NASR-M board bring-up, USB adventures

It’s USB show time! Xilinx documents suggest the USB3320 as a good USB PHY that works, and I’ve seen multiple development boards that use that chip. If it works for them, it will probably work for me, I thought. Despite being defined in Vivado hardware design, USB was not detected by the Linux kernel. Looking closely, I saw that a reference clock was ok, but there was no signal on the CLKOUT pin.