Decrease I/O Pull up current consumption

[michaeld]

Hello,

I found this on the forum:

According to nRF51822 PS v3.1 section 8.23 pullup and pulldown resistors have a typical value of 13kohm. if you have e.g. a button that connects a certain pin to ground when pressed, and the pin is configured as input with pullup, and you have supply power of 3.0 volts for the nRF51, then the current consumption is 3/13k = 230uA when the button is pressed.

Now my question is what would be the highest possible pull up resistor value for the I/O to still be detected?

I tried using a 10M, 1M and 470K ohm resistor for external pull up with the internal pull up disabled but then the I/O change event is not detected. I want to reduce the current consumption of 230uA would be to high for my application.

Thanks!

[RK]

I can't see why that would happen. Perhaps at 10M but 470K? You have VCC, the your pullup resistor, then the pin, then the switch and then ground right? So when you press the switch you must absolutely be grounding the pin, it's a direct connection, it must be at 0V. So the only way you wouldn't be detecting an event is if the switch is open and the voltage doesn't rise enough to register a logic-'1'. For that to happen, the resistance to GND of the GPIO input would have to be lower than about 1M at 470K pullup. I find it hard to believe it's anywhere near that low. Sure you have the pin configured correctly as input, connected etc? What does a meter measure the voltage at with the switch open (and closed which must be 0V)?

[michaeld]

Thanks for you response.

Here you can find my schematic: http://i.imgur.com/E9M6FVs.png

I measured the voltage with the switch open its is 3.2V and the supply voltage is also 3.2V. So I really don't understand why it's not triggering at 470K ohm. When pressed it is 0V as expected. Only with the 1M and 10M I measured 2.2V and 1.45V at the pin.

I configured the pin like this:

		nrf_gpio_cfg_input(LEFT_BUTTON, NRF_GPIO_PIN_NOPULL);

[RK]

ok those voltages are pretty much what I'd expect. Surprised it sags as low as 2.2V with just 1M, that's worth knowing.

Going back to your original question, you say the I/O change event 'is not detected'. What are you using to detect it? I'm pretty sure if you read the GPIO INPUT register it will change when you press the button, if you are using a GPIOTE PORT event however you need to set up the pin to have some kind of SENSE on it, the nrf_gpio_cfg_input() function sets it to SENSE_Disabled.

[michaeld]

That last thing what is. I thought nrf_gpio_cfg_input() left my GPIOTE configuration alone. But it did not thank you!

470K works :)

[michaeld]

1M also works BTW.