BQ24157 Datasheet, PDF (23/39 Pages) Texas Instruments – Fully Integrated Switch-Mode Charger With USB Compliance and USB-OTG Support

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SERIAL INTERFACE DESCRIPTION
bq24157
SLUSB80 – SEPTEMBER 2012
I²C is a 2-wire serial interface developed by Philips Semiconductor (see I²C-Bus Specification, Version 2.1,
January 2000). The bus consists of a data line (SDA) and a clock line (SCL) with pull-up structures. When the
bus is idle, both SDA and SCL lines are pulled high. All the I2C compatible devices connect to the I²C bus
through open drain I/O pins, SDA and SCL. A master device, usually a microcontroller or a digital signal
processor, controls the bus. The master is responsible for generating the SCL signal and device addresses. The
master also generates specific conditions that indicate the START and STOP of data transfer. A slave device
receives and/or transmits data on the bus under control of the master device.
The IC works as a slave and is compatible with the following data transfer modes, as defined in the I²C-Bus
Specification: standard mode (100 kbps), fast mode (400 kbps), and high-speed mode (up to 3.4 Mbps in write
mode). The interface adds flexibility to the battery charge solution, enabling most functions to be programmed to
new values depending on the instantaneous application requirements. Register contents remain intact as long as
supply voltage remains above 2.2 V (typical). I²C is asynchronous, which means that it runs off of SCL. The
device has no noise or glitch filtering on SCL, so SCL input needs to be clean. Therefore, it is recommended that
SDA changes while SCL is LOW.
The data transfer protocol for standard and fast modes is the same; therefore, they are referred to as F/S-mode
in this document. The protocol for high-speed mode is different from the F/S-mode, and it is referred to as HS-
mode. The bq24157B device supports 7-bit addressing only. The device 7-bit address is defined as ‘1101010’
(6AH).
F/S Mode Protocol
The master initiates data transfer by generating a start condition. The start condition is when a high-to-low
transition occurs on the SDA line while SCL is high, as shown in Figure 28. All I²C-compatible devices should
recognize a start condition.
DATA
CLK
S
P
START Condition
STOP Condition
Figure 28. START and STOP Condition
The master then generates the SCL pulses, and transmits the 8-bit address and the read/write direction bit R/W
on the SDA line. During all transmissions, the master ensures that data is valid. A valid data condition requires
the SDA line to be stable during the entire high period of the clock pulse (see Figure 29). All devices recognize
the address sent by the master and compare it to their internal fixed addresses. Only the slave device with a
matching address generates an acknowledge (see Figure 29) by pulling the SDA line low during the entire high
period of the ninth SCL cycle. Upon detecting this acknowledge, the master knows that communication link with a
slave has been established.
DATA
CLK
Data Line
Stable;
Data Valid
Change
of Data
Allowed
Figure 29. Bit Transfer on the Serial Interface
Copyright © 2012, Texas Instruments Incorporated
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