Specifications

Intel
®
810E2 Chipset Platform
R
Design Guide
149
6.2. Pull-up and Pull-down Resistor Values
The pull-up and pull-down values are system dependent. The appropriate value for a system can be
determined from an AC/DC analysis of the pull-up voltage used, the current drive capability of the output
driver, the input leakage currents of all devices on the signal net, the pull-up voltage tolerance, the pull-
up/pull-down resistor tolerance, the input high-voltage/low-voltage specifications, the input timing
specifications (RC rise time), etc. Analysis should be performed to determine the minimum/maximum
values usable on an individual signal. Engineering judgment should be used to determine the optimal
value. This determination can include cost concerns, commonality considerations, manufacturing issues,
specifications, and other considerations.
A simplistic DC calculation for a pull-up value is:
R
MAX
= (Vcc
PU
MIN - V
IH
MIN) / I
LEAKAGE
MAX
R
MIN
= (Vcc
PU
MAX - V
IL
MAX) / I
OL
MAX
Since I
LEAKAGE
MAX is normally very small, R
MAX
may not be meaningful. R
MAX
also is determined by
the maximum allowable rise time. The following calculation allows for t, the maximum allowable rise
time, and C, the total load capacitance in the circuit, including the input capacitance of the devices to be
driven, the output capacitance of the driver, and the line capacitance. This calculation yields the largest
pull-up resistor allowable to meet the rise time t.
R
MAX
= -t / (C * In(1-(V
IH
MIN / Vcc
PU
MIN) ) )
Figure 88. Pull-up Resistor Example
Vcc
pu
min.
R
max
V
IH
min.I
Leakage
max.
Vcc
pu
max.
R
min
V
IL
max.I
OL
max.
pwr_pullup_res