what's exactly the difference (on hardware-level) between the register- and the DMA-transfer mode of the I2C-interface in FX3?
I understood, the register-mode should be used if only some bytes are to be transferred and the DMA-mode in case of more data (which should be multpile of 16 bytes).
Is the behaviour on the I2C-wire the same using either register- or DMA-mode or are there some differences? Are there any differences in transfer-speed?
The behavior on the I2C lines is going to be the same irrespective of the method. The 2 methods are based on how the firmware handles the transfer internally hence the size of transfer based recommendation.
You should be able to find more information through the comments in cyu3i2c.h