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1.3. PHY Selection
The timing characteristic of ULPI PHYs vary across the spectrum of available devices.
Arria® V and Cyclone® V USB 2.0 OTG controllers support the following clock modes:
- Output clock mode—the PHY drives the clock to the controller
- Input clock mode—an external clock source on the board or a clock input sourced from the FPGA fabric drives the PHY.
All ULPI PHYs support output clock mode.
Newer ULPI PHYs support input clock mode. This mode compensates for timing mismatches between the PHY and the controller.