nRF52 maximum capacitive load on GPIO pins

[Otso Jousimaa]

Hello,

We're thinking of powering external sensors through GPIO pins configured as high-drive. The sensors themselves consume only a few milliamps at most, which is easily within the drive range of the GPIO pins. 

Our concern is with the charging of the decoupling capacitors of the sensors: In theory there would be infinite current spike when the capacitors start charging from 0 V to VDD. The potential issues I can see are:

• Latch-up of GPIO.
• Collapse of nRF52 internal voltage, even if external VDD is ok.
• Collapse of external VDD if capacitance of sensors is large compared to capacitance on VDD.

Is there any kind of specification on how much capacitive load there can be on GPIO pin? Or any recommended workaround to turn-on transient, such as charge capacitors with standard drive first and then switch to high-drive mode?

[Turbo J]

Otso Jousimaa said:

In theory there would be infinite current spike when the capacitors start charging from 0 V to VDD

Nope. The GPIOs are current limited - ~4mA in standard drive and ~20mA in high drive.

If possible, I'd use standard drive for the initial ramping, and then switch to high drive. This way one could charge an arbitrary sized capacitor given a VCC source with more than 5mA current.

Note: A CR2032 has trouble delivering this much current at rated voltages and would need at least some large capacitors "helping" out.

[Otso Jousimaa]

Turbo J said:

Nope. The GPIOs are current limited - ~4mA in standard drive and ~20mA in high drive.

Can you point me to a section of datasheet which says it's safe to short-circuit GPIO because of this current limit? If not, there must be some limit to the load on GPIO. I'd prefer to find the limit in some other manner than having end-users report failures under some conditions or after some time.

[Jared]

Hi,

The current will be limited by the nRF, which in effect will increase the charging time of the capacitor. There is no specified limit on how much capacitance you can place on the GPIO pins. 

regards

Jared

[Otso Jousimaa]

Ok, Thanks. We'll consider charging any capacitors safe