20.5.8 Connection Considerations For I2C Bus

Because the I2C bus is a wired-AND bus connection, Pull-up Resistors (RP) on the bus are required, as illustrated in Figure 20-40. The Series Resistors (RS) are optional and are used to improve the Electrostatic Discharge (ESD) susceptibility.

The values of the resistors, RP and RS, depend on the following parameters:

  • Supply voltage
  • Bus capacitance
  • Number of connected devices (input current + leakage current)
  • Input level selection (I2C or System Management Bus (SMBus))

Because the device must be able to pull the bus low against RP, current drawn by RP must be greater than the I/O pin minimum sink current, IOL at VOLMAX, for the device output stage. Equation 20-2 shows the formula for computing the minimum pull-up resistance.

Equation 20-2. Minimum Pull-up Resistance

In a 400 kHz system, a minimum rise time specification of 300 ns exists; in a 100 kHz system, the specification is 1000 ns. Because RP must pull the bus up against the total Capacitance, CB, with a maximum rise time of 300 ns to (VDD – 0.7V), the Maximum Resistance for the Pull-up (RPMAX) is computed using the formula as shown in Equation 20-3.

Equation 20-3. Maximum Pull-up Resistance

The maximum value for RS is determined by the desired noise margin for the low level. Rs cannot drop enough voltage to make the device VOL and the voltage across RS more than the maximum VIL. Equation 20-4 shows the formula for computing the maximum value for RS.

Equation 20-4. Maximum Series Resistance
Note: The SCLx clock input must have a minimum high and low time for proper operation. Refer to the “Electrical Characteristics” chapter for more information on the high and low times of the I2C bus specification and requirements of the I2C module and I/O pins.
Figure 20-40. Sample Device Configuration for I2C Bus