High voltage on open drain IO

[chethans]

Hi,

Can we give a voltage more than VDD+0.3V on an open drain high impedance configured IO pin? By open drain, I mean:

nrf_gpio_cfg(pin_num, NRF_GPIO_PIN_DIR_OUTPUT,
NRF_GPIO_PIN_INPUT_DISCONNECT,
NRF_GPIO_PIN_NOPULL,
GPIO_PIN_CNF_DRIVE_S0D1,
NRF_GPIO_PIN_NOSENSE );

Since its an open configuration, can it still damage the controller?

Due to a design error, we are getting 3.6V on an IO pin whereas the NRF VDD is at 2.5V.

We will fix this design error for future productions but for the current produced boards (which is quite a lot), I am wondering if the above workaround can be done.

Or is there any other pin configuration which can mitigate this damage?

Thank you,

Chethan

[Dmitry]

chethans said:

So the ESD diode clamps the voltage? Something like this

Yes (the cathode of ESD protection diode is tied to VDD). The regulator is not still running, so it's output is 0V, and you have 0.3V at enable pin - your circuit is in a deadlock...

chethans said:

The circuit gets started up with strong pull up or a diode on R1. Would this damage the IO pin or the controller?

Is your battery is LiPo (it can have 3.6-4.2V at output) or you're sure that it's output not higher than 3.6V? In first case, you will have a high current leakage from battery through R1 and ESD diode to VDD - it may cause damage (a diode with high voltage drop instead of R1 can help). In second case there shouldn't be any damage, as voltage at your IO pin is VDD+0.3. You may encounter a higher current consumption than you expected, and you loose the ability to measure battery voltage with ADC.

[chethans]

Thanks Dmitry for the details.

We are using Lithium Thionyl Chloride batteries which usually does not go beyond 3.7V.

With a diode, we are seeing a 3.4v- 3.5v on the adc line and current increase of about 1-2uA. This is with the pin configured as Hi-Z.

Another query on this: is there a risk of controller hanging/ going to some unknown state when voltage on any pin goes beyond VDD+03V?

If a pin goes to higher voltage (>VDD+0.3) and then returns to normal voltage (<= VDD), what are the risks here?

[chethans]

ketiljo Can you please provide clarification on this?

[ketiljo]

What's the current flowing into the pin?

It's not the CPU that's the problem here but the current that goes through the ESD protection diode. If the current is high enough, you will lift the VDD net as well.