Calculating The Physical Segment Pull-Up Resistor - Intel Pentium M Processor Design Manual

® ®
Intel Pentium M Processor and Intel
I/O Controller Hub 3 (Intel
Added Considerations for Mixed Architecture:
•
The bus switch must be powered by VCC_SUSPEND.
•
Devices that are powered by the VCC_SUSPEND well must not drive into other devices that
are powered off. This is accomplished with the bus switch.
•
The bus bridge may be a device like the Phillips PCA9515.
9.5.4

Calculating The Physical Segment Pull-Up Resistor

Table 80
be used for a physical bus segment. When any physical bus segment exceeds 400 pF, then a bus
bridge device such as 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 80. Bus Capacitance Reference Chart
Device
®
Intel ICH3-S
CK408
DIMMS
PCI Slots
SMBus Trace
Length in
inches
Table 81. Bus Capacitance/Pull-Up Resistor Relationship
Physical Bus Segment Capacitance
162
®
E7501 Chipset Platform
®
ICH3-S)
and Table 81 present references for calculating the value of the pull-up resistor that may
No. of Devices/
Trace Length
1 Pin Capacitance
1 Pin Capacitance
2 Pin Capacitance (10 pF) + 1 inch of trace capacitance
(2 pF/inch) per DIMM and 2 pF connector capacitance per
3
DIMM
2
3
Each PCI add-in card is allowed up to 40 pF + 3 pF per each
4
connector
5
6
≥
24"
≥
36" 2 pF per inch of trace length
≥
48"
0 to 100 pF
100 to 200 pF
200 to 300 pF
300 to 400 pF
Capacitance Includes
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
Cap (pF)
12
10
28
42
86
129
172
215
258
48
72
96
Design Guide