ISO1541: Side 1 SDA line held low during transmission

Panashe Madzivanyika

Prodigy 20 points

Part Number: ISO1541
Other Parts Discussed in Thread: TMP102, TCA9406

Tool/software:

Hi All,

I am working on a design that uses I2C to communicate to three devices:

1. TPL0102 (digital potentiometer) through an I2C isolator ISO1541

2. TMP102 (temp sensor) and 24AA64TE/OT (EEPROM) through a voltage-level translator TCA9406

The following schematic is what I implemented:

When I talk via U46 without connecting/powering U47, there are no issues. When I talk via U47, there are no issues. However, sometimes when I power both U46 and U47 and try to talk to U46 I can an error reading. On examining the signals, it seems U46 holds SDA1 low, the only way for SDA1 to be released is if power is reset. This keeps happening as long as both U46 and U47 are powered and I attempt to communicate via U46.

I am not sure what I am missing, can anyone offer assistance?

Regards,

Panashe

3 months ago

0 Clemens Ladisch 3 months ago

Guru 287370 points

What is the voltage of that stuck SDA1?

0 Aaditya V. 3 months ago in reply to Clemens Ladisch

TI__Genius 12395 points

Hello,

Thank you for your question.

I noticed that you have more than one device connected on the VCC1 side of the isolator, which is not recommended. The ISO1541 device supports only one device on the VCC1 side. Considering this, we recommend to please switch the sides of the isolator to where the VCC1 side of the isolator has one device connected and the VCC2 side has more than one device connected.

Additionally, please make sure logic voltage levels between controller and isolator are compatible with each other.

Regards,
Aaditya Vittal

0 Panashe Madzivanyika 3 months ago in reply to Aaditya V.

Prodigy 20 points

Hello,

Thank you for your response. I flipped the isolator, see the schematic below.

However, now I cannot talk to U45 (which is on Side 1) at all, I can still communicate to everything on Side 2. On measuring the signal integrity on both sides, I observe that SDA2 remains low at 0.64 V at startup and during communication even when SDA2 is high.

0 Aaditya V. 3 months ago in reply to Panashe Madzivanyika

TI__Genius 12395 points

Hello,

Thanks for your reply.

The issue you have described now seems to be a logic voltage level compatibility related issue. The controller on the left side and the ISO1541 device may have a certain logic voltage level that does not allow for easy communication. Please check this E2E FAQ for more information: [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

Please check the VOL voltage and see if this is compatible to the other devices.

Regards,
Aaditya Vittal