LSF0108: Communication issue + schematic review

Yan Pui Chan

Intellectual 520 points

Part Number: LSF0108
Other Parts Discussed in Thread: SN74LVC1T45

Hi,

I am implementing a new sensor (supply voltage is 1.8V) that uses I2C interface, and our host MCU is using 3.3V.

Besides the I2C (SDA, SCL), this sensor also has the following pins:-

1 x open-drain (RST)

2 x push-pull (PROG, DIR)

1 x INT (direction control is required)

For this application, I am using LSF0108 + SN74LVC2T45DCTR.

So far, I am not able to get successful communication and not sure what went wrong.

Attached is the connection diagram, can you please review it and let me know if you have any recommendations?

over 4 years ago

0 Yan Pui Chan over 4 years ago

Intellectual 520 points

0 Clemens Ladisch over 4 years ago in reply to Yan Pui Chan

Guru 320720 points

R10 and R11 are required.

The DIR input is overvoltage tolerant; it is not necessary to translate this signal.

I see no reason why the direction control would be necessary for the INT signal. Why did you not simply route it through the LSF?

0 Yan Pui Chan over 4 years ago in reply to Clemens Ladisch

Intellectual 520 points

Thanks for the inputs!

I also tried adding R10 and R11 with 2.2k but still having issues.

Just to confirm, I thought the DIR pin on SN74LVC2T45DCTR is referenced to VCCA (1.8V)?

Are you saying the DIR pin on SN74LVC2T45DCTR can support both 3.3V and 1.8V?

The INT from the sensor is neither a push-pull nor open-drain, the sensor manufacturer is recommending to use level shifter with direction control.

0 Yan Pui Chan over 4 years ago in reply to Yan Pui Chan

Intellectual 520 points

Besides the R10 and R11 are required, do you think the rest of the connections are good?

0 Clemens Ladisch over 4 years ago in reply to Yan Pui Chan

Guru 320720 points

In theory, all other connections should work.

What exactly does not work? Do you have oscilloscope traces?

What is the sensor model?

0 Yan Pui Chan over 4 years ago in reply to Clemens Ladisch

Intellectual 520 points

From what I am seeing so far, all the pins work as expected except the SDA pin from low side (1.8V) to high side (3.3V).

Seem like few data are missing.

Below have 2 captures, one good and one bad, for your reference.

The I2C that we are using is fast mode - 400khz, do you think this would cause this issue?

The address response from the sensor is incorrect, attached is the capture with LSF. Above is the capture.

The above capture is with another voltage translator that got from Sparkfun. The sensor response is correct.

0 Dylan Hubbard over 4 years ago in reply to Yan Pui Chan

TI__Mastermind 46745 points

Hey Yan,

It would be better to measure these signals with an oscilloscope so we can see exactly what is happening.

0 Yan Pui Chan over 4 years ago in reply to Dylan Hubbard

Intellectual 520 points

After some tests with our firmware team, the issue may be caused by our firmware.

We will spend more time troubleshooting.

I want to confirm with you one more time, is the "DIR" pin on SN74LVC2T45DCTR okay to direct control by our host MCU (3.3V level)?

Even VCCA is 1.8V?

0 Yan Pui Chan over 4 years ago in reply to Yan Pui Chan

Intellectual 520 points

Attach the schematic here again.

0 Yan Pui Chan over 4 years ago in reply to Yan Pui Chan

Intellectual 520 points

I think I missed the 74LVC series is not require VCCA>VCCB or VCCA<VCCB.

I have updated the schematic, can you please review it and let me know if it is good?

Thanks

0 Dylan Hubbard over 4 years ago in reply to Yan Pui Chan

TI__Mastermind 46745 points

Hey Yan,

The Dir input is over-voltage tolerant and can handle up to 5.5 V so the older configuration was fine. As for the current schematic, I recommend adding weak pull-downs to channel 2 I/O or switching to the SN74LVC1T45 (1 channel version). This is to avoid floating nodes. Other than that, everything looks good.

+1 Yan Pui Chan over 4 years ago in reply to Dylan Hubbard

Intellectual 520 points

Thanks for the input.

Yes, we are planning to switch to the 1 channel version eventually.

Thansks

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LSF0108: Communication issue + schematic review