TCA6408A: Interface timing requirements

Min Hisen Kao

Part Number: TCA6408A

Hi Support team

We use the TI I2C EXP TCA6408ARSVR and implement the SI measurement.
We checked the spec and found the criteria of fall time as below; It shows the I2C spec is 20+0.1Cb~300ns.
As I know the latest I2C spec is (20*vdd/5.5~300ns) = (12ns~300ns).
It’s very difficult to calculate the Cb value for every PCB board.
Can we follow the latest I2C spec 12ns~300ns?
If you have any concern, please let me know.
Thanks a lot.

1 month ago

0 Tyler Townsend 1 month ago

TI__Guru 54238 points

Hi Min,

These rise-times can be used to determine the PU resistor needed for your system.

Please see this app note: https://www.ti.com/lit/an/slva689/slva689.pdf?ts=1720204726016&ref_url=https%253A%252F%252Fwww.google.com%252F

Regards,

Tyler

0 Min Hisen Kao 1 month ago in reply to Tyler Townsend

Prodigy 90 points

We need to know if we can follow the latest I2C standard spec (20*vdd/5.5~300ns) = (12ns~300ns) for SI measurement.

0 Tyler Townsend 1 month ago in reply to Min Hisen Kao

TI__Guru 54238 points

Hi Min,

I would recommend sticking to the minimum fall time listed in the datasheet.

However, TCAL6408RSVR follows the minimum timing spec. TCAL is p2p, but can only operate up to 3.6 V on VCCI and VCCP.

Regards,

Tyler

0 Eric Yang 1 month ago

Prodigy 30 points

Hi Tyler:

Did you mean we need to set the I2C criteria for signal integrity need to calculate the Cb firstly based on the Tr result? Why IT I2C requirement different with other vendor?

0 Tyler Townsend 1 month ago in reply to Eric Yang

TI__Guru 54238 points

Hi Eric,

Calculate with an estimated bus capacitance. Say 1 pf / 1 cm of PCB trace, and 6-8 pF of input capacitance per device on the I2C bus to calculate total Cb.

It looks like the difference is a result of when this part was defined. The 20 +0.1Cb is from an older version of the I2C specification:

TCA6408A: I2C falling time of TCA6408A

Regards,

Tyler

0 Eric Yang 1 month ago in reply to Tyler Townsend

Prodigy 30 points

Thanks Tyler. Checked last TCA6048 spec, the I2C ris and fall time requirement is still showing "20+0.1Cb" for min. requirement. Could you advice the 20+0.1Cb min. requirement have update or not?

Spec version: SCPS192E – APRIL 2009 – REVISED JANUARY 2023

0 Tyler Townsend 1 month ago in reply to Eric Yang

TI__Guru 54238 points

Hi Eric,

The part was characterized to the older spec of the I2C standard back in 2009.

The datasheet revision in 2023 is adding a constraint on the power sequence of VCCI and VCCP, but no change to the I2C spec in the EC table.

Regards,

Tyler

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TCA6408A: Interface timing requirements