BQ40Z50: SMBus communication problem/ sink capability

Edward Lee

Part Number: BQ40Z50
Other Parts Discussed in Thread: EV2400, PCA9515A

Hi Sir,

My customer recently encountered a problem. They try to communicate the gauge IC BQ40Z50 by Apollo lake SOC through SMBus.

They found it received the read command and responded acked but it seems the sink capability is not enough to pull down the voltage at SDA to under 0.4V.

This caused SOC failed to transmit the other commands.

The internal pull high resistors in SOC is around 20Kohm.

Would you kindly give us some directions?

Thanks

Edward

over 5 years ago

0 Kang Kang over 5 years ago

TI__Mastermind 24870 points

Please try a stronger pull-up resistor here.

0 Edward Lee over 5 years ago in reply to Kang Kang

TI__Intellectual 1655 points

Hi KangKang,

Thanks for your prompt reply.

The minimum pull-up resistor is 20Kohm that is inside the Apollo Lake SOC.

There are external 2.2kohm pull-up resistors in the original circuits. With them, the voltage acked by BQ40Z50 is ever higher.

It seems to be related to the sink capability of BQ40Z50. But the customer had tried to communicate it with EV2400 and it worked normally.

After checking the pull-up resistor in EV2400, it's around 20Kohm too. What seems to be the problem that caused the difference?

Thanks

Sincerely,

Edward

0 Kang Kang over 5 years ago in reply to Edward Lee

TI__Mastermind 24870 points

Hello Edward,

You may want to check the min SCL and SDA logic levels VIH, VIL for the bq40z50.

I believe it should be intermittent at best for some ICs and other ICs it may not work.

0 Edward Lee over 5 years ago in reply to Kang Kang

TI__Intellectual 1655 points

Hi KangKang,

Thanks for your reply.

Does that mean to check the min logic levels VOL for the BQ40Z50? The gauge IC can response to the read command from SMBus host (SOC) while could not pulled low the SDA to be lower than 0.4V.

We try to add a SMBus repeater PCA9515A between the SOC and BQ40Z50 and the issue is solved.

But we could not find too much info about the pull low capability of BQ40Z50 in datasheet, it only has a table described below.

Would you kindly advise?

Thanks

Sincerely,

Edward

0 Edward Lee over 5 years ago in reply to Edward Lee

TI__Intellectual 1655 points

Hi KangKang,

Sorry for bothering. Would you mind suggest us how to improve without I2C repeater?

Thanks

Sincerely,

Edward

0 Kang Kang over 5 years ago in reply to Edward Lee

TI__Mastermind 24870 points

Hello Edward,

How much capacitance do you have on SMBus? Are there multiple devices and really long traces in your application?

0 Edward Lee over 5 years ago in reply to Kang Kang

TI__Intellectual 1655 points

Hi KangKang,

The capacitance may be less than 100pF. The trace is short than 5 cm and no other multiple devices on the bus.

They also try to test with our BQ40Z50 EVM and the issue is the same.

Thanks

Edward

0 Kang Kang over 5 years ago in reply to Edward Lee

TI__Mastermind 24870 points

Hello Edward,

What is your pull-up resistor? I think in this case, your pull up resistors may be too strong. That would be my only theory at this point for why you would need a repeater.

0 Edward Lee over 5 years ago in reply to Kang Kang

TI__Intellectual 1655 points

Hi Kangkang,

Thanks for your feedback. I try to add a pull low resistor and measure the divided voltage when the bus is idle.

That's to estimate the value of pull-up resistor and it is around 0.8Kohm,

I may try to help customer to increase the resistance of pull-up resistor to see how it improves.

Thanks

Sincerely,

Edward

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BQ40Z50: SMBus communication problem/ sink capability