How does the USB3300 control the HostDisconnect bit found in the ULPI RXD command byte?
The host disconnect bit is asserted only in Hi-Speed Host mode when a device is disconnected. Here’s how it works. The end of packet delimiter (EOP) for the host start of frame packets (SOF’s) is a 40bit long sequence of J’s or K’s. The USB3300 PHY has a comparator that samples the 32nd to 36th bit times of the EOP. If an enumerated device’s 45 ohm terminations to ground are not present, the amplitude of the EOP will be approximately 800 mV indicating a disconnect and the HostDisconnect bit will be asserted. If the 45 ohm device terminations are present, the EOP amplitude will be approximately 400 mV and the HostDisconnect bit will not be asserted. The PHY will queue an RXD command byte with the state of the HostDisconnect bit once HostDisconnect has been asserted. This is done so the HostDisconnect assertion won’t be missed. It is important that the USB3300 be placed in exactly the HOST Hi-Speed mode called out in the ULPI rev 1.1 specification table 24. In device mode, HostDisconnect
