Hub Interface
R
Intel
®
855PM Chipset Platform Design Guide
177
8. Hub
Interface
The Intel 855PM MCH and Intel 82801DBM ICH4-M pin-map assignments have been optimized to
simplify the hub interface routing between these devices. Intel recommends that the hub interface
signals be routed directly from the MCH to the ICH4-M with all signals referenced to VSS. Layer
transitions should be kept to a minimum. If a layer change is required, use only two vias per net and
keep all data signals and associated strobe signals on the same layer.
The hub interface signals are broken into two groups: data signals (HI) and strobe signals (HI_STB). For
the 11-bit hub interface, HI[10:0] are associated with the data signals while HI_STB and HI_STB# are
associated with the strobe signals.
Figure 94. Hub Interface Routing Example
Intel
ICH4-M
Intel
855PM
MCH
CLK
Synthesizer
CLK66
CLK66
HI[10:0]
HI_STB
HI_STB#
8.1. Hub
Interface
Compensation
This hub interface connects the 82801DBM ICH4-M and the Intel 855PM MCH. The hub interface uses
a compensation signal to adjust buffer characteristics to the specific board characteristic. The hub
interface requires resistive compensation (RCOMP).
The trace impedance must equal 55-
± 15%
Table 44. Hub Interface RCOMP Resistor Values
Component
Trace Impedance
HICOMP Resistor Value
HICOMP Resistor Tied to
ICH4-M 55
± 15%
36.5
± 1%
VSS
MCH 55
± 15%
36.5
± 1%
Vcc1_8
8.2.
Hub Interface Data HI[7:0] and Strobe Signals
The hub interface HI[7:0] data signals should be routed on the same layer as hub interface strobe
signals. There are two options for routing and include either an external layer or an internal layer.