Real Time Calendar/Clock Suggestion?

MioTheGreat

Does anyone have any recommendations for an RTCC chip?

There seem to be quite a few choices, and quite frankly, I'm not sure what to pick. I just need to be able to keep track of the date and time with one of the projects I'm planning with a Propeller.

This is something that would likely never lose power for more than a few hours. Would I need a battery for the RTCC, or would a really big capacitor work?

Oldbitcollector (Jeff)

Personally, I've played with the DS1302, which has a nice object already in obex.
obex.parallax.com/objects/89/ You will want to use a small 3v watch cell
to maintain the time when the power is off.

There's also been a recent mention of using a PCF8583 (vampyre) which also incorporates
a little extra ram (240 bytes?) and can be communicated with I2C routines.

There's also this ready-made solution from futurelec...
www.futurlec.com/Mini_DS1307.shtml
OBC

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Post Edited (Oldbitcollector) : 6/23/2008 3:47:39 AM GMT

Drone

Have a look at the DS3232 Extremely Accurate I2C-Integrated RTC/TCXO/Crystal and SRAM at www.maxim-ic.com. This looks like a really cool RTCC. I tried the DS1302. It drifts, especially if you're device is going to move around a lot between indoors/outdoors or is in an automobile. The on-board TCXO on the DS3232 should mitigate this problem substantially. It only comes in a SOIC-20 package. Today Digikey has them in-stock for $7.63 in unit quantity. Can't seem to find them in-stock elsewhere. I have not experimented with the DS3232 yet.

Good Luck, David

Timmoore

I am using the ds3231 also an accurate I2C rtc. No crystal needed and haven't had any drift problems.

Rayman

I'd recommend the DS1307. You can connect it directly to the existing I2C bus thereby not using any free pins. Don't need any resistors for the connection even though it is a 5V device.

parsko

Mr Great,

I've played with a PCF8583 myself. You can see the post at:

Philips PCF8583

I really like it. The added ram can be pretty usefull. I'm very impressed that it does 1/100sec!!!

-Parsko

Leon

I've just ordered a couple of DS1307s. They aren't specifically for the Propeller, although I might try one with it.

Leon

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Philldapill

What kind of comparison is there between the accuracy of an RTC, like DS1307, and the propellers internal clock? Are we talking about 0.01% error for the RTC vs. 1% for the prop?

Timmoore

In the ds1307 app notes they talk about +-20ppm, ds3231 datasheet says +-2ppm. The ds1307 is mostly dependent on the crystal. I couldn't find a ppm for the prop but it would also be dependent on the crystal. Also the crystal varies with temperature, the ds3231 is handles this over a temp range(+-2ppm for 0-40C), the others dont. So it depends on your crystal, whether its close to a component that will get warm, the temp range you want to run etc.
PS the other reason to look at the ds3231 is its 3.3V and I2C (ds1307 software works fine, same I2C address). One decoupling capacitor suggested is all it needs and a backup battery if wanted. If you need more accurary you can trim the oscilator with an internal register.

MioTheGreat

Thanks for the suggestions, guys.

I think I'm going to go with the DS3231, and try to find a SOIC to DIP adapter. I'm liking not having to put an extra crystal in the circuit.

Unless, of course, anyone can suggest a reason why the extra RAM in the DS3232 would be useful?

Post Edited (MioTheGreat) : 6/24/2008 3:32:45 AM GMT

Timmoore

Look at the breakout boards at sparkfun.com, one of those fits as a soic to dip adapter. You might also try asking for a sample, thats how I got my ds3231.