TCA6424A: About VCCI, VCCP, and RESET pin

Hi Y.Ottey,

Y.Ottey said:

1. Is it okay to connect the power supplies to the VCCI and VCCP pins directly without separating them?

Yes, you can directly connect the power supplies to VCCI and VCCP. What do you mean by separating them?

Y.Ottey said:

2. If 3.3V is applied to the RESET pin before raising the voltage of the VCCI and VCCP pins, is there a problem in operation?
(If you check the rating of the input voltage, it is 6.5V at the maximum, so I think that it will not be damaged.)

You are absolutely correct. As long as you operate within the absolute maximum conditions you can't do long term damage to the device. However the device may not operate as you expect it to. For this device the RESET will not do anything unless there is power on VCCP. So you can apply power to RESET first and then the power supplies. However even if you did it the other way around it would still work. RESET simply just resets all the register values to their defaults and resets the I2C state machine. So as long as you wait for RESET to go high to start sending data there will be no issue with operation.

Y.Ottey said:

3. As shown in the figure below, is there any problem if the following procedure is performed at startup?
(I think that RESET should be set to High after starting VCCI and VCCP.)
3.3V can be applied to VCCI
→ Release RESET (Set to High)
→ 3.3V can be applied to Vccp
→ Set only Vccp to 0V

There is nothing wrong with this procedure. What is the goal of setting VCCP to 0?

Best,

Chris

Hi Chris 

Thank you for your reply.

When we asked our customers how to set up RESET, VCCI, and VCCP, they said they would start up as follows.
The order is as follows.
1. Set RESET to High
2. Start up VCCI and VCCP at the same time
3. After communication is completed, turn down VCCI and VCCP (RESET remains High)

Will there be any problems in the above cases?
(Probably not a problem.)

Regards,
Y.Ottey

Hi Y.Ottey,

How are they applying power to the RESET pin before applying it to the supply pins? We recommend that they just use a pull up resistor from VCCI to RESET. This would turn on VCCI and RESET at the same time. Then when you apply power to VCCP it won't automatically reset your device.

Either way, if they were able to power RESET first I don't see there being an issue, as long as they wait for all 3 pins to be HIGH to start sending data.

However, as I stated before I definitely recommend they just use a pull up resistor and tie RESET to VCCI.

Best,

Chris

Hi Chris 

Thank you for your reply.

I will ask our customers how they are applying voltage to the reset pins.

I would like to ask you an additional question.
You answered as follows.
"I stated before I definitely recommend they just use a pull up resistor and tie RESET to VCCI."

It's okay to set RESET High before powering VCCI, VCCP, but why is it recommended to pull up the RESET pin to the VCCI pin and have the RESET pin and VCCI pin up at the same time?

Regards,

Y.Ottey

Y.Ottey,

There are multiple benefits to attaching RESET to the VCCI pin. I would say that the most important reason is that VIL on RESET is equal to 0.3*VCCI. By pulling RESET to VCCI you are ensuring that the device will not randomly reset.

Every time there is a power cycle on VCCI the device will automatically reset. This could be seen as a feature or a drawback. However, this could not be done if the RESET was attached to VCCP since RESET requires VCCP to be active in order for a reset to be recognized.

Now this doesn't mean that RESET has to be pulled up to VCCI. You could have RESET connected to a separate power supply but that just adds more variables where things could go wrong. You would also have to make sure that this power supply stays above 0.3*VCCI. However, by attaching RESET to an external power supply you are ensuring that if there is a power cycle on VCCI, and VCCP is still active, the device will not reset.

Hopefully this cleared some things up, let me know if you have any further questions.

Best,

Chris