Datasheet

PCA9600 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2011. All rights reserved.
Product data sheet Rev. 5 — 5 May 2011 21 of 31
NXP Semiconductors
PCA9600
Dual bidirectional bus buffer
Question: We have 2 meters of cable in a bus that joins the TX/RX sides of two
PCA9600 devices. When one TX drives LOW the other PCA9600 TX/RX is driven to
−0.8 V for over 50 ns. What is the expected value and the theoretically allowed value of
undershoot?
Answer: Because the cable joining the two PCA9600s is a ‘transmission line’ that will
have a characteristic impedance around 100 Ω and it will be terminated by pull-up
resistors that are larger than that characteristic impedance there will always be negative
undershoot generated. The duration of the undershoot is a function of the cable length
and the input impedance of the connected IC. As shown in Figure 24
, the transient
undershoot will be limited, by the diodes inside PCA9600, to around −0.8 V and that will
not cause problems for PCA9600. Those transients will not be passed inside the IC to
the SX/SY side of the IC.
Question: If we input 800 mV undershoot at TX, RX pins, what kind of problem is
expected?
Answer: When that undershoot is generated by another PCA9600 and is simply the
result of the system wiring, then there will be no problems.
Question: Will we have any functional problem or reliability problem?
Answer: No.
Fig 24. Wiring transients limited by the diodes in PCA9600
002aaf082
time (ns)
2
0
4
6
voltage
(V)
−2
horizontal scale = 62.5 ns/div
send
receive
5 V
RX
TX
SX
PCA9600
send
300 Ω
5 V
2 meter
cable
5 V
300 Ω
GND
receive
SX
RX
TX
5 V