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BQ24765 SMBus timeout

Kazuya Nakai59
Guru 20430 points
Other Parts Discussed in Thread: BQ24765

 Hello guys,

 My customer is evaluating BQ24765(SMBus-Controlled Multi-Chemistry Battery Charger With Integrated Power MOSFETs). They rarely found that SDA was fixed low when BQ24765 was controlled by MCU via SMBus and the low level is kept until the IC power is turned off. BQ24765 SMBus release timeout is 25ms (typ) as described on page 10 of the device datasheet.    

 The customer checked what condition causes the situation and  they found the condition.

When SCL is fixed low by MCU at BQ24765 ACK output timing (SDA is low by BQ) in the write mode, SDA is released after about 25ms from SCL low level fixing.

 But when SCL is fixed low by MCU at BQ24765 L level output timing on SDA in the read mode, SDA is not released no matter how much time passes.

 My questions are the below.

1. Is the SDA no releasing BQ24765 original specification?

2. Is there any method to have BQ24765 release SDA?

 Your comment would be much appreciated.

Best regards,

Kazuya Nakai 

 

  • 0
    TI__Guru 56650 points
    Are they evaluating BQ24765 on an EVM or their board?
    Charger only occupy SDA when received ‘read’ command or ‘ACK’.
    If SCL rising time is longer than 1us spec and the switching noise impacts SCL, charger could get an extra clock. And then, SMBus command is missed up and charger occupies SDA.

    To avoid this is issue,
    1. Let SCL have a sharp rising time. Reduce the pull-up resister to 2.2kohm instead of 10k.
    2. Reduce the switching noise on SCL. Check the ground noise. May add a 100pF near charger IC from SCL to ground.

    After SDA is locked up, there are two options to unlock it:
    1. If the host holds SCL low for more than 35ms, SMbus is reset and SDA is released.
    2. Host send more extra clock to charger finishes sending out data. It needs at least 20 extra clocks to release SDA.
  • 0 in reply to Wang5577
    Guru 20430 points
    Hello Wang,

    They are evaluating BQ24765 on thier own board.
    I checked their SDA and SCL waveform but I couldn't find any big noise at the rising/falling edge. Also the rising/falling time is less than 1us.
    I will recommend them to implement the two option you tell me to avoid SDA locking out.

    Thank you and best regards,
    Kazuya Nakai.