Datasheet
P82B96_8 © NXP B.V. 2009. All rights reserved.
Product data sheet Rev. 08 — 10 November 2009 11 of 32
NXP Semiconductors
P82B96
Dual bidirectional bus buffer
Figure 13 shows how a master I
2
C-bus can be protected against short circuits or failures
in applications that involve plug and socket connections and long cables that may become
damaged. A simple circuit is added to monitor the SDA bus, and if its LOW time exceeds
the design value, then the master bus is disconnected. P82B96 will free all its I/Os if its
supply is removed, so one option is to connect its V
CC
to the output of a logic gate from,
say, the 74LVC family. The SDA and SCL lines could be timed and V
CC
disabled via the
gate if one or other lines exceeds a design value of ‘LOW’ period as in
Figure 28 of
AN255
. If the supply voltage of logic gates restricts the choice of V
CC
supply then the
low-cost discrete circuit in Figure 13 can be used. If the SDA line is held LOW, the 100 nF
capacitor will charge and the Ry input will be pulled towards V
CC
. When it exceeds 0.5V
CC
the Ry input will set the Sy input HIGH, which in practice means simply releasing it.
In this example the SCL line is made unidirectional by tying the Rx pin to V
CC
. The state of
the buffered SCL line cannot affect the master clock line which is allowed when
clock-stretching is not required. It is simple to add an additional transistor or diode to
control the Rx input in the same way as Ry when necessary. The +V cable drive can be
any voltage up to 15 V and the bus may be run at a lower impedance by selecting pull-up
resistors for a static sink current up to 30 mA. V
CC1
and V
CC2
may be chosen to suit the
connected devices. Because DDC uses relatively low speeds (< 100 kHz), the cable
length is not restricted to 20 m by the I
2
C-bus signalling, but it may be limited by the video
signalling.
Fig 13. Extending a DDC bus
P82B96
SCL
Rx
Tx
002aab989
Sx
V
CC
Ry
Ty
Sy
I
2
C-bus/DDC
master
SDA
V
CC1
GND
Rx
Tx
Ry
Ty
V
CC
Sx
Sy
SCL
SDA
V
CC2
GND
I
2
C-bus/DDC
slave
4.7 kΩ
BC
847B
100 nF
100
kΩ
+V cable drive
470 kΩ
470 kΩ
BC
847B
3 m to 20 m
cables
I
2
C-bus/DDC
R
G
B
video signals
PC/TV receiver/decoder box
P82B96
monitor/flat TV
+V cable drive