I'm making an interface board using FT2232H in sync mode with a small FPGA, and thought how everything could be much easier if I can just use FT2232H as my clock source. Unfortunately I need different frequency and deriving my clock from 60 MHz would be way too much trouble.

So... can I swap 12 MHz crystal for something between 6 and 12 MHz or is it somehow tied to how USB works?

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    \$\begingroup\$ It is very much tied to how USB works, but see the data sheet for your options, if any. Modern FPGAs have a lot of capability to manipulate clocks, are you sure you can't use those? I'm assuming you want to be able use the ftdi to program the fpga so generating its clock in the fpga would be tricky. \$\endgroup\$ – Chris Stratton Nov 30 '15 at 17:47

Why are USB devices slower than 480 MBit/s

Oh, I see now. If I reduce bit rate I won't be able to hit the allocated time slot. Also USB 2.0 spec requires +/- 500 ppm accuracy.

Oh, well.. back to 2 clocks.

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