P82B96_8 © NXP B.V. 2009. All rights reserved.
Product data sheet Rev. 08 — 10 November 2009 20 of 32
NXP Semiconductors
P82B96
Dual bidirectional bus buffer
10.2.1 Example with questions and answers
Question: On a falling edge of Tx we measure undershoot at −800 mV at the linked Tx,
Rx pins of the P82B96 that is generating the LOW, but the P82B96 data sheet specifies
minimum −0.3 V. Does this mean that we violate the data sheet absolute value?
Answer: For P82B96 the −0.3 V Absolute Maximum rating is not intended to apply to
transients, it is a DC rating. As shown in Figure 22, there is no theoretical reason for any
undershoot at the IC that is driving the bus LOW and no significant undershoot should
be observed when using reasonable care with the ground connection of the ‘scope. It is
more likely that undershoot observed at a driving P82B96 is caused by local stray
inductance and capacitance in the circuit and by the oscilloscope connections. As
shown, undershoot will be generated by PCB traces, wiring, or cables driven by a
P82B96 because the allowed value of the I
2
C-bus pull-up resistor generally is larger
than that required to correctly terminate the wiring. In this example, with no IC
connected at the end of the wiring, the undershoot is about 2 V.
Fig 22. Transients generated by the bus wiring
002aaf064
time (ns)
2
0
4
6
voltage
(V)
−2
horizontal scale = 62.5 ns/div
send
receive
5 V
Rx
Tx
Sx
P82B96
send
300 Ω
5 V
2 meter
cable
5 V
300 Ω
GND
receive
