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Rajan Arora

Oct 15, 2015

Repeater

I2C
Hub

TCA9548A

TCA9617B

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How to simplify I2C tree when connecting multiple slaves to an
I2C master

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Electronic products such as smartphones, tablets, TVs, cars … even
refrigerators and washing machines have added more and more features over the
last few years to make life easier for users. To play games and monitor your
health on a smartphone, you need haptic control and a touchscreen interface,
which require additional sensors. Other systems in the automotive, industrial,
communication markets have also added more sensors. Example systems include:

Automotive, such as infotainment, body and advanced driver assistance
systems (ADAS).

Enterprise and communications, such as servers, network switches, and base
stations.

Industrial, such as building or factory automation, and smart
grid.

Many times these sensors communicate with the microprocessor/microcontroller
(MCU) or with one another using the I2C
interface. These sensors can be integrated circuits (ICs) with different
functionality (e.g. temperature sensor, pressure sensor, touchscreen interface),
or they can be the same IC needed at multiple locations on the board (such as
multiple temperature sensors). When system engineers draw a schematic of such a
system, the I2C “tree” can get complicated, as shown in Figure 1. A
complicated I2C tree can sometimes cause communication errors between
the device and MCU, as degrading signal integrity can result in missed bits.
Such an error can be pretty painful to debug, or worse, lead to product
recalls.

Figure 1: An electronic
system with multiple IC’s interfacing with an MCU using an I2C
bus

Using I2C switches
and repeaters
significantly simplifies an I2C tree. You can use an I2C
switch to split the I2C bus into multiple downstream I2C
buses, as shown in Figure 2, splitting the I2C slaves into multiple
I2C buses. You can select each downstream I2C path based
on which slave needs to communicate with the MCU. In Figure 2, multiple IC’s of
the same slave are split between different I2C downstream
channels.

Figure 2: Using an
I2C switch to split an upstream I2C bus into multiple
downstream buses

If only one downstream channel is turned on at a time, there should not be
any issues (unless more than 15 slaves are connected on one channel). There
could be scenarios, however, where multiple paths need to be on at the same
time. If multiple slaves are on the upstream I2C bus at the same
time, there could be signal-integrity issues, which could result in
miscommunication. Assume that each slave device will contribute ~15pF of
capacitance on the I2C bus, and the overall capacitance on the bus
cannot exceed the I2C specification level. Figure 3 shows how using
I2C repeaters resolves such issues. The I2C repeater
buffers the I2C signal, allowing the connection of multiple slaves to
the downstream bus.

Figure 3: Using repeaters
on the downstream I2C bus to improve signal integrity

Figure 4 shows the clock (SCL) waveform at the input (SCL_in) and the output
of the switch (SCL_out1 from Figure 2). Multiple slaves (> 25) are connected
to the I2C bus, with a TCA9617B repeater added to the
downstream path to improve signal quality (SCL_out2, Figure 3). As you can see
from the figure, the SCL_out1 waveform has signal-integrity issues and can
result in missed bits at a higher capacitance load. The SCL_out2 signal with the
addition of the repeater shows much-improved signal-integrity
characteristics.

Figure 4: The
I2C SCL signal at the input (SCL_in) and output of the switch without
a repeater (SCL_out2), and at the output with a repeater on the downstream
I2C bus (SCL_out2)

Each channel may not require the addition of a repeater; you can look at the
waveforms on the oscilloscope to determine this. Another issue you may see, when
many slaves on the I2C bus increase capacitance, is related to
pull-up strength. To improve the signal integrity caused by increasing
capacitance, system engineers usually decrease the size of the pull-up resistor.
However, this can cause issues pulling down the signal, as the slaves may not
have strong enough pull-down. There is a limit to how much current a slave
device can safely sink to ground (IOL). Adding a repeater helps
increase the pull-down strength.

This technique is a useful strategy to implement into complex I2C
trees and avoid any signal-integrity-related issues.

Additional resources

Read TI’s application note, “Maximum
Clock Frequency of I2C Bus Using Repeaters.”

Read TI’s application note, “Understanding
the I2C Bus.”

Find out more about TI’s TCA9617B
repeater.

Brian Burk

We used the TCA9548A on our haptic design with 8x DRV2605L haptic drivers.
Check out the design below.  It works great!

http://www.ti.com/tool/drv2605levm-md

Oct 15, 2015

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