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Thank you for contacting Intel® technical support. I will be more than happy to help you.
I understand that you want to be able to change the I2C clock frequency in an Intel® NUC Board NUC5i3MYBE. For better assistance with your request, I am escalating this thread to a higher level of support.
You will be contacted back as soon as the next level of support has reviewed your request and found a resolution.
Originally, the I2C-bus was limited to 100 kbit/s operation. Over time there have been several additions to the specification so that there are now five operating speed categories. Standard-mode, Fast-mode (Fm), Fast-mode Plus (Fm+), and High-speed mode (Hs-mode) devices are downward-compatible — any device may be operated at a lower bus speed so I would assume that it at least should work at 100 kbit/s. I dont believe there is any "default speed" but speed sensing.
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I hope this helps,
I would add that it is the destination device that actually controls the speed of the bus. When a slow device is being talked to, devices capable of much faster speeds must twiddle their thumbs waiting for the bus to become available.