SLUA475 November   2016 BQ2060A , BQ20Z80 , BQ40Z50-R1 , BQ40Z50-R2 , BQ40Z60 , BQ78350-R1 , BQ78350-R1A

 

  1.   SMBus Made Simple
    1.     Trademarks
    2. 1 Getting to Know SMBus
      1. 1.1 Closer Inspection
      2. 1.2 Final Considerations
        1. 1.2.1 Clock Stretching
        2. 1.2.2 Broadcasting (Master Mode Messages)
        3. 1.2.3 PEC
          1. 1.2.3.1 How to Calculate PEC
        4. 1.2.4 Examples
          1. 1.2.4.1 Example 1
          2. 1.2.4.2 Example 2
          3. 1.2.4.3 Example 3
          4. 1.2.4.4 Example 4
    3. 2 Most Common Problems
    4. 3 Glossary
    5. 4 References
  2.   A SMBus Reference Sheet
    1.     19

1.2.2 Broadcasting (Master Mode Messages)

Sometimes, a slave can become the master of the bus. The SMBus specification allows for this possibility. All of TI's advanced fuel gauges have this ability. Depending on the fuel gauge (FG), this feature can be enabled or disabled in different ways. If enabled, the FG as master causes the bus to send alarms and charging voltage and charging current every 10 to 20 seconds to the host (0x10) and charger (0x12) device addresses

An SMBus master can only start a packet if the SMBus has been idle for more than 50 µs. Once this requirement has been met, the master immediately takes control of the bus by sending a start bit. All TI FGs function exactly like this, and because they are hardware-controlled SMBus engines, the FGs easily detect idle.

This type of behavior is difficult to create for an SMBus engine implemented in firmware. It is easy to detect 50 µs of idle time, but the port pins used to create the bus must be switched to outputs, and the start bit must be sent. During this time frame, another master can actually take control of the bus. If another master controls the bus, the firmware-controlled bus does not detect that the bus is no longer busy, which causes arbitration to be lost.

Because of this difficulty, most host systems do not recognize this part of the SMBus specification and, instead, act as the only master on the bus. Therefore, TI recommends that broadcasting messages be disabled for every application unless absolutely necessary.