BBB RTC

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sdas86
 
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Joined: Tue Sep 23, 2014 9:35 pm

BBB RTC

Post by sdas86 »

Dear all,
I have BBB Rev B3. I am looking for RTC for this BBB. I found this RTC on adafruit.
https://www.adafruit.com/products/264
The item above is 5V output SCL and SDA. I am afraid if I bought the RTC item mentioned above, the 5V might spoil my BBB board.

I am not sure BBB I2C2 SDA(Pin20) and I2C2 SCL(Pin19) pin is 5V or 3.3V. May I know how to check the voltage of the two pins?

Thanks.

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jlbeck
 
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Re: BBB RTC

Post by jlbeck »

The I2C lines are 3.3V. You'll need to use a logic level converter such as

https://www.adafruit.com/products/757

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jlbeck
 
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Re: BBB RTC

Post by jlbeck »

If you leave R1 and R2 off the RTC then the I2C operates at 3.3V and there shouldn't be a problem and you don't
need a logic level converter.

It's all covered in the tutorial

https://learn.adafruit.com/adding-a-rea ... bone-black

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sdas86
 
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Re: BBB RTC

Post by sdas86 »

Hi,
Thanks for the reply. I read through the DS1307 Datasheet but I do not find the 3.3V information.
http://datasheets.maximintegrated.com/en/ds/DS1307.pdf

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adafruit_support_mike
 
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Re: BBB RTC

Post by adafruit_support_mike »

At the top of page 2, in the table "Recommended Operating Conditions", it lists the logic 1 input (V.ih) as 2.2v. It will read 3.3v input as HIGH and 0v as LOW.

I2C is an active-low protocol that uses pull-up resistors and open-drain devices. Devices transmit data by pulling the bus LOW and then releasing it. Devices decide whether the bus is pulled low or still high by comparing it to their HIGH and LOW thresholds.

The BBB's pull-up resistors wlil hold the I2C bus at 3.3v when no device is pulling it down. That's higher than the DS1307's HIGH threshold, so the DS1307 will be able to read signals generated by the BBB.

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