Calculating The Physical Segment Pull-Up Resistor; Table 91. Bus Capacitance Reference Chart; Table 92. Bus Capacitance/Pull-Up Resistor Relationship - Intel 855GM Design Manual

R
Added Considerations for Mixed Architecture
1. The bus switch must be powered by V
2. Devices that are powered by the V
powered off. This is accomplished with the "bus switch".
3. The bus bridge can be a device like the Phillips PCA9515.
11.6.1.4.

Calculating the Physical Segment Pull-Up Resistor

The following tables are provided as a reference for calculating the value of the pull-up resistor that may
be used for a physical bus segment. If any physical bus segment exceeds 400 pF, then a bus bridge
device like the Phillips PCA9515 must be used to separate the physical segment into two segments that
individually have a bus capacitance less than 400 pF.

Table 91. Bus Capacitance Reference Chart

Device
ICH4-M
CK408
SO-
DIMMS
PCI
Slots
Bus
Trace
Length
in inches

Table 92. Bus Capacitance/Pull-Up Resistor Relationship

Physical Bus Segment Capacitance
®
Intel 855GM/855GME Chipset Platform Design Guide
# of Devices/ Capacitance Includes
Trace Length
1 Pin Capacitance
1 Pin Capacitance
2 Pin Capacitance (10 pF) + 1 inch worth of trace
capacitance (2 pF/inch) per SO-DIMM and 2 pF
3
connector capacitance per SO-DIMM
2 Each PCI add-in card is allowed up to 40 pF + 3 pF per
each connector
3
4
5
6
≥24 2 pF per inch of trace length
≥36
≥48
0 to 100 pF
100 to 200 pF
200 to 300 pF
300 to 400 pF
_
CC SUSPEND.
_ well must not drive into other devices that are
CC SUSPEND
Pull-Up Range (For Vcc = 3.3 V
8.2 k Ω to 1.2 k Ω
4.7 k Ω to 1.2 k Ω
3.3 k Ω to 1.2 k Ω
2.2 k Ω to 1.2 k Ω
I/O Subsystem
Cap (pF)
12
10
28
42
86
129
172
215
258
48
72
96
215