ISO1644: ISO1644 issue

Jay Yang

Prodigy 10 points

Part Number: ISO1644

Hi,

I used TI ISO1644 but found a issue when testing them.

The figure below shows the schematic for ISO1644.

The left side is connected to the MCU and the right side is connected to the sensor.

I2C frequency: 400kHz.

This is the measured I2C_CLK waveform, which is very nice.

This is a measurement of the I2C_SDA waveform. Some of the signals in the waveform don't reach full 0V.

Although this has no effect on ST MCU, if we use the different brand of MCU, it will cause data readings to be abnormal.

Please help me with this issue.

Thanks.

over 1 year ago

+1 Vikas Jeevannavar over 1 year ago

TI__Intellectual 2160 points

Hi Jay,

Thanks for reaching out.

The signals that you are measuring are behaving as expected. The ISO1644 has a diode in the design in order to have a bi-directional communication. It is a common practice in industry for isolated i2c communication. For more details, please go through the Designing an Isolated I2C Bus interface by using digital isolators (Rev. A) (ti.com) and [FAQ] Why is the logic LOW level output voltage, VOL1, up to 0.8V on Side1 of the ISO1540/ISO1541 and ISO1640/ISO1641 bidirectional I2C isolators? - Isolation forum - Isolation - TI E2E support forums

You need to select the MCU that supports the ISO1644 V_OL1= 0.71V specification.
Hope this answers your query.

Thanks

Vikas J

0 Jay Yang over 1 year ago in reply to Vikas Jeevannavar

Prodigy 10 points

Hi Vikas,

Thank you for your note.

There seems to have been a problem with the forum the other day and I am unable to reply, sorry.

I did see this description in the datasheet and understand the reason why this happened.

But I would like to ask why this only happens in SDA, SCL is normal.

Thanks!

Jay

+1 Vikas Jeevannavar over 1 year ago in reply to Jay Yang

TI__Intellectual 2160 points

Hi Jay,

Sorry for the inconvenience caused with the forum.

To answer your question, the behavior is observed only on the MCU side(left) and only when the data is sent from Sensor side (right).
In your case, the Sensor data being transmitted will cause that issue while clock is being sent from MCU to Sensor always. Hence, SCL is normal.

Thanks

Vikas J

0 Jay Yang over 1 year ago in reply to Vikas Jeevannavar

Prodigy 10 points

Hi Vikas,

Okay, I understand.
Thank you for your reply.