BQ24138 uses I²C compatible interface for flexible charging parameter programming and instantaneous device status reporting. I²C is a bi-directional 2-wire serial interface. Only two open-drain bus lines are required: a serial data line (SDA), and a serial clock line (SCL).

The device has 7-bit I²C address 0x6B, receiving control inputs from a host device such as a micro-controller or digital signal processor through register addresses defined in the Register Map. The host device initiates all transfers and the charger responds. Register reads outside of these addresses return 0xFF. When the bus is free, both SDA and SCL lines are HIGH.

The I²C interface supports standard mode (up to 100kbits/s), fast mode (up to 400kbits/s) and fast mode plus (up to 1Mbits/s.) These lines are pulled up to a reference voltage using a pullup resistor. The device I²C detection thresholds support a communication reference voltage between 1.2V - 5V.

Due to the ultra low I_Q when the device operates in low power mode, verifying a minimum of 90μs between a START command and any subsequent START command on the I²C bus is necessary. The recommended minimum t_buf (bus free time between a STOP and START condition) depends on the I²C mode:

• Standard mode (100kbits/s):
  • No additional requirements
• Fast mode (400kbits/s):
  • Increase I²C t_buf to at least 68μs
  • If using repeated start commands, verify I²C tsu:STA is at least 68μs
• Fast mode plus (1Mbits/s):
  • Increase I²C t_buf to at least 81μs
  • If using repeated start commands, verify I²C tsu:STA is at least 81μs

These recommendations assume a successful I²C transaction. Verify that a minimum 90μs time between two START commands in the case of a NACK.