Tiny RTC clock module w/Supercap backup

Rayman

Just finished testing this new I2C RTC clock module and it appears to work perfectly.

This module features the smallest SMT chip, crystal, and supercapacitor I could find...
Here it is soldered to the Propeller Platform Express's second eeprom socket:

It actually only needs pins 5,6,7 and 8 to function, so it could be used as a 4-pin SIP module instead of 8-pin DIP...

These Mill-Max stackable headers seem to be pefect for this as I can still attach something else on top:

These ones are a little too long for my taste though, so I'm ordering some shorter ones from Digikey to try out.
The Digikey part number is ED90249-ND.

Since this is perhaps the smallest supercapacitor out there, I was a bit worried about how long it could backup the clock. But, I left it overnight and it still had the time in the morning. So, it's good for at least 10 hours. I'd have to break out some datasheets and a calculator to figure out how long it can go...

cavelamb

Nicely done, Ray!

Martin_H

I bought an RTC clock a while back and I was wondering about backup power. Are there any guidelines on capacitor choice?

Don M

Ray- what RTC chip are you using?

Rayman

I'm using DS1339 here.

It has a very flexible trickle charger with options to support many types of backup capacitor...

Tubular

Really nice work, Ray. The 4 pin (SIL) function is great too

If you do a revision perhaps some pads for external connection (so a larger supercap can be optionally connected)

Rayman

I thought about pads for larger supercap or battery, but was more interested in making it smaller and I thought that would make it bigger...

One can remove this supercap pretty easily though and expose the pads that could be used for something else...