RE: GPIO high drive on nRF52, any limitations?

Ladasky — Tue, 13 Dec 2016 17:52:03 GMT

Thank you Mr. Bauck, that was very helpful information.

It would appear that the nRF52 actually has a lower current driving limit than the nRF51. That's fine, I'm sure most people don't need much current driving capability, and it can always be added.

A few weeks ago while I was waiting for your reply, I found some NPN transistors to drive the relay. It appears that I need two of them, since the relay is a latching type. I tried to drive those transistors with GPIO. But I didn't have flyback diodes in my first circuit. And it appears that I actually damaged an nRF52 DK board with this circuit. Oops. Live and learn.

RE: GPIO high drive on nRF52, any limitations?

Ole Bauck — Tue, 06 Dec 2016 13:57:08 GMT

Hi,

You should use a MOSFET transistor. A SOT-23 transistor does not take much space. Driving a relay directly from the gpio pin will drive the gpio pin out of spec when it comes to current. We recommend 15mA total max current on the nRF52 gpio pins. This can be on one pin if you want. If you go outside this we cannot guarantee the stability of the device. We also recommend that you sink current rather than source, because there are more ground pads on the device than VDD pads.

Regarding the figure, you are reading it the other way. This figure tells you the pad voltage when you are sinking current. If you sink 25mA the pad voltage will be 3V, as 25mA is the absolute maximum current you can sink from a pin. If you want 2V on the pin you can sink 20mA, but again this is not recommended, especially when doing this on two pins.

Here is a related case discussing nRF52 max gpio current: devzone.nordicsemi.com/.../.

You will have to make room for that transistor. And also don't forget to add a flyback diode across the relay input.