GPIO Current Drive

RE: GPIO Current Drive

awneil — Wed, 15 May 2019 16:39:01 GMT

You really do need a hardware engineer on your team!

The analogue inputs see a voltage. So you need some way to translate the variable resistance of your sensor into a voltage for the analogue input ...

This is basic electronics - not specific to Nordic.

https://en.wikipedia.org/wiki/Voltage_divider#Sensor_measurement

RE: GPIO Current Drive

lance338 — Wed, 15 May 2019 16:29:16 GMT

I know what a voltage divider is, just don't understand how that is required for the analog comparator (which admittedly I just learned about today)

RE: GPIO Current Drive

awneil — Wed, 15 May 2019 16:26:24 GMT

You need a hardware engineer on your team!

https://en.wikipedia.org/wiki/Voltage_divider

RE: GPIO Current Drive

lance338 — Wed, 15 May 2019 16:24:01 GMT

Thanks. We will migrate to nRF52 eventually.

BTW, you said we need to use a potential divider. Can you please elaborate?

RE: GPIO Current Drive

awneil — Wed, 15 May 2019 16:15:44 GMT

Yes.

I would read the section on the comparator in the Product Specification, and then look for an example in the SDK examples.

Note that the nRF51 chips are not recommended for new BLE designs.

RE: GPIO Current Drive

lance338 — Wed, 15 May 2019 16:10:12 GMT

Are you suggesting to use the GPIO as analog with the LPCOMP logic? I came to that conclusion as well. If that is the case, can you give/point to more details on how to set it up?

RE: GPIO Current Drive

awneil — Wed, 15 May 2019 16:06:19 GMT

So you want to detect whether the resistance is "high" (1M) or "low" (20k)

The arrangement you showed will not do that.

What you need is a potential divider, and user either the ADC or Comparator to look at the voltage and, thus, decide whether the resistance is "high" or "low".

RE: GPIO Current Drive

lance338 — Wed, 15 May 2019 15:57:30 GMT

Just detect whether chemical present/not present.

RE: GPIO Current Drive

awneil — Wed, 15 May 2019 15:47:17 GMT

So are you actually trying to measure the resistance, then?

Or just detect whether the chemical is present ("low" resistance) or not ("high" resistance)?

RE: GPIO Current Drive

lance338 — Wed, 15 May 2019 15:33:02 GMT

Thanks for your response. I used the word load loosely. Let's call them external resistor whose resistance varies depending on the amount of chemical substances it interacted with. The circuit is exactly as shown. Both gpios or on the same nrf51822. For our purpose, if there is no chemical substances, the external resistor is 1M ohm. With chemical present, the resistance is 20K ohm.

RE: GPIO Current Drive

awneil — Wed, 15 May 2019 13:52:01 GMT

See below about your meaning of "load".

As a straightforward resistor between OUT and IN, it should work - so there must be something else going on...

RE: GPIO Current Drive

awneil — Wed, 15 May 2019 13:49:57 GMT

The way you use the word "load" is suspicious.

The connection you've shown would not normally be described as a "load":

[quote userid="62040" url="~/f/nordic-q-a/47289/gpio-current-drive"]GPIO_OUT ------------------ Rload -----------------------GPIO_IN [/quote]

That is simply a resistor connecting the OUT to the IN.

As Kenneth said, the IN has a very high impedance - so virtually no current wil flow through the "load"

But does that diagram fully represent your situation?

Why are you calling this thing a "load" ?

Are the OUT and IN pins on the same nRF51822 chip?

Can you post a full schematic of your setup?

Or some good, clear, in-focus photographs.

RE: GPIO Current Drive

lance338 — Wed, 15 May 2019 12:55:20 GMT

Thanks Kenneth. Our goal is to be able to detect '1' at the input gpio with a load of 20K ohm. Do you have any suggestion to achieve that?

RE: GPIO Current Drive

Kenneth — Wed, 15 May 2019 12:01:14 GMT

Hi,

The drive strength configure the max current the GPIO _may_ provide, to sink or source external circuitry.

In this case I think you have a 6Kohm resistor in series with an input pin with very high impedance (>>1Mohm). So there will be close to no current floating through the 6Kohm resistor.

Best regards,
Kenneth