TCA9555: Question on Open Drain pins

Hey Jeff,

To clarify, are you talking about SDA/SCL or another pin on the TCA9555? The GPIOs are push-pull not open drain. The only other output is INT which is open drain.

"Examples, TCA9555 at 5 volts, micro at 1.8v, or TCA9555 at 2.5v and micro at 3,3v."

-I'm having some trouble understanding this. Can you provide a block diagram for me?

The way I am interpreting your question is as follows:

Is this correct or are you also trying to connect SDA1 to SDA2 to form an I2C line?

Note: This set up should not be used.

-Bobby

Hi Bobby

We are concerned about the interrupt pin, not the I2C pins. Although it could potentially apply to the I2C as well. If the tca is providing 5 volt I/O (hence on a 5v rail) and the processor (with I2C interface) has a 2.7 volt I/O rail that is a possibility also. Another possibility is the processor is on a digital 3.3v and the TCA is on a analog 3.3v. Now we have a possible issue if the power rails do not come up and down simultaneously.


Thanks
Jeff

Hey Jeff,

"We are concerned about the interrupt pin, not the I2C pins."
-The INT pin can be pulled up to the same voltage as your micro-controller and does not need to be pulled to the 5V rail.

"If the tca is providing 5 volt I/O (hence on a 5v rail) and the processor (with I2C interface) has a 2.7 volt I/O rail that is a possibility also."
-If you are trying to communicate between a 5V TCA and a 2.7V processor through the SCA/SCL lines, you will need to use a voltage translator. But I think what you are asking about is the TCA GPIO pins being 5V while the processor is 2.7V. If this is the case please take a look at the TCA6416A. It has separate voltage inputs for it's GPIO rail(Vccp) and it's I2C rails(Vcci). Vcci can connect to the 5V rail and Vccp can connect to 2.7V rail. The Voltages on Vcci and Vccp can also be swapped (they can are independent of each other).

Can you provide a schematic or block diagram. We are still confused about if this is a communication (I2C bus) concern or you are trying to use the GPIO of TCA with the processor.

-Bobby

Bobby,

Jeff originally submitted this post on my behalf. My question to Jeff was only about the INT pin, but the conversation went a little broader and didn't come back around the whole way.

In your most recent post, you wrote, "Problems could occur if you pull INT higher than Vcc(TCA)." I'd like some elaboration. Two questions specifically:

1) Can INT be powered up before VCC? (i.e. INT pulled up to 3.3V while VCC is at 0V)
2) Can INT be powered up to a rail higher than VCC? (i.e. INT pulled up to 3.3V while VCC is at 1.8V)

If the answer to these question is “no”, then we can work with that, but I’d appreciate an explanation why so that I understand it.

Regards,
Jon

Hey Jon,

Sorry for the late response, I've been out of the office and didn't get a chance to take a look at this until now.

I suspect you are asking about the TCA9555 as my most recent post was in reference to TCA6416A (it has two Vccs one for I2C and one for port pins) though the responses to your specific questions should still be similar.

"1) Can INT be powered up before VCC? (i.e. INT pulled up to 3.3V while VCC is at 0V)"

This should be okay as long as your voltage is below the 5V tolerance stated in the datasheet. I suspect the INT pin is connected to an NFET and in the case where the gate could be floating (it shouldn't be though) make sure the pull up resistor value is large enough to meet the max output low current allowed.

The calculation for this is simple: Vpullup/I(max@expectedtemp)=Rmin

The pull up resistor should be larger than Rmin.

"2) Can INT be powered up to a rail higher than VCC? (i.e. INT pulled up to 3.3V while VCC is at 1.8V)"

-Yes, usually the answer to this in I2C architecture is no but because this pin is an output only it does not look at a logical high or low. It's only purpose is to pull low when an INT event has occurred. The datasheet states the pins are 5V tolerant with a 6V absolute max so as long as you keep the voltage on the INT below these values (recommend 5V) then you should not see any issues. Make sure the max allowable current is not violated (see answer to first question).

Thanks,

-Bobby

Bobby,

Thanks!  Yes, I was asking about TCA9555 only and the INT pin only.  As the desired pullup voltage is 3.3V and the max Tj is 85C in this application, 3.3/.006 = 550 ohms.  We will use a 10k pullup to 3.3V, and per your response this is acceptable when VCC is off or at 1.8V.  That's what I hoped to hear and this kind of open-drain architecture is true for INT pins on many devices.  The absence of clarity in the TCA9555 datasheet is what led to this question.

Regards,

Jon

Bobby and Jon,
For the TCA9555 it is ok to do this because the part was designed such that it can't be back biased from the INT pin. Some parts are not, so care must be taken to look at the internals of the device to make sure it will not back bias Vcc. You are fine for this case but do not assume that it is always true. I looked at the TCA9555 schematics and NO back bias path was found from the INT pin to Vcc, usually the back bias path is through a ESD structure via a body diode of a FET or something like that.
-Francis Houde