It has been two years since my last blog post, and things have changed. The focus of my electronics hobby has shifted from collecting cheap parts and tools to designing my own PCBs with KiCad. I’m also considering turning it into something more than just a hobby, but let’s do this one step at a time.
This is a breakout board for the Intersil ISL12022M real-time clock, with optional I²C pull-ups and a CR1225 backup battery. The circuit is based on the design recommended by Intersil, with a few tweaks. It’s the second board I’ve designed so far. My first board, an OLED clock, had a more elaborate circuit, but working on several smaller projects that are useful to me and other people is a good way to continue to improve my skills, while I wait for the larger project boards to get fabbed.
Back in my eBay/AliExpress electronics days, I often chose one of the notoriously cheap DS323x RTC breakouts for my projects. It was quite a shock when I discovered the actual price of these ICs at Digi-Key and Mouser. Compared to the more popular Maxim RTCs with similar features, the ISL12022M is slightly cheaper. It also has an integrated temperature compensated crystal, and offers built-in support for daylight saving time.
One of the goals for this board was to keep it small and tight, with just enough room for hand soldering and legible silkscreen text. Successfully routing the traces in the limited space available, while respecting all applicable design guidelines, turned out to be a satisfying experience.
I specifically avoided running signal traces underneath the package, and kept the ground copper pour on the top layer intact as much as possible, with multiple paths to the ground pin. At first glance the layout might look more complicated than it needs to be, and I admit it would’ve been easier to switch the SDA and SCL pins and move all traces to the pads to the top layer, but that would’ve wrecked one of the paths to ground. (Update: I spent some more time swapping pins and rerouting traces, and have come up with a more elegant layout. I’ll post a detailed update when I get my new boards in.)
If you’d like to get your own breakout, you can use this shared OSH Park project to either download the KiCad board layout, or order some bare PCBs.
The board was designed with a CR1225 coin cell battery in mind, but either a CR1216 or a CR1220 will work just fine as well (max. 1.6 µA backup battery current), and fits the Keystone 3000 battery holder.
I²C pull-up resistors
The solder jumper on the back of the board can be used to enable or disable the I²C pull-up resistors. You’ll want to disable them if you already have multiple pull-ups on SDA/SCL elsewhere in your circuit. Make sure you always have at least one pair of pull-up resistors hooked up to the I²C bus, but be careful not to exceed the maximum sink current (3 mA for standard and fast mode).
I’ve chosen 4.7 kΩ resistors, because those best fit my typical breadboard circuit (3.3 V, short wires, at most one other board with pull-ups). You may want to use lower value resistors depending on the supply voltage, bus speed and capacitance, or higher values (10 kΩ) if you often have several breakout boards connected with their pull-up resistors enabled.
Bill of Materials
|BT1||1||CR1225 battery holder||–||Keystone||3000|
|C1,C2||2||MLCC 0.1 µF X7R 10%||0805||Murata||GRM21BR71H104KA01|
|R1,R2||2||Thick film 4.7 kΩ||0805||Panasonic||ERJ6ENF4701|
|R3||1||Thick film 10 kΩ||0805||Panasonic||ERJ6ENF1002|
You can get in touch with me on Twitter at @golemparts. I would love to see photos of your projects!
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