Rajan Arora

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 (for example, 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.

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 (I_OL). 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.

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)

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.