I2C connection problems on M8T XL board

I’m having an issue with the i2c connection on my M8T XL board. The M8T module is configured for the default i2c address 0x42. The u-center software confirms that the i2c port is enabled and configured to signal TXREADY on the disabled UART TX pin. I’m only seeing an address 0x1E responding on the i2c bus–I presume that is the magnetometer. Have I overlooked something or is the i2c connection inoperable for the GPS receiver?

I’m developing a low-power device that’s intended to run for days on battery power. My goal is to quickly fetch buffered data packets when TXREADY interrupts and wakes up my microcontroller. Monitoring a streaming serial line requires lighter sleep states and higher power consumption.

Another minor point: the resistance between +5V and SCL or SDA is around 8M ohms, rather than the typical 4.7K-10K ohms. My i2c master was reporting phantom devices at every address until I diagnosed the problem and added a pullup resistor.
David Pletcher


The I2C address of the HMC5983 magnetometer is 0x1E indeed.

I would recommend you to ensure you have the I2C port properly configured in CFG-PORT abd CFG-MSG settings in U-Center. 0x42 is he default address but it can be changed with those settings. The documentation says that SDA and SCL lines have internal pull-ups, and these resistors are very critical in an I2C bus. 8MOhms seems way too high, could you give a try with 10k and nothing else on the bus except the M8T?

Thank you for your response.

I can’t think of any non-destructive way to test the M8T i2c connection in isolation. The tiny magnetometer is connected to i2c and surface mounted on the XL board; I don’t have the tools or skills to remove it.

Is there a board layout diagram available? I’d like to double-check that the M8T is connected to the i2c bus at all. I read a note in the u-blox forums suggesting that drone market boards usually connect i2c only to the MEMS sensors, not to the GPS module. The simplified schematic in the RTK document (found in the Docs section) doesn’t include that level of detail.

Kind regards,

Wait sorry, I thought you had reworked the board by yourself. On XL board, M8T is NOT connected to the I2C bus. Only magnetometer is connected to the I2C bus. The only available communication interface with M8T is UART or USB (not present in the picture below).

Ahh, too bad, thank you for the information!

Feel free to disregard or close the ticket without further action; I submitted it just before you responded here.

To answer your question, power LED is connected to power supply directly, so the only way to disable it is to remove it.

Feel free to ask any question!