P82B715_8

© NXP B.V. 2009. All rights reserved.

Product data sheet

Rev. 08 — 9 November 2009

6 of 23

NXP Semiconductors

P82B715

levels on the buffered bus. (The standard open-collector configuration is retained.) The

value and number of pull-up resistors used is flexible and depends on the system

requirements and designer preferences.

If P82B715 ICs are to be permanently connected into a system it could be configured with

design will be simplified, and performance improved, by fitting separate pull-ups on each

section of the bus. When a sub-system using P82B715 may be optionally connected to an

8.1.1 Pull-up resistance calculation

When calculating the pull-up resistance values, the gain of the buffer introduces scaling

factors which must be applied to the system components. In practical systems the pull-up

resistance value is usually calculated to achieve the rise time requirement of the system.

As an approximation, this requirement will be satisfied for a standard 100 kHz system if

the time constant of the total system (product of the net resistance and net capacitance) is

set to 1 microsecond or less.

In systems using P82B715s, the most convenient way to achieve the total system

nodes, and the buffered bus node, is designed by selecting its pull-up resistor to provide

the required rise time by setting its time constant (product of the pull-up resistance and

the system requirement will also be met with a small safety margin.

This arrangement, using multiple pull-ups as in Figure 6, provides the best system

extended system are disconnected or re-connected. For each bus section the pull-up

resistor for a Standard-mode system is calculated as shown in Equation 1:

(1)

Where: C device = sum of any connected device capacitances, and C wiring = total wiring

and stray capacitance on the bus section.

Remark: The 1

µs is an approximation, with a safety factor, to the theoretical

time-constant necessary to meet the Standard-mode 1

µs bus rise time specification in a

apply. The actual RC requirement can be shown to be 1.18

µs. For a Fast-mode system,

and the same safety margin, replace the 1

µs with its 300 ns requirement.

If these capacitances cannot be measured or calculated then an approximation can be

made by assuming that each device presents 10 pF of load capacitance and 10 pF of

trace capacitance and that cables range from 50 pF to 100 pF per meter.

If only a single pull-up must be used then it must be placed on the buffered bus (as R2 in

Figure 6) and the associated total system capacitance calculated by combining the

individual bus capacitances into an equivalent capacitive loading on the buffered bus.

R

1

µs

C device

C wiring

+

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=