Another RTC Question...

matt redmond · Joined: 26 Sep 2003 Posts: 6

Any thoughts on RTC/Quartz/Lithium integrated devices (like the DS12C887) versus RTCs w/ external crystals and backup batteries (e.g., DS1306)? The integrated device costs $2.40 more than the other - but I don't have to supply backup battery or crystal & I avoid interference issues. BUT the interface looks a bit more complex (multiplexed bus instead of SPI/I2C).

Does anyone who has made this choice have any input?

Thanks!

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Embedded designers who use small micro-controllers don't normally
use byte-wide devices. We use serial devices, either i2c or SPI.
The major reason is the pin count. Typically, we will choose the
smallest PIC that can do the job, in order to keep the cost down.
That means that we only have a limited number of Ports and pins.
We can't afford to waste them on a byte-wide RTC chip.

The device you're talking about might be OK for a larger embedded
design, with a micro that has a dedicated 8 bit data bus (ie., eZ80).
Even then, my bias would still be for the DS1307-like chip, just
because it takes less board space and costs less.

matt redmond · Joined: 26 Sep 2003 Posts: 6

Okay, thanks for the info - that makes sense. Bolstering the case for a DS1306 is the fact that I have in hand an Application Note w/ schematic and code for a PIC interfacing w/ the 1306. No such document exists for the 12C887. A 32.768 crystal is only 30 cents qty. 1 and other than a battery, that looks like all I need.

elizabeth ann · Joined: 16 Feb 2008 Posts: 33

dtl · Joined: 16 Jan 2008 Posts: 5

Check out STMicro, they make SPI & I2C real time clocks that use the SNAPHAT battery crystal. They are a small SMT package that has pin holes in the top of the IC, after reflow you put the SNAPHAT on which includes the crystal and battery. We use them in a number of products, I am going to use the 3.3v I2C version in the product I am developing now.

Cheers,
Dave