Seemingly great RTC IC - where's the catch? - Page 1
Forum section: electronics / projects

author: daqq
author_rank: Super Contributor
timestamp: March 14, 2021, 10:13:42 pm
content: Hi,I'm looking around RTCs and I found this:RX8900CE UAhttps://sk.farnell.com/epson/x1b000301000112-rx8900ce-ua/rtc-40-to-85deg-c/dp/3253613RLBasically an RTC with everything included aside from the battery/cap which you connect externally. Has built in crystal, temperature compensation, cap/battery charging. Average pricing. The only problem is the current consumption, which is slightly above average at 700nA typical. And it's small (3.2 x 2.5 mm). Seems like a great fit for my application. Am I missing something?Because everything I've seen so far was either huge, or needed an external crystal or some other issue.Thanks,David

author: wizard69
author_rank: Super Contributor
timestamp: March 15, 2021, 04:15:36 am
content: It is an Epson device, a reputable company, what don't you believe about their specs? clocks are like anything else electronic these days, as the processes shrink, functionality goes up while size goes down.By the way I do believe others have chips with integrated crystals.

author: capt bullshot
author_rank: Super Contributor
timestamp: March 15, 2021, 07:17:38 am
content: Nothing wrong with the device in general, but ...it's got too many registers. I'd really like an RTC chip that just has a 32 (or more) bit seconds counter. All that wall clock time / calendar stuff is better done in software today. In my typical firmware, I'm keeping time as unix time (seconds since epoch start) anyway, so I'd have to convert the RTC calendar time to seconds each time reading the RTC. So why shouldn't the RTC provide me with unix time from the beginning anyway?

author: floobydust
author_rank: Super Contributor
timestamp: March 15, 2021, 06:56:48 pm
content: I dislike the part. You get backfeedapp manual 8.8.8from the battery into +VDD, so the IC needs an extra diode and ends up running at 2.7V or 3V. FAIL."Warning: When accessing this device, all communication from transmitting the start condition to transmitting the stop condition after access should be completed within 1 seconds.If such communication requires 1 seconds or longer, the I2C bus interface is reset by the internal bus timeout function. When bus-time-out occur, SDA turns to Hi-Z input mode.Note:During access to the time registers, the time counting is on hold!This means that up to 1 second can be “lost” in case of unsuccessful communication as mentioned above!Please make sure to send I2C start condition before actual transmission of the RTCs slave address as otherwise the slave address appears to be shifted by 1 bit!"That seems so 1980's when the cost of time holding registers was too much.

author: DrG
author_rank: Super Contributor
timestamp: March 15, 2021, 07:02:48 pm
content: I have been meaning to play around with a RV-3028-C7https://www.microcrystal.com/en/products/real-time-clock-rtc-modules/rv-3028-c7/(they kindly consented to sending me some for freeI was just looking for what's new and is something "better" than the maxim parts or the old mchip. Data sheet looks pretty good (to me).