32 kHz Clock required for USB ? | Cypress Semiconductor
32 kHz Clock required for USB ?
Hi, we designed and assembled two prototyping board mounting a ES of the EZ-FX3. The board plugs directly into a Xilinx virtex6 development board trough an FMC connector.
The “thing” is now working well end delivers 220 MB/s using the modified BulkSource reference firmware posted by aasi on this forum.
On our original board design we simple connected the 32 kHz Watchdog Timer Clock Input to ground as one would do if there is no interest on using the watchdog functionality (also considering that in the datasheet the 32 kHz clock is referred as optional). Using this approach unfortunately the USB bus do not works. The CPU boots correctly and tries to load the firmware from the I2C (if configured so) but the load from USB simply doesn’t work since the USB interface doesn’t work.
Well, we mounted a second prototyping board but this was affected from the same problem. In the end, in order to make our system as much similar as the Cypress development board we connected the Watchdog Timer Clock input to a 32KHz clock source … and … miracle … The USB 3 interface works perfectly, with the above mentioned performances.
My question: why is there no mention about this important detail in the FX3 datasheet ? Do we missed something ? Did somebody ever tried to use an USB 3.0 FX3 without the 32 KHz clock ?