GPIO output voltage for small drain current

[XavierYin]

nRF51822 PS says standard drive GPIO output voltage will be VDD-0.3V ~ VDD
In our previous project w/ nRF51822, I  found it to be very close to VDD when connected to GND via 200 kohm resistor.
I blindly trusted this and it seems OK in mass production.

nRF52832 PS says standard drive GPIO output voltage will be VDD-0.4V ~ VDD.
And on all DUTs I played with so far similar phenomenon is observed as long as drain current is small.

Is this generally true that "GPIO output voltage will be very close to VDD as long as drain current is far less than I_OL,SD"?

The reply of this question mentioned an estimation in a resistive-load-view.
Assume drain current is less than 1% I_OL,SD, can I quantitatively trust the output voltage drop will be no more than an order above (0.4V)*1%?

[Joakim Jakobsen]

Hi. 

I've made an internal request to confirm if your statements above are correct or not, and if we have made any measurements on this. 

Will get back to you with more information.

Best regards, 
Joakim Jakobsen

[XavierYin]

Hi Joakim Jakobsen , any update?

[Joakim Jakobsen]

Hi again.
Got some feedback from our developers:

Yes, if the output current load is small, the output voltage will be close to VDD.

In standard drive, the GPIO at nRF52-series is able to deliver 0.5mA with no more than 0.4V of voltage drop. This means that the output resistance is approximately (It is really nonlinear, since it is a MOS transistor) 0.4V/0.5mA = 800ohm. With a 200k rload, the output voltage should be close to 200kohm*1.8V/(200kohm+800ohm) = 1.79V. Or similar for 3.3V operation.

They did some simulations as well to verify the simple calculation.

So, yes, as a first order view, the voltage drop will follow the resistive-load-view from the other question you mention.

Best regards,
Joakim