Intel 815 - Manual (Page 108)

Hub Interface
R
108 Intel
®
815 Chipset Platform Design Guide
9.1.1 Data Signals
Hub interface data signals should be routed with a trace width of 5 mils and a trace spacing of
20 mils. These signals can be routed with a trace width of 5 mils and a trace spacing of 15 mils for
navigation around components or mounting holes. To break out of the GMCH and the ICH, the
hub interface data signals can be routed with a trace width of 5 mils and a trace spacing of 5 mils.
The signals should be separated to a trace width of 5 mils and a trace spacing of 20 mils, within
0.3 inch of the GMCH/ICH components.
The maximum trace length for the hub Interface data signals is 7 inches. These signals should each
be matched within ±0.1 inch of the HL_STB and HL_STB# signals.
9.1.2 Strobe Signals
Due to their differential nature, the hub interface strobe signals should be 5 mils wide and routed
20 mils apart. This strobe pair should be a minimum of 20 mils from any adjacent signal. The
maximum length for the strobe signals is 7 inches, and the two strobes should be the same length.
Additionally, the trace length for each data signal should be matched to the trace length of the
strobes, within ±0.1 inch.
9.1.3 HREF Generation/Distribution
HREF is the hub interface reference voltage. It is 0.5 * 1.85V = 0.92V ± 2%. It can be generated
using a single HREF divider or locally generated dividers (see Figure 54 and Figure 55). The
resistors should be equal in value and rated at 1% tolerance (to maintain 2% tolerance on 0.9V).
The value of these resistors must be chosen to ensure that the reference voltage tolerance is
maintained over the entire input leakage specification. The recommended range for the resistor
value is from a minimum of 100
Ω
to a maximum of 1 k
Ω
(300
Ω
shown in example).
The single HREF divider should not be located more than 4 inches away from either the GMCH or
ICH. If the single HREF divider is located more than 4 inches away, then the locally generated hub
interface reference dividers should be used instead.
The reference voltage generated by a single HREF divider should be bypassed to ground at each
component with a 0.01
µ
F capacitor located close to the component HREF pin. If the reference
voltage is generated locally, the bypass capacitor must be close to the component HREF pin.