Comparison of four real-time clocks
A real-time clock (RTC) is necessary for any project requiring accurate time-keeping, especially when you don't always have an Internet connection – for example, if you're sending a Raspberry Pi up in a balloon. The Raspberry Pi keeps pretty good time, but only if it is connected to the Internet. If you're not on the Internet, and you turn it off at, say, 10:43am and then turn it on at 12:34pm, the clock will resume at 10:43am and go forward.
The Pi uses an Internet service called NTP to set the clock on power-up if the Internet is available. It then calls the NTP servers (some are hosted by the National Institute of Standards and Technology [NIST] and linked to their atomic clocks). You aren't going to get atomic clock accuracy using NTP, but it's pretty good. Here, I used NTP to update the Raspberry Pi so I can use it to estimate the error rates of these RTCs.
Introduction to the Clocks
I chose four popular RTCs to test. They had to meet the following criteria:
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