DS1302 with a SuperCap

Chicago Mike

SO I think I've gone through every piece of material I could on the DS1302, and I'm missing something. I have the DS1302 RTC working great with my BS2, and I went off and bought a aerogel 5V .47F supercap from digikey to hold the backup charge. All the manuals, including the parallax one show how to enable 1 or 2 diodes, and 2000,4000, or 8000 ohm resistors on the backup circuit which changes the charge time and drain time. OK, makes sense, but no manual explains why to use 1 or two diodes or resistors. Why would you ever want your charge time to be really long and drain time really fast? (Which seems to be an option). Is there a formula somewhere I'm missing? I saw the formulas in the DS1302 datasheet which explained how the the charge and discharge times relate, but it doesn't explain why you wouldn't leave it as fastest charge, slowest drain for the DS1302....·I've hooked the cap up with 1 diode and 2 resistors and it seems to work just fine. (Though I haven't measured the charge/drain). Is there some rule of thumb I'm breaking or should follow with these types of caps on any device (especially the DS1302).

Thanks!

Chris Savage

Mike,

·· I don't use the charging circuit because it's simpler to hang a CR2032 battery off the DS1302, however if memory serves (not looking at the data sheet) the diodes control the charging voltage and the resistors control the charge current.· Other than leakage I don't believe it will discharge through the resistors (The diodes should prevent that).· Again, I would have to get the datasheet out bug as I recall that was the way it worked.· And as for needing to change those parameters that would depend on what you had connected, since that circuit would also charge a NiCad battery.

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Chris Savage
Parallax Tech Support
csavage@parallax.com