8.4.1.1 BAT INSERT CHECK Mode

This mode is a halted-CPU state that occurs when an adapter, or other power source, is present to power the fuel gauge (and system), yet no battery has been detected. When battery insertion is detected, a series of initialization activities begin, which include: OCV measurement, setting the Flags() [BAT_DET] bit, and selecting the appropriate battery profiles.

Some commands, issued by a system processor, can be processed while the fuel gauge is halted in this mode. The gauge wakes up to process the command, then returns to the halted state awaiting battery insertion.

8.4.1.2 NORMAL Mode

The fuel gauge is in NORMAL mode when not in any other power mode. During this mode, AverageCurrent() , Voltage() , and Temperature() measurements are taken, and the interface data set is updated. Decisions to change states are also made. This mode is exited by activating a different power mode.

Because the gauge consumes the most power in NORMAL mode, the Impedance Track algorithm minimizes the time the fuel gauge remains in this mode.

8.4.1.3 SLEEP Mode

SLEEP mode is entered automatically if the feature is enabled (Op Config [SLEEP] = 1) and AverageCurrent() is below the programmable level Sleep Current. Once entry into SLEEP mode has been qualified, but prior to entering it, the fuel gauge performs a coulomb counter autocalibration to minimize offset.

During SLEEP mode, the fuel gauge periodically takes data measurements and updates its data set. However, a majority of its time is spent in an idle condition.

The fuel gauge exits SLEEP mode if any entry condition is broken, specifically when:

• AverageCurrent() rises above Sleep Current, or
• A current in excess of I_WAKE through R_SENSE is detected.

In the event that a battery is removed from the system while a charger is present (and powering the gauge), Impedance Track updates are not necessary. Hence, the fuel gauge enters a state that checks for battery insertion and does not continue executing the Impedance Track algorithm.

8.5.1.1 Standard Data Commands

The bq27531-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 bq27531-G1 Technical Reference Manual (SLUUA96).

8.5.1.2 Charger Data Commands

The charger registers are mapped to a series of single byte Charger Data Commands to enable system reading and writing of battery charger registers. During charger power up, the registers are initialized to Charger Reset State. The fuel gauge can change the values of these registers during the System Reset State.

Each of the bits in the Charger Data Commands can be read/write. It is important to note that System Access can be different from the read/write access as defined in bq2419x charger hardware. The fuel gauge may block write access to the charger hardware when the bit function is controlled by the fuel gauge exclusively. For example, the [VREGx] bits of Chrgr_Voltage_Reg4 are controlled by the fuel gauge and cannot be modified by system.

The bq27531 reads the corresponding registers of System_Stat_Reg8() and Fault_Reg9() every second to mirror the charger status. Other registers in the bq2419x are read when registers are modified by the bq27531.

8.5.2.1 I²C Interface

The 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 8. Supported I²C Formats

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 bq27531-G1 or the I²C master. “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:

Attempt to write a read-only address (NACK after data sent by master):

Figure 9. Attempt to Write a Read-Only Address

Attempt to read an address above 0x6B (NACK command):

Figure 10. Attempt to Read an Address Above 0x6B

8.5.2.2 I²C Time-Out

The I²C engine releases both SDA and SCL if the I²C bus is held low for 2 seconds. If the bq27531-G1 is holding the lines, releasing them frees them for the master to drive the lines. If an external condition is holding either of the lines low, the I²C engine enters the low-power SLEEP mode.

8.5.2.3 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 bq27531-G1 . 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 11. I²C Command Waiting Time

8.5.2.4 I²C Clock Stretching

A clock stretch 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
( BAT INSERT CHECK , NORMAL, SLEEP+ ) 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. However, less frequent yet more significant clock stretch periods may occur as blocks of Data Flash are updated. The following table summarizes the approximate clock stretch duration for various fuel gauge operating conditions.