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8.2
Pullup and Pulldown Resistor Values
Pullup and pulldown values are system dependent. The appropriate value for your system can be
determined from an AC/DC analysis of the pullup voltage used, the current drive capability of the
output driver, input leakage currents of all devices on the signal net, the pullup voltage tolerance,
the pullup/pulldown resistor tolerance, the input high/low voltage specifications, the input timing
specifications (RC rise time), etc. Analysis should be done to determine the minimum/maximum
values that may be used 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 pullup value is:
Since I
LEAKAGE
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 input
capacitance of the devices to be driven, output capacitance of the driver, and line capacitance. This
calculation yields the largest pullup resistor allowable to meet the rise time t.
Figure 8-1. Pullup Resistor Example
I
Leakage
®
Intel
810A3 Chipset Design Guide
R
= (Vcc
MIN - V
MAX
PU
R
= (Vcc
MAX - V
MIN
PU
MAX is normally very small, R
R
= -t / (C * In(1-(V
MAX
V c c
Min
pu
R
M a x
Max
MIN) / I
MAX
IH
LEAKAGE
MAX) / I
MAX
IL
OL
may not be meaningful. R
MAX
MIN / Vcc
MIN) ) )
IH
PU
V
Min
I
Max
IH
O L
Design Checklist
is also
MAX
V c c
Max
pu
R
Min
V
Max
IL
pull_res.vsd
8-15
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