Download TB-101: Optically Isolating an I2C Interface

Technical Brief: TB-101
INTEGRATED CIRCUITS DIVISION
One way to avoid the latch-up problem is to use a
separate input and output pin on all devices at SideB
(so that when the repeater output is driven low it does
not drive the repeater input low). There are some
peripherals, which have been intentionally designed to
run with standard optoisolators, that do utilize this extra
wire to provide separate data input and output wires at
devices at SideB. This does require an extra package
Figure 4
pin, and prohibits the use of most I2C devices (without
the extra pin) at the isolated SideB bus. This also
complicates debugging of the bus because most I2C
bus analyzers do not have a provision for the third wire,
and thus can only be used on SideA of the isolated
bus.
Separate In and Separate Out at SideB
VDDB
VDDA
Bus Repeater
Separate IN / OUT
499Ω
OB
499Ω
20pF
IOA
20pF
VDDB
499Ω
IB
VIN
SideB Driver
0V
SideB Receiver
4
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December 5, 2012