Intel 820E manual ISA Support, Disabling the Native USB Interface of ICH2

Intel® 820E Chipset

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Figure 56. USB Data Signals

	Driver	15	Ω	Motherboard Trace
P+		< 1"		45	Ω	Connector
				15k	Optional 47 pF		90 Ω
				Motherboard Trace
	Driver
		15	Ω			USB
P-		< 1"		45	Ω
				15k	Optional 47 pF
	ICH2		Transmission Line				USB Twisted Pair Cable

Recommended USB trace characteristics

•	Impedance Z0	=	45.4 Ω
•	Line delay	=	160.2 ps
•	Capacitance	=	3.5 pF
•	Inductance	=	7.3 nH
•	Resistance at 20 ° C	=	53.9 mΩ

2.14.3.Disabling the Native USB Interface of ICH2

The ICH2 native USB interface can be disabled. This can be done when an external PCI based USB controller is being implemented in the platform. To disable the native USB Interface, ensure the differential pairs are pulled down thru 15 kΩ resistors, ensure the OC[3:0]# signals are de-asserted by pulling them up weakly to VCC3SBY, and that both function 2 and 4 are disabled via the D31:F0;FUNC_DIS register. Ensure that the 48 MHz USB clock is connected to the ICH2 and is kept running. This clock must be maintained even though the internal USB functions are disabled.

2.15.ISA Support

Implementations that require ISA support can benefit from the enhancements of the ICH2, while “ISA- less” designs are not burdened with the complexity and cost of the ISA subsystem. For an implementation of an ISA design, contact external suppliers.

Design Guide 93