Question about interfacing DS1307 to beagleboard black

General Discussion
1

Hi,
I’m pretty new to understanding how the beagleboard black works but I had a question regarding the gpio pins.

From what I’ve found, the gpio pins for the board are only 3.3v tolerant and I wanted to ask if my understanding on the following was correct

If I had a DS1307 rtc and I wanted to connect it to the board, would it break it?

According to the datasheet…

https://www.analog.com/media/en/technical-documentation/data-sheets/ds1307.pdf

it operates at a supply voltage between 4.5 and 5.5 volts and has a high logic lvl input between 2.2 and at a maximum 5.7 volts and a low logic level input of 0.3 to 0.8

Would it be safe for me to connect the board to it?

I think it would be since the most that the ds1307 can output is 0.4v.

2

logic level 1 is Vcc + .3V max, if Vcc is 5 then it will most likely break it.

now if ya just pull the I2C lines to 3v3 with 2.2k to 5.1k then it should be ok

do so at your own risk

3

I would either find an RTC that operates at 3.3V or you have to look into I2C level shifting.

You’re almost certain to damage your BBB if you proceed. You’ve been warned.

4

But IoL (logic low output) is only 0.4V at most, would it still affect the board. As was said, as long as I have vcc at 3.3v. It should be fine right? Im mostly asking to check my understanding of it.

5

as long as all voltages applied to the RTC are 3.3v, then the BBB will be ok, but the RTC may not work being it needs 5v to operate.

i do agree with @lranders that you should look for the 3.3 volt part or use voltage level shifting.

6

The DS3231 is a 3.3v rtc, I believe it uses the same driver as the DS1307.

1 Like
7

The VOL is just a measure of how strong their output FET is.

I took another look and there’s a lot of caveat’s, but technically you shouldn’t need
any level shifting because I2C is an open-drain bus, so as long as you drive the
two pull-up resistors from the BBB 3.3V pin, you should be ok, even when supplying
the DS1307 with it’s intended 5V.

As I said, this only applies to I2C, so don’t go and think you can do the same on say, SPI.

While it’s not uncommon to intermix 3.3V and 5V devices on I2C,
often you will see people using a PCA9545A or similar to keep the busses
separate in case of a device shorting out.
That way the damage will be contained in the local bus segment.