RTC breakout at 3v?

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djszk
 
Posts: 28
Joined: Mon May 14, 2012 3:43 pm

RTC breakout at 3v?

Post by djszk »

Good morning,

Can the RTC breakout board run at 3v?

Thanks,
Dave

djszk
 
Posts: 28
Joined: Mon May 14, 2012 3:43 pm

Re: RTC breakout at 3v?

Post by djszk »

...ah, just found this thread: http://forums.adafruit.com/viewtopic.php?f=31&t=15984

I'm driving a Teensy at 8 MHz with 2x AA batteries. I have an SD and an OLED screen (which work fine) on SPI, but the RTC isn't working on I2C. Seems from that thread like it might be an I2C issue, since I2C stops working below 3.85v.

Any suggested workarounds for using the I2C RTC breakout with 3v?

Thanks!

djszk
 
Posts: 28
Joined: Mon May 14, 2012 3:43 pm

Re: RTC breakout at 3v?

Post by djszk »

I'm still having issues with this: I tried putting extra pull-up resistors (in addition to the internal ones) on the I2C clock and data channels and it appeared that the data always got pulled high rather than low (instead of presenting 00:00:00 it presented 89:89:89). Any other ideas?

Thanks!

stephanie
 
Posts: 295
Joined: Sat Dec 11, 2010 1:17 am

Re: RTC breakout at 3v?

Post by stephanie »

I have lots of I2C stuff going at 3v without any trouble. I think the 3.85v cutoff you mentioned for I2C is assuming Vcc is 5v.

When you say you added extra pullups to the additional ones, assuming you added them alongside (in parallel) that would reduce the total amount of pullup resistance for each line. If the data is always being pulled high that suggests that the pullup resistance is too low.

I've seen values from 2.2k to 10k listed for pullups on I2C but I have had the most success with 10k and personally won't go below 4.7k for my pullups.

One other thing that might be obvious but I will mention it - you are using the Teensy, do you have the I2C on the correct pins? Unlike a regular Uno where you'd be using A4 and A5, on the Teensy the pins are PD0 for SCL and PD1 for SDA.

Could you post your code and photos of your wiring? That might help folks figure out where the problem is.

Cheers!

djszk
 
Posts: 28
Joined: Mon May 14, 2012 3:43 pm

Re: RTC breakout at 3v?

Post by djszk »

Thanks for the input, Stephanie--I'll post photos of my wiring later tonight.

Just for your reference, yes, I do have I2C plugged into the right pins and am running the device fine when the Teensy is plugged into 5v via USB. It's only when I run 3v directly from 2 AA batteries to the voltage inputs that the I2C device doesn't work (though everything else works fine on the batteries: the Teensy, a MicroSD breakout, and an OLED screen).

I will pick up some 10k resistors on the way home tonight and try that.

Thanks!

stephanie
 
Posts: 295
Joined: Sat Dec 11, 2010 1:17 am

Re: RTC breakout at 3v?

Post by stephanie »

Ah ok - that makes all the difference. Since it is working on 5v from the USB but not working when on the 3v source, maybe the part itself is not 3v friendly...

Yep, I just looked up the DS1307 datasheet and it says minimum voltage is 4.5. Sorry, I should have thought to check that before. I'm just so used to working with 3.3v stuff lately I assumed the DS1307 would run at either 3.3 or 5.0 volts.

For a more or less drop-in-replacement, you could look at the Chronodot. It is a 3.3v part that is 5v tolerant so it runs at either level. It will work with the DS1307 library too if you don't feel like redoing anything in your code.

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