I2C Pull-Up Resistor Calculator: How to Choose

This calculator determines the allowable range of pull-up resistor values for an I2C bus based on your supply voltage, bus capacitance, and required rise time. You enter the I2C supply voltage, the total bus capacitance (from all devices and traces), and the maximum rise time for your chosen speed mode (for example, 1000 ns for Standard mode or 300 ns for Fast mode). The calculator returns the minimum and maximum pull-up resistance values that keep the bus within specification.

Formula

The I2C bus uses open-drain outputs with pull-up resistors to VCC. When a device pulls the line low, the resistor determines how fast the line returns high. The rise time is approximately:

t_rise = 0.8473 × R_p × C_bus

Where:

• t_rise – maximum allowed rise time in seconds (from the I2C specification for your speed mode).
• R_p – pull-up resistance in ohms.
• C_bus – total capacitance of the bus line in farads (sum of device pin capacitance, trace capacitance, and any other capacitance).

Rearranging gives the maximum resistance:

R_max = t_rise / (0.8473 × C_bus)

The minimum resistance is set by the need to limit current when the line is low. The maximum low-level output voltage (V_OL) is 0.4 V for most devices. The current through the resistor when the line is low must not exceed the maximum sink current of the device (typically 3 mA for standard I2C, but check your device datasheet).

R_min = (V_CC – V_OL) / I_max

For a typical 3.3 V supply and 3 mA sink current, R_min ≈ (3.3 – 0.4) / 0.003 = 967 Ω. Many designers use 1 kΩ as a safe minimum.

Worked Example

Using the default values from the calculator:

• Supply voltage V_CC = 3.3 V
• Total bus capacitance C_bus = 200 pF
• Maximum rise time t_rise = 1000 ns (Standard mode)

First, find the maximum resistance:

R_max = (1000 × 10^-9) / (0.8473 × 200 × 10^-12) = 1000 / (0.8473 × 200) = 1000 / 169.46 ≈ 5.90 kΩ

So the maximum pull-up is about 5.9 kΩ. For the minimum, assume V_OL = 0.4 V and I_max = 3 mA:

R_min = (3.3 – 0.4) / 0.003 = 2.9 / 0.003 = 967 Ω

A good choice is around the geometric mean or the midpoint: (967 + 5900) / 2 ≈ 3.4 kΩ. Many designers pick 4.7 kΩ or 3.3 kΩ as a standard value.

Choosing the Right Pull-Up Value

In practice, you want a resistor that is low enough to meet the rise time requirement but high enough to limit current and power dissipation. Here are the steps:

1. Determine the speed mode (Standard 100 kHz, Fast 400 kHz, Fast+ 1 MHz). The rise time limits are: Standard: 1000 ns, Fast: 300 ns, Fast+: 120 ns.
2. Estimate the total bus capacitance. Each device typically adds 5-10 pF of pin capacitance, plus trace capacitance (about 1-2 pF per cm on a PCB).
3. Use the formula to find R_max.
4. Check the minimum resistance from the sink current capability.
5. Pick a standard resistor value between R_min and R_max.

For a typical 3.3 V system with 200 pF capacitance, a 4.7 kΩ resistor is common and works for Standard and Fast modes. For Fast+ mode, you might need 1 kΩ to 2 kΩ.

Common Mistakes and Pitfalls

• Ignoring capacitance: The total bus capacitance includes every device on the line, not just one. A long PCB trace or a connector adds significant capacitance.
• Using the wrong rise time: Each speed mode has a different rise time limit. Using Standard mode limits for a Fast mode design will cause timing violations.
• Forgetting the minimum resistance: Too low a resistor can overheat the open-drain output or cause excessive current draw. Always check the sink current rating.
• Not considering voltage: The pull-up voltage may be different from the logic voltage. For level shifting, the pull-up might go to a different supply.

Multiple Devices and Bus Length

The more devices you have, the higher the capacitance. Each device adds pin capacitance (often 5-10 pF) and possibly some trace capacitance. A long ribbon cable or a bus with many connectors can easily exceed 200 pF. In such cases, you may need lower pull-up resistors to meet rise time, but watch the minimum limit.

For buses longer than about 10 cm, consider using a buffer or a bus extender. Some designs use a series resistor in addition to the pull-up to reduce ringing, but that adds complexity.

Comparing to Other Bus Standards

I2C is an open-drain bus, so pull-ups are required. In contrast, SPI uses push-pull outputs and typically does not need pull-ups. UART uses push-pull or open-drain depending on the implementation. The pull-up calculation for I2C is unique because of the open-drain structure and the defined rise time limits.

Frequently Asked Questions

What happens if I use too large a pull-up resistor?

If the resistor is too large, the rise time becomes too slow. The bus may still work at lower speeds, but at higher speeds (400 kHz or 1 MHz) the signal may not reach the high level in time, causing communication errors or data corruption.

Can I use 10 kΩ pull-ups?

Yes, but only if the total bus capacitance is very low (e.g., less than 50 pF) and you are using Standard mode. For most modern designs with multiple devices, 10 kΩ is too high and will fail at 400 kHz.

Do I need pull-ups on both SDA and SCL?

Yes, both lines are open-drain and need a pull-up resistor to VCC. They can be the same value, but sometimes you might use different values if one line has more capacitance.

What is the typical pull-up value for a 5V I2C bus?

For 5V, the minimum resistance is higher because the voltage drop is larger. With a 5V supply and 3 mA sink current, R_min = (5 – 0.4) / 0.003 ≈ 1.5 kΩ. A common value is 4.7 kΩ or 10 kΩ, but you must check the rise time.

Can I use a single pull-up for both lines?

No, each line (SDA and SCL) needs its own pull-up resistor to VCC. They are independent lines.

How do I measure bus capacitance?

You can measure it with an LCR meter or a capacitance meter. Alternatively, you can estimate it from the datasheets of the devices and the PCB trace length. For a rough estimate, sum the pin capacitance of all devices and add about 1 pF per centimeter of trace.

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in vcc | Supply voltage (V) | V | 3.3 | 0
in cap | Total bus capacitance (pF) | pF | 200 | 0
in tr | Max rise time (ns) | ns | 1000 | 0
out rmin | Minimum pull-up (Ω) | Ω | (tr * 1e-9) / (0.8473 * cap * 1e-12) | 0
out rmax | Maximum pull-up (Ω) | Ω | (tr * 1e-9) / (0.8473 * cap * 1e-12) | 0
out rp | Recommended pull-up (Ω) | Ω | (rmin + rmax) / 2 | 0