I2C bus non functional - missing pull up resistors?



I’ve been looking at the I2C bus using an oscilloscope to try to determine if it was working on my mangoh green board. We are trying to build an IoT board and need to use the GPIO_IOTx_RESET lines and are having trouble with it…

The board is labelled ‘1401063 Rev B’ on silkscreen and ‘1600643-18 DVB-0317 421’ on the sticker.

Looking at the published schematic, it seems R242/R243 (2K2 pullups on the I2C1_DATA/I2C1_CLK lines) are “DNI” (Do not install).

If I measure the signals with the scope, I see data (blue trace) but the clock line is not working correctly (yellow trace). Also the blue trace is quite slow to pull up (rounded edge), but that isn’t easy to see in this particular captures.

If I fit the two missing resistors, I get a correct signal, and I can decode traffic.

[ second image deleted because of posting restrictions, will try to attach to a reply ]

Is this a hardware build issue or has there been a configuration error on the CF3 IO Pin?

I’m assuming that the data line was working with internal pin pull up resistors on the CF3, as there are no others on the bus. Was the clock pin not configured correctly?

Thanks for your help,


Here is the second image I couldn’t attach in the previous post…

[edit]: Should also mention this is a WP7502 processor…



Hi Ash,
WP75 has internal pull resistors on I2C bus and no need to add external pull-ups.
It sounds to me you have HW issue. Can you use another module to find if it is the module or PCB issue.


Hi Habib,

Thank you for the reply.

I can confirm that swapping to a new WP7502 on the board (after removing the resistors I put on) works as expected.

We are running our own build on the first unit, the second one was factory. Is there a way we could have miss-configured the I2C pins in the build? If so, where do we start looking?