TMP75-Q1: PEC operation/support question

Shih Stone

Part Number: TMP75-Q1
Other Parts Discussed in Thread: TMP75, TCA9548A

Dear,

In our design(diagram below), we have 2 TMP75-Q1 and one sensor with PEC(Packet Error Checking) function support.

When we write 0x94 value data(PEC enabled) to the 0x03 register of sensor, it seems TMP75-Q1 response it and run into a wrong operation.

After remove 2 TMP75 parts on the I2C bus, we write 0x94 value data(PEC enabled) to the 0x03 register of sensor correctly.

The question we have is - is it expect behavior of TMP75? is it because TMP-75 can't recognize PEC command of sensor?

over 2 years ago

0 Shih Stone over 2 years ago

Prodigy 65 points

0 Jalen Tate89 over 2 years ago

TI__Genius 9675 points

Hi Shih,

Considering the slave address is 0x2A in your I2C protocol, the TMP75-Q1 should not be responding to the data byte. Is the TMP75-Q1 sending an ACK after the data byte?

Can you please provide a scope capture of the I2C transfer to help with debug?

Jalen

0 Shih Stone over 2 years ago in reply to Jalen Tate89

Prodigy 65 points

Scope picture.(same as above)

0 Shih Stone over 2 years ago in reply to Shih Stone

Prodigy 65 points

One future question.

If we add an TCA9548A (I2C switch) to isolate our sensor(PEC support) and 2 TMP75 devices.

Is it can work? Do "TCA9448A" can support PEC command communicatiob?

0 Jalen Tate89 over 2 years ago in reply to Shih Stone

TI__Genius 9675 points

Hi Shih,

I found more information on this topic. The PEC as referenced by the SMBus standard is actually an optional feature that the majority of our temperature sensors (including the TMP75) do not support.

More details can be found in the Software Differences and PEC sections of the SMBus compatibility with an I2C Device application note.

Jalen

0 Shih Stone over 2 years ago in reply to Jalen Tate89

Prodigy 65 points

Hi Jalen,

Is this means we can put an PEC support I2C device with non-PEC support I2C device in the same bus?

If it's true, can we use I2C switch - TCA9548A to isolate it(my future question)?

If I2C switch can't isolate them, what we can do to solve it?

Do TI have solution for this situation?

0 Josh Wyatt over 2 years ago in reply to Shih Stone

TI__Guru* 91086 points

Shih -

the PEC is an SMBus "feature" which has no corresponding equivalent in I2C. Page 4 of the app note Jalen pointed you to above describes this. You can use a mux or not use the PEC byte (which might be optional in your other sensor) or not use the other sensor.

0 Shih Stone over 2 years ago in reply to Josh Wyatt

Prodigy 65 points

Hi Josh,

Your answer can't clear our concern.

We send an PEC command to our PEC support sensor(0x2A), but we saw your device -TMP75 latch the SDA line in low.(wrong response??)

Is it expect behavior??

If it's true, is this means we can't have both PEC support sensor and non-PEC support sensor in the same bus?

This question will clarify - can we use I2C switch to isolate it ? because your I2C switch TCA9548A can't support PEC function too.

BTW,

can we have an contact window for direct discussing? This have gating our schedule.

0 Shih Stone over 2 years ago in reply to Shih Stone

Prodigy 65 points

Hi all,

It seems I close this ticket in accident.

But actually, our concern still not clear and solution not found.

I don't know how to re-open it.

Can someone help to to re-open it and answer my question above?

+1 Josh Wyatt over 2 years ago in reply to Shih Stone

TI__Guru* 91086 points

Shih -

what is the part number of the other sensor? As the application indicates, there is an incompatibility between I2C and SMBus, in this regard.

0 Shih Stone over 2 years ago in reply to Josh Wyatt

Prodigy 65 points

Hi Josh,

Another sensor(PEC support) is Maxim - MAX20480.

0 Josh Wyatt over 2 years ago in reply to Shih Stone

TI__Guru* 91086 points

Dear Shih -

This device supports PEC as optional feature -

From their datasheet ==> https://datasheets.maximintegrated.com/en/ds/MAX20480.pdf , page 16

Packet Error Checking (PEC) In order to increase fault coverage on the I2C interface, an optional PEC byte is supported. This follows the SMBus 3.0 implementation, which has a CRC-8 polynomial of x8 + x2 + x + 1. If the PEC byte is enabled and a supervisor system attempts to read more than 2 bytes (one data and one PEC) from the IC in a single communication packet, the IC will return 0xFF for the remaining bytes read. If a master device transmits a byte and an incorrect PEC, the IC replies with a NACK and discards the attempted write.

this means the best path forward is probably to not use the PEC in the first place, and as it is optional, this means it is possible.

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TMP75-Q1: PEC operation/support question