Function of  Discon bit in EZ-USB

Question: What is functioning of DISCON bit ?



A USB hub or host detects attachment of a full speed device by sensing a high level on the D+ wire. A USB device provides this high level using a 1500-ohm resistor between D+ and 3.3V (the D+ line is normally low, pulled down by a 15 K-ohm resistor in the hub or host). The 1500-ohm resistor is internal to the EZ-USB. The EZ-USB accomplishes ReNumeration by selectively driving or floating the 3.3V supply to its internal 1500-ohm resistor. When the supply is floated, the host no longer looks the EZ-USB; it appears to have been disconnected from the USB. When the supply is then driven, the EZ-USB appears to have been newly-connected to the USB. From the hosts point of view, the EZ-USB can be disconnected and re-connected to the USB, without ever physically disconnecting. The connect state of EZ-USB is controlled by a register bit called DISCON (USBCS.3), which defaults to 0, or connected. This default may be overridden by setting the DISCON bit in the EEPROM configuration byte to 1, which allows the EZ-USB to come up disconnected. The EZ-USB core sees that this DISCON bit is set, and sets the USBCS.3 bit before the CPU is taken out of reset. The DISCON bit in the EEPROM configuration byte cannot be used to instruct the EZ-USB to connect to the USB bus. Once the CPU is running, firmware can modify this bit.