# Can I2C work without pull up resistor?

I'm working with nrf51 board, I'm using I2C to communicate with an accelerometer (see the scheme), the communications are successful but I dont see any pull up resistors in the lines SDA and SCL.

could I2C work without pull-up resistors or is the scheme wrong or does the accelerometer has internal pull up on lines SDA and SCL

• At leas highlight the lines, so we don't have to break our eyes.. Anyway the PUs can be internal to a chip. I2C can't work without them. – Eugene Sh. Dec 16 '15 at 17:36
• you can click on the image and zoom, it gives a good look – Makhlouf GHARBI Dec 16 '15 at 17:38
• And then to parse all of the lines? Thank you. I'll pass. – Eugene Sh. Dec 16 '15 at 17:39
• @EugeneSh. SDA and SCL are written in red it's really clear – Makhlouf GHARBI Dec 16 '15 at 17:40
• Don't be so lazy - do the job properly - it's impossible to read the part numbers because the resolution is bad. – Andy aka Dec 16 '15 at 17:41

## 1 Answer

Real IIC requires pullup resistors. Many microcontrollers have optional internal pullup resistors on some pins. If there is a micro on the bus, and that's usually the case, then it could be switching its pins between pullup and active low.

The pullups in a micro are usually higher resistance than what you'd use for IIC, but for a short bus all on one board when you know the bit rate is low enough, this can be a legitimate thing to do. I've done it.

• I've done I2C with internal pull-ups too. But just in case, I had pads for discrete resistors which were DNL (do not load). – Nick Alexeev Dec 16 '15 at 17:52
• @Nick: That sounds like a good approach. – Olin Lathrop Dec 16 '15 at 18:34