TinyCircuits Forum

General Category => General Discussion => Topic started by: EKMallon on February 05, 2014, 09:45:41 PM

Title: The cheapest RTC board out there?
Post by: EKMallon on February 05, 2014, 09:45:41 PM
I was about to order some more chronodots for my datalogger project,  but came across this DS3231 AT24C32 combined RTC & Memory Module just about everywhere from new egg to ebay:


I was also looking into separate eeproms, but this might allow me to kill two birds with one stone. Has anyone tried one of these yet?  I am thinking that this thing might be a power hog somehow.

(and of course, I am still eagerly awaiting the promised TinyRTC to make my unit more compact...nudge nudge)
Title: Re: The cheapest RTC board out there?
Post by: tinybuilder on March 03, 2014, 06:46:53 PM
The DS1307 (http://datasheets.maximintegrated.com/en/ds/DS1307.pdf) is a nice choice. It is small ~3mm x 3mm(?). It is inexpensive, Digi-Key (http://www.digikey.com/product-detail/en/DS1307Z%2BT%26R/DS1307Z%2BT%26RCT-ND/3647910) has it at $3.05 for 1 and $2.01710 for 100. It needs a 32.768 kHz external crystal which Mouser (http://www.mouser.com/ProductDetail/ABRACON/AB26T-32768KHZ/?qs=m%252bUhWDcpCfZq/BFfwzExYQ==) has for $0.25 for 1 and $0.14 for 100. SparkFun's RTC Module (https://www.sparkfun.com/products/99) is a breakout board that is 20x20mm, so that could easily be made into a TinyDuino Shield but adding the connector in place of the SPI connections. The schematic (http://dlnmh9ip6v2uc.cloudfront.net/datasheets/BreakoutBoards/RTC-Module-v13.pdf) is extremely simple.

The DS3234 (http://datasheets.maximintegrated.com/en/ds/DS3234.pdf) a low-cost an all-in-one package extremely accurate SPI bus real-time clock with an integrated temperature-compensated crystal oscillator and crystal. It is ~12mm x 10mm, and Digi-Key (http://www.digikey.com/product-detail/en/DS3234S%23/DS3234S%23-ND/1197582) has them for $9.07 for 1 and $4.39 for 100. SparkFun's DeadOn RTC (https://www.sparkfun.com/products/10160) is a breakout board which looks to be about 20x20mm. The schematic (https://www.sparkfun.com/datasheets/BreakoutBoards/DeadOn%20RTC%20-%20DS3234%20Breakout-v11.pdf) is extremely simple.

Note: RTC's use separate batteries, so an RTC shield would look like the TinyDuino Processor shield with battery holder. The 2 above SparkFun examples use a small CR1225 battery (12mm) so height would be the only concern for making it stackable. A battery clip from Farnell (http://www.farnell.com/cad/1458706.pdf) has specs of  19 x 13 x 3.6 mm. Mounting could take up to 19.6 mm, which should really work nicely.
Title: Re: The cheapest RTC board out there?
Post by: EKMallon on March 10, 2014, 08:11:38 PM
Just posting an update on these RTC's. I have been testing 6 different units (from 2 suppliers) and they seem to be working Ok, including the 32k I2C eeprom.  I did some modifications to them: First was to de-solder the power led, and the second was to remove the charging circuit (by desoldering one resistor) , and replace the rechargeable lir2032 button cell with a non rechargeable cr2032.

I have taken them down to 2.8 volts, and not yet seen any problems with the RTC, or read/writes to the 32k eeprom, even though the units original spec was for 4-5 volts. More tests are ongoing, but so far they seem to be working. Of course, at this price, I am still a bit uncertain of the quality of that DS3231. Lots of talk on the web of ghost runs, or people grabbing out of spec chips, that were supposed to be destroyed, and sending them back to market in these discount boards from China...