In I2C communication, pull-up resistors are essential components to ensure optimal signal integrity for reliable and efficient data exchanges between devices. In this blog, we'll dive into the role pull up resistors play in I2C communication as well as how to determine the best resistor value for your I2C application.

What is the I2C Protocol?

I2C, which stands for Inter-Integrated Circuit, is a popular communication protocol used in embedded systems for connecting low-speed peripheral devices to processors and microcontrollers. This protocol operates using a simple two-wire interface that includes the Serial Data Line (SDA) and the Serial Clock Line (SCL), where data is synchronously transmitted.

I2C two-wire bus layout

More on the background of I2C can be found here.

The Role of Pull-Up Resistors

In an I2C system, both the SDA and SCL lines are open-drain, meaning they can pull the line low but cannot drive it high. Instead, an external component is required to pull the line back to a high state – this is where pull-up resistors come in.

Image by Gosia K. from Pixabay

Pull-up resistors are connected between the SDA and SCL lines – one on the SDA line and one on the SCL line – and the positive supply voltage.

Their primary function is to ensure that the lines are pulled high when no device is pulling them low. Without these resistors, the lines would “float”, resulting in undefined states. The addition of the pull-up resistors creates a clear distinction between low (logical 0) and high (logical 1) states to help ensure stable communication.

Choosing the Right Pull-Up Resistor Value

Pull-up resistors can have various resistance values (in ohms). This value must be chosen to balance the rise time, power consumption, and signal integrity to ensure reliable I2C communication.

Relationship Between Pull-Up Resistor Value and Rise Time

The rise time is the time it takes for a signal to transition from a low voltage level to a high voltage level. The bus capacitance, or its ability to store a charge, affects rise time because it determines how quickly a circuit can charge. Overall, the total bus capacitance and the communication speed should be taken into consideration when choosing the value of the pull-up resistor.

Lower resistance values (Ex: 1 kΩ to 4.7 kΩ) allow more current to flow through the pull-up resistors. This increased current charges the capacitance of the bus lines more quickly, resulting in faster rise times which is vital in higher speed communications. While lower resistance values improve rise times, they can also lead to an excessive flow of current, causing unnecessary power consumption and potential damage to the components. 

Higher resistance values (Ex: 10 kΩ) reduce the current flowing through the pull-up resistors, resulting in a slower charging of the bus line capacitance. This results in longer rise times, which can cause delays in signal transitions and potentially lead to bus errors or unreliable communication.

Calculating Pull-Up Resistor Values

For most standard I2C communication, both 4.7 kΩ and 10 kΩ resistors should work well. However, there may be instances where calculating exact pull-up resistor values is optimal for system performance, and this depends on factors like speed, capacitance, and the number of devices connected on the bus. Some device datasheets may specify the recommended pull-up resistor value that is optimal for device performance.

Otherwise, the I2C specification suggests two methods to determine the correct pull-up resistor value: calculating the maximum pull-up resistor value and the minimum pull-up resistor value, both of which can be used to determine an appropriate range for pull-up resistors in specific applications.

The minimum pull-up resistor value is based on bus capacitance and rise time.

The bus capacitance includes the capacitance of bus wires, connection points, and pins. For the most accurate pull-up values, it is best to measure bus capacitance, though an estimated value or the maximum allowable capacitance specified by the I2C standard can also be used.

The rise time is the period during which the signal transitions from V_IL(MAX) (0.3*V_DD) to V_IH(MIN)(0.7*V_DD). The rise time value can typically be found on the device's data sheet.

The formula for the maximum pull-up resistor value is:

R_p(max)= T_rise/0.8473×C_bus

Where T_rise​ is the maximum rise time and C_bus is the total bus capacitance.

The minimum pull-up resistor value is based on the supply voltage (Vdd).

The minimum resistor value is constrained by the supply voltage, as it must meet the minimum sink current requirements of 3 mA for Standard mode (100 kHz) or Fast mode (400 kHz), and 20 mA for Fast mode Plus (1 MHz).

The formula for the minimum pull-up resistor value is:

R_p(min)= V_DD-V_OL(max)/I_OL

Where ​V_DD is the supply voltage, V_OL(max) is the maximum output low voltage, and I_OL is the minimum sink current.

Min and Max Resistor Value Trade-Offs

Once you've established the minimum and maximum range for the pull-up resistor's value, you can determine the exact resistor value by weighing the trade-offs between speed and power consumption. As mentioned earlier, lower resistance values reduce power consumption and improve speed but increase current draw, while higher values save power but may slow down signal transitions and lead to communication errors. It’s important to consider these factors when selecting the resistor value for specific applications and what is most optimal for its purpose and operation.

I2C Host Adapters and Protocol Analyzers from Total Phase

Total Phase supports I2C debugging and development by offering a line of host adapters and protocol analyzers.

The Beagle I2C/SPI Protocol Analyzer enables users to capture I2C bus communication up to 4 MHz in real time, providing detailed data exchanges, bus info, timing, and signaling any bus errors for further troubleshooting analysis.

Our host adapters, including the Aardvark I2C/SPI Host Adapter and Promira Serial Platform, both enable users to emulate I2C master and slave devices to prototype and test the performance of entire I2C systems. Users can also utilize these tools to perform high-speed programming of I2C memory devices.

These host adapters come in a range of supported bus speeds and capabilities. Our Aardvark adapter is our best-selling general-purpose host adapter while the Promira platform is our most advanced serial device with the ability to support master speeds up to 3.4 MHz along with many other advanced features.

For more information on our line of I2C/SPI tools, please see our I2C/SPI Product Guide.

Conclusion

Overall, understanding the role of pull-up resistors in I2C communication is a crucial component in effectively debugging and developing I2C-based systems. These components are critical in facilitating the communication between devices by ensuring signals rise quickly and correctly for proper data exchange. While there are commonly used resistor values for many standard I2C applications including 4.7 kΩ or 10 kΩ, calculating the exact value of pull-up resistors for a specific application can ensure the optimal performance.

For more information on our I2C embedded solutions, please email us at sales@totalphase.com.