![]() 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. Such a circuit would be mostly limited by the CR2032’s self-discharge rate anyway. Still, a common 220mAh CR2032 battery could power the chip for at least a decade with no problem. The one (very minor) downside is that it draws about twice the current, a bit less than 1 μA, than the DS1307. It’s specified to keep time within 2ppm from 0☌ to +40☌, and 3.5ppm from -40☌ to +85☌, which means the clock would only drift 63 and 110 seconds per year, respectively. 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. Not so great.įortunately, Maxim also offers the DS3231, which is advertised as an “Extremely Accurate I2C-Integrated RTC/TCXO/Crystal”. A 20ppm error in the frequency of the crystal adds up to about a minute of error per month. Thus, any change in temperature will cause the clock to drift. ![]() 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).Īs great as it is, the DS1307 has a major drawback: it relies on an external crystal and lacks any sort of temperature compensation. It’s probably a good idea to buy chips for serious purposes from legitimate distributors.ĭallas Semiconductor, now owned by Maxim Integrated, is well known for making some excellent real-time clocks (RTCs). and the chips are wildly inaccurate even though they’re marked as DS3231. Today, it appears that whatever stocks of real DS3231 chips have been depleted and many people have contacted me to say they’ve ordered the common “DS3231 and EEPROM” items from eBay, AliExpress, Amazon, etc. At some point they started selling DS3231M chips without updating their listings (which described the item as being the more-accurate -SN variant). ![]() Update : At the time this post was originally made, most of the Chinese sellers on eBay were selling real (but not guaranteed by Maxim) DS3231 chips that generally kept good time.
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