High voltage on open drain IO

RE: High voltage on open drain IO

ketiljo — Mon, 22 Jun 2020 13:02:12 GMT

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.

RE: High voltage on open drain IO

chethans — Mon, 22 Jun 2020 07:20:05 GMT

ketiljo Can you please provide clarification on this?

RE: High voltage on open drain IO

chethans — Mon, 15 Jun 2020 06:53:11 GMT

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?

RE: High voltage on open drain IO

Dmitry — Sat, 13 Jun 2020 11:40:44 GMT

[quote userid="85690" url="~/f/nordic-q-a/62467/high-voltage-on-open-drain-io/254828"]So the ESD diode clamps the voltage? Something like this[/quote]

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...

[quote userid="85690" url="~/f/nordic-q-a/62467/high-voltage-on-open-drain-io/254828"]The circuit gets started up with strong pull up or a diode on R1. Would this damage the IO pin or the controller?[/quote]

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.

RE: High voltage on open drain IO

chethans — Sat, 13 Jun 2020 09:14:34 GMT

So the ESD diode clamps the voltage? Something like this:

Can you please elaborate on the complex start up?

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

Thank you,

Chethan

RE: High voltage on open drain IO

Turbo J — Sat, 13 Jun 2020 08:52:15 GMT

[quote userid="85690" url="~/f/nordic-q-a/62467/high-voltage-on-open-drain-io/254825"]But with NRF52833 in picture, the voltage gets held up at 0.3V-0.5V.[/quote]

That would be correct with NRF chips VDD at 0 Volt (LDO off).

You will need a more complex startup curcuit here.

RE: High voltage on open drain IO

chethans — Sat, 13 Jun 2020 08:43:53 GMT

Ok. Thanks for the clarification Dmitry and ketiljo.

To give you more idea on the issue, here is our configuration:

We operate on a 3.6V battery, which goes to a resistor divider. The output of resistor divider is about 2/3rd of the battery voltage.

The output of voltage divider is fed to the enable pin of regulator and the adc pin (AIN0) of NRF52833. The output of regulator is configured to be for 3.3V.

Ideally, the voltage divider should give us 2.4V with 3.6V battery. This works fine without NRF52833 connected.

But with NRF52833 in picture, the voltage gets held up at 0.3V-0.5V.

What could be the reason for this?

Is the state of IO pin during startup causing the voltage to be clamped?

We were thinking of bypassing this issue by shorting R1. But with the above behaviour, there might be more serious issues which can effect long term operation.

Thank you,

Chethan

RE: High voltage on open drain IO

ketiljo — Fri, 12 Jun 2020 10:38:22 GMT

Correct. The ESD diodes will start to conduct of the GPIO voltage is VDD+0.3 V. If you have a series resistor, the current will be limited and the voltage is clamped to VDD+0.3 V on the pin. Depending on the size of the resistor, this is fine. But of you pull the voltage higher than spec, you will start to lift the whole VDD net as well, and the current through the ESD diode will become to high and may damage the structure in the GPIO.

RE: High voltage on open drain IO

Dmitry — Thu, 11 Jun 2020 15:47:00 GMT

Hi Chethan,

A voltage higher than VDD+0.3V coming from external chip will pass through an internal ESD protection diode and can damage the nRF chip if the signal is strong enough, despite of pin configuration. If there's only 3.6V coming from pull-up resistor, the bus will be clamped by nRF chip at 2.6-2.8V level - the damage is unlikely but other weird things can happen, nobody can guarantee a stable behavior in this case.