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bq24295 pulls 500ma before enumeration of standard downstream port?

Hedley Davis
Prodigy 215 points
Other Parts Discussed in Thread: BQ24295

Perhaps this is a dumb question, but I am confused about the case of detecting a standard downstream port. This is from reading the datasheet, not from actual measurements.

The bq24295 datasheet seems to say that once it detects a SDP via primary detection, it decides it can pull 500mA for charging the battery. The USB specs (for 2.0 and earlier at least) says that powered devices can only pull 100mA until after they are enumerated and granted the additional power. Given that the bq24295 does not enumerate, how can it legally pull 500mA?

Have I misinterpreted the bq24295 datasheet, or the USB specs, or is there something I am missing (like you don't need to really pay attention to the 100mA current limit) or what?

Thanks in advance for a little clarity.

Hedley

  • 0
    TI__Guru 50811 points

    During the input current limit detection, the input current limit is set to 100 mA initially. After the detection is completed, the input current limit is set based on

    D+/D-. Thanks!

  • 0 in reply to Ning T
    Prodigy 215 points

    NT35830,

    Ti's own literature, for example www.ti.com/.../slyt118.pdf, tates:

    "The host first recognizes the peripheral as low-power, allowing it to draw less than 100 mA of current. The peripheral can ask the host to recognize it as a high-power device in a process called “enumeration.” Once enumeration is completed and permission is granted, the allowed peripheral current is increased to 500 mA.

    The process of enumeration consists of the host reading descriptors from the device (which may contain a request for power beyond 100mA), and the host configuring the device (implicitly granting that power). 

    If the BQ24295 starts to consume 500mA purely in the basis that it has detected that it is plugged into a standard downstream port, then it would appear to be in violation of this aspect of the USB specifications. 

    I know TI is a super smart company, I have nothing but respect, but this violation of well known time tested specifications is confusing. So I am hoping you can point me to some revision of a USB spec that eases the "no 500mA before enumeration completes" requirement of USB. Or, at least confirm that TI feels that portion of the spec is old, and folks have widely been violating it, so its okay.

    Thanks,

    bq24295

  • 0 in reply to Hedley Davis
    TI__Genius 10240 points
    The literature you referred was published in 2002. The original USB2.0 budgets 100mA to be drawn by a peripheral for 100ms was not enough for most portable devices with dead batteries to power up and be ready to enumerate. The “USB 2.0 Connect Timing ECN Update” published in April 2013 allows 500mA drawing after the attachment.