Cypress Perform

Question 1: Why resistors of 330 ohm are required on I2C lines?

Answer: The series resistors of 330 ohm added on the I2C lines is to increase the RF noise immunity.

The resistor along with the pin capacitance forms a low pass filter and filters out any high frequency signals which may get coupled to the I2C lines.

Question 2: Pull-up resistor sizing on I2C lines?

Answer: The bus capacitance which is the total capacitance of wire, connections and pins. This capacitance limits the maximum value of Rp (Poll – up resistors) due to the I2C specified rise time (rise time of both SDA and SCL signals). The supply voltage limits the minimum value of resistor Rp due to the specified minimum sink current of 3 mA for Standard-mode and Fast-mode, or 20 mA for Fast-mode Plus. The equivalent formula of Rp-max and Rp-min list as following:

Rp(max) = t_r / 0.8473 × C_b

Rp(min) = (V_DD – V_OL(max)) / I_OL

• t_r - Rise time of both SDA and SCL signals, it specified in I2C Spec
• C_b - Estimated bus capacitance
• V_DD - Supply voltage of I2C bus, it specified in I2C Spec
• I_OL - Minimum sink current of I2C bus, it specified in I2C Spec
• V_OL(max) - LOW-level output voltage, it specified in I2C Spec