EYE on NPI: EPSON RX4901CE and RX8901CE Temperature Compensated Real-Time Clocks #EYEonNPI @DigiKey
Channel: Adafruit Industries
Date: 2022-10-27

Tick tock, it's time for this week's EYE ON NPI - this week we're on time and on budget with the EPSON RX4901CE and RX8901CE Real-Time Clocks. (https://www.digikey.com/en/product-highlight/e/epson/rx4901ce-and-rx8901ce-real-time-clocks). These are two  affordable yet ultra-high high precision temperature compensated Real Time Clocks - from EPSON who are well known for their quality crystal offerings (https://www.digikey.com/short/b0t2j2c5).
Accurate time-keeping is something folks ask us about a lot - it's a lot harder than most people think! (It's sorta up there with voice recognition and 3D-indoor-spacial-tracking with "things that seem really common but actually are super tough) Most folks are used to alarm clocks that use the power line 60Hz sine wave to keep track of seconds, we've done that for plug-in clocks for almost a century (https://en.wikipedia.org/wiki/Electric_clock) And while the utility's mains frequency does fluctuate with load, its compensated overnight so average out to a near-perfect 60Hz (http://leapsecond.com/pages/mains/).

If you don't have access to that 60Hz calibrated signal, say because you're battery powered, or using a switching supply, you have a few options for externally-validated time keeping. Some folks like the WWVB radio-signal (https://en.wikipedia.org/wiki/WWVB) synchronization modules (https://www.sparkfun.com/products/retired/10060) that receive an atomic-clock-accurate signal from Colorado. Others like to use a GPS module, which will get atomic-clock-accurate signals from spaaaaaaaaaace (https://www.adafruit.com/product/4279), finally of course, if you have Internet connectivity you can get time from NTP (https://www.ntppool.org/en/). All of these require a lot of power, and for GPS you need to have an outside antenna - or at least close to a window. WWVB works best in the middle of the country, but we've always had a little difficulty in a NYC apartment.

If you want low-power or battery-powered time stamping and logging, a real time clock is the way to go. But these chips aren't sync'd to some global standard and they tend to have a lot of drift (https://en.wikipedia.org/wiki/Clock_drift) - seconds a day, which adds up! This is due to the timing crystal having some variation, usually 10 to 20 ppm. 20 parts-per-million is the same as 20 / 1,000,000 * (24 * 60 * 60 seconds/day) = 1.7 s/day variability, or about a minute a month. So it really can add up!

The RTC counter circuitry itself is perfectly accurate, so if we could make sure the crystal was perfect, we'd have no timing issues. To do that, we have to account for the temperature...thus bringing forth the invention of the TCXO Temperature Compensated Xtal Oscillator (https://www5.epsondevice.com/en/products/tcxo/) something that Epson is really good at making! So it's no surprise that they've packaged an RTC chip into one of their crystal-sized enclosures to create a sealed high precision RTC. And the price is really good too! We often see TCXO RTC's for $6-$8, whereas the Epson RX8901CE are $3.25 in quantity and they're also nice and compact, 3x2x2.5mm in size. (https://www.digikey.com/short/mw38mqrz)

Both the Epson RX8901CE I2C version (https://www.digikey.com/short/z4rhvbw0) and the RX8901CE SPI version (https://www.digikey.com/short/hmwj9djc) are in stock right now for immediate shipment from Digi-Key! Order today and you'll get it by tomorrow afternoon - so you can stick to your production schedule with a tip-top RTC that won't waste your time.