I2C (Inter-Integrated Circuit) bus is a commonly used serial communication protocol for short-distance data transmission between devices. The I2C bus consists of two lines: the serial data line SDA and the serial clock line SCL. In order to ensure that these two lines remain at a high level when idle, the I2C bus design usually requires external pull-up resistors. Let's discuss in detail why pull-up resistors are needed and how to choose the appropriate resistance value. The role of pull-up resistors

  1. Ensure that the line is high by default: The devices on the I2C bus indicate logic 0 (i.e., transmitting data or clock signals) by pulling the line low. When all devices release the line, the I2C bus cannot return to the high level (logic 1) state. I2C uses open-drain output and has no output (logic 1) capability, which requires a pull-up resistor to provide the necessary pull-up to ensure that the SDA and SCL lines remain high when no device actively pulls the line low.
  2. Current limiting and protection: Why not use push-pull output directly? When two devices output at the same time, one is (logic 1) and the other device outputs (logic 0), the power supply is short-circuited at this time, and one device must burn out. Using open-drain output can completely solve this problem. The pull-up resistor can also limit the current when the line is pulled low, preventing excessive current from flowing into the device that outputs low level, thus protecting the device.
  3. Improve noise suppression capability: A proper pull-up resistor helps reduce noise interference and improve system stability. It forms an RC charging loop with the capacitance of the line (mainly parasitic capacitance) to filter out high-frequency noise.

How to select the resistance value of the pull-up resistor The following factors need to be considered when selecting the resistance value of the pull-up resistor:

  1. Rise time: The larger the resistor, the slower it will pull up to a high level. This will result in a limited data transfer rate. Therefore, it is necessary to determine the appropriate rise time based on the maximum frequency of the I2C bus and select the resistance value accordingly. Generally speaking, the I2C bus standard recommends a rise time of less than 300ns.
  2. Maximum current sink capability: The I2C bus specification defines the maximum current (sink current) that each device on the bus can withstand when pulling the line low. The resistance of the pull-up resistor should ensure that even if all mounted devices try to pull the line low at the same time, the maximum sink current specification of any single device is not exceeded.
  3. Power consumption considerations: A certain amount of power will continue to be consumed in the resistor (P = I²R). Although this part of the power consumption is usually small, it still needs to be considered in low-power designs. A larger pull-up resistor can reduce static power consumption, but it must also balance the need for data transfer speed.
  4. Number of devices: The increase in devices on the I2C bus will also appropriately reduce the resistance value to ensure signal integrity.

Typical resistance selection: Common I2C pull-up resistors range from 1kΩ to 10kΩ. For I2C communication with a standard rate of 100kHz, 4.7kΩ or 10kΩ is usually more appropriate. For fast mode (400kHz) or faster high-speed mode (3.4MHz), it may be necessary to reduce the resistance to several thousand ohms to ensure a fast rising edge of the signal. In short, choosing the resistance of the pull-up resistor is a trade-off process that requires comprehensive consideration of data transmission rate, power consumption, bus load capacitance, and device driving capability. In practical applications, the data manual of the specific I2C device and the I2C bus specification should be referred to for reasonable selection.