PCA9518: PCA9518

Hi Bobby,

Yes, Only A side of TCA9617 is connected with PCA9518.

Signal Quality:
As mentioned in data sheet of TCA9617- page 11, I would expect "Inverted Pedestal" on B side (SCL/SDA) falling edges. But I am seeing it on both A and B sides falling edges.

-Kanava

Hey Kanava,

Do you have a scope shot you can provide?

"I would expect "Inverted Pedestal" on B side (SCL/SDA) falling edges. But I am seeing it on both A and B sides falling edges. "
I believe you are seeing this on A side because PCA9518 also has a static voltage offset of about 500mV.

If you look at figure 4 of the PCA9518, you can also see the inverted pedestal. (page 13)

This shouldn't be a problem as long as A side's pull down voltage is below 30% of Vcc when PCA9518 is passing a low and TCA9617 needs to pull below 0.45V on A side when B side pulls low because PCA9518's ViLc is 0.52-0.070V.

Thanks,
-Bobby

Hi Bobby,

Attached A side (SCL/SDA) scope shot.

"Inverted pedestal" appears only on channel2 (SDA2/SCL2) of PCA9518 where TCA9617 is connected, not on other channels of PCA9518 where only slave devices are connected.

-Kanava

Hey Kanava,

I suspect yellow is B side of the TCA9617 and green is A side of the TCA9617 and the master/slave is pulling low on some other channel on PCA9518?

Do you have a schematic showing the PCA9518 and the TCA9617 (pull up resistors or any series resistors)?

Can you remove the TCA9617 and populate a new one and check to see if this still occurs?

Are you having any issues with the signal integrity?

Thanks,
-Bobby

Hi Bobby,

I suspect yellow is B side of the TCA9617 and green is A side of the TCA9617 and the master/slave is pulling low on some other channel on PCA9518?

[Kanava]: No,  Yellow is A side - SDA, Green is A side -SCL.

Attached screen shot of A and B side of SDA.

Do you have a schematic showing the PCA9518 and the TCA9617 (pull up resistors or any series resistors)?

[Kanava]: I will share the details later.

Can you remove the TCA9617 and populate a new one and check to see if this still occurs?

[Kanava]: I will try to probe the signals in other boards.

Are you having any issues with the signal integrity?

[Kanava]:  Signal doesn't look clean. I am trying to understand why  A side signals are also getting "inverted pedestal" when we are expecting it only on B side

Can I connect A side of TCA9617 to EXPSDAx/EXSCLx of PCA9518?

-Kanava

Hey Kanava,

Can you also specify which TCA9617 you are using? A revision or B?

"Can I connect A side of TCA9617 to EXPSDAx/EXSCLx of PCA9518?"
It depends, where does B side of TCA9617 is. The EXPSDAx/EXSCLX pins were meant to be connected to other PCA9518 pins, not used for direct I2C communication. The EXPSXX pins therefore should either be pulled high or connect to other PCA9518 devices and not to the TCA9617 though you should be able to do so if the B side is not in contact with a device using a static voltage offset.

Does changing the pull up resistor on A side of TCA9617 clean the signal up? I suspect using a larger pull up resistor will help with the overshoot we are seeing in the yellow curve of your second picture. If you have a spot for a series resistor on the SDA/SCL lines between the PCA9518 and TCA9617, you can populate a very small 10~20 ohm resistor to try to further reduce the overshoots.

From my opinion though, the signal looks clear only worry would be if there are slave devices which are edge rate triggered (most are threshold based from my experience).

-Bobby