8.4.1.1 Standard Data Commands

The bq27621-G1 uses a series of 2-byte standard commands to enable system reading and writing of battery information. Each standard command has an associated command-code pair, as indicated in Table 1. Because each command consists of two bytes of data, two consecutive I²C transmissions must be executed both to initiate the command function, and to read or write the corresponding two bytes of data. Additional details are found in the bq27621-G1 Technical Reference Manual (SLUUAD4).

8.4.1.2 Control(): 0x00 and 0x01

Issuing a Control() command requires a subsequent 2-byte subcommand. These additional bytes specify the particular control function desired. The Control() command allows the system to control specific features of the fuel gauge during normal operation and additional features when the device is in different access modes. Additional details are found in the bq27621-G1 Technical Reference Manual (SLUUAD4).

8.4.3.1 I²C Interface

The bq27621-G1 fuel gauge supports the standard I²C read, incremental read, quick read, one-byte write, and incremental write functions. The 7-bit device address (ADDR) is the most significant 7 bits of the hex address and is fixed as 1010101. The first 8 bits of the I²C protocol are, therefore, 0xAA or 0xAB for write or read, respectively.

Figure 5. I²C Format

The quick read returns data at the address indicated by the address pointer. The address pointer, a register internal to the I²C communication engine, increments whenever data is acknowledged by the fuel gauge or the I²C master. The quick writes function in the same manner and are a convenient means of sending multiple bytes to consecutive command locations (such as two-byte commands that require two bytes of data).

The following command sequences are not supported:

Figure 6. Attempt To Write a Read-only Address (Nack After Data Sent By Master)

Figure 7. Attempt To Read an Address Above 0x6B (Nack Command)

8.4.3.2 I²C Time Out

The I²C engine releases both SDA and SCL if the I²C bus is held low for 2 seconds. If the fuel gauge is holding the lines, releasing them frees them for the master to drive the lines.

8.4.3.3 I²C Command Waiting Time

To ensure proper operation at 400 kHz, a t_(BUF) ≥ 66 µs bus-free waiting time must be inserted between all packets addressed to the fuel gauge. In addition, if the SCL clock frequency (f_SCL) is > 100 kHz, use individual 1-byte write commands for proper data flow control. The following diagram shows the standard waiting time required between issuing the control subcommand the reading the status result. For read-write standard command, a minimum of 2 seconds is required to get the result updated. For read-only standard commands, there is no waiting time required, but the host must not issue any standard command more than two times per second. Otherwise, the gauge could result in a reset issue due to the expiration of the watchdog timer.

Figure 8. I²C Command Wait Time

8.4.3.4 I²C Clock Stretching

A clock stretch of up to 4 ms can occur during all modes of fuel gauge operation. In SLEEP and HIBERNATE modes, a short clock stretch occurs on all I²C traffic as the device must wake-up to process the packet. In the other modes (INITIALIZATION, NORMAL) clock stretching only occurs for packets addressed for the fuel gauge. The majority of clock stretch periods are small as the I²C interface performs normal data flow control.