of the devices simultaneously, and I had valid data for that arbitrary time and date. Excellent. It worked!

Using the Raspberry Pi synchronized to a local NTP server (another Raspberry Pi running NTP with a GPS reference clock) within less than a millisecond, I send the command to set the date and time on all the modules to the current time on Friday 8 Sep 11:18:16 UTC 2017 (unix time: 1504869496). Reading the date and time from all the modules confirms they all have the correct date and time with no errors.

Now I’ll let them run for a while to see how they drift. A few have hand-tuned aging registers, so they should hopefully drift less than the others, while others use the default aging register of 0.


Major differences between the DS3231 and DS3231M RTC chips

As should be clear from one of my earlier posts, I’m really interested in clocks and precision timekeeping. In particular, I rather like the Dallas Semiconductor DS3231 series of temperature compensated RTC/TCXO (real-time clock/temperature compensated crystal oscillator) modules.

Recently, I had ordered several DS3231 boards from my regular eBay vendor in Shenzhen for some testing, only to find two oddities: first, the factory had evidently gotten an incorrect chip with the same sized 0.300″ SOIC package as the DS3231. This chip was the wholly-incompatible DS1315. It happens, particularly at this price point and via gray market suppliers. No worries, I contacted the seller and they sent me a replacement board.

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A look inside the DS3231 real-time clock

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.

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Well, that was an interesting failure mode…

I have a bunch of eBay-sourced DC-DC converters that I use for a bunch of purposes around the house. Most are ordinary “LM2596” (in scare quotes, as most seem to be clones: they’re marked as LM2596 and generally work well, but have different switching frequencies. Supposedly this is an issue with such things.) buck converters configured as adjustable, constant voltage power supplies where the output voltage is set by a multi-turn potentiometer. Very handy.

Others can be used in either constant voltage mode or constant current mode. For the latter, a serpentine strip of PCB trace acts as a low-value sense resistor. An LM358 dual op-amp integrates the difference between the voltage across the trace and a voltage set by a potentiometer, with the output connected to the regulator’s feedback pin via an LED so you can tell when the regulator is in constant current mode. Another potentiometer sets when the “charging” LED lights up; this is purely cosmetic, and the LED turns off when the current through the regulator drops below the setpoint set by the potentiometer.

Caleb Engineering has an excellent teardown of such a regulator here.

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My Daughter’s First Circuit

My daughter turns three in June. Yesterday, we were playing and an idea popped into my mind: she likes to help me build various electronic things at my desk, but she’s never really built anything of her own. I asked if she wanted to make something with me and she energetically agreed.

Here it is:

It’s a simple two-transistor astable multivibrator that alternates between the red and green LEDs at around 2Hz. Everything to the right of the red wire is pretty bog-standard: 5% tolerance 470 ohm current-limiting resistors for the LEDs and 100k ohm resistors for charging the 10uF capacitors. Two BC548 transistors do the switching. Some 24 AWG wire connects parts too far apart (or awkwardly placed) for component leads to reach.

In retrospect, I could have laid things out better, but she didn’t mind. The only major thing I’d change is using ceramic capacitors instead of electrolytic, as I’d like to keep this circuit around until she’s older and have it still work without the capacitors drying out, but I didn’t have any 10uF ceramics at hand. I’ll order some, have her pick them out, and swap them out.

On the left is a simple terminal block for connecting a power supply. I wanted the circuit to be robust in terms of polarity, so I used a bridge rectifier so it can operate regardless of how the DC power supply is connected (I could have added a filter cap so AC could be used too, but I don’t have any wall warts with AC out, and she likes batteries, so this was not a major design consideration). I could have used a cheap diode, but the bridge rectifier uses Schottky diodes and so drops only 0.6V compared to a 1N400x’s 0.7V, plus it means the circuit will work (rather than simply not be destroyed) regardless of how it’s connected, so that was an easy and robust choice.

A 50mA polyfuse provides protection from faults (important when using old cellphone Li-Ion batteries as a power source). All the exposed underside contacts of the unfused section (i.e. terminal blocks and rectifier) are liberally coated with hot glue for insulation, with the jumper wires on the top and bottom tacked down with hot glue as well. All solder and components are lead-free, with burrs and other sharp points on connections filed smooth for minimal danger.

My daughter loved picking the components out of the parts drawers, listened attentively while I explained what they did and how they work, and helped me put them in the correct places on the breadboard. After things worked and she (later) went to bed, I moved the same parts over to a protoboard for a bit more durability. Now she’s running around the house waving it (and the 1000mAh cellphone battery stuck to the bottom with double-sided tape) around, blinking it at her baby brother, and integrated it into playing with her other toys.

This makes me happy.