On the ML40x board, GPIO_31 and 30 are connected to EEPROM and tied to Vcc 3.3v. It means that the USB chip will operate in the standalone mode. For standalone mode to work, you need to program theEEPROM.
Xapp925 and other Xilinx reference designs indicate that the device operates in HPI Mode.
The reason is that, sometimes, Xilinx isnot sure if the EEPROM has been configured with USB firmware or not. Therefore, by leaving the board connections as it is will not affect the mode of operation. As at power on, the USB chip tries to load the configuration data from the EEPROM and finds no firmware.
So, in the current reference system where we are using the OPB EPC core, Xilinx configures the Cypress USB device through HPI mode. By doing this, Xilinx forces the Cypress USB chip to operate in co-processor mode.
In other words, the USB worksin co-processor mode.
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