How can i tell if a peripheral is connected to GPIO? - android-things

Question

I want to be able to detect when a peripheral sensor is NOT connected to my Raspberry Pi 3.
For example, if I have a GPIO passive infrared sensor.
I can get all the GPIO ports like this:
PeripheralManagerService manager = new PeripheralManagerService();
List<String> portList = manager.getGpioList();
if (portList.isEmpty()) {
Log.i(TAG, "No GPIO port available on this device.");
} else {
Log.i(TAG, "List of available ports: " + portList);
}
Then I can connect to a port like this:
try {
Gpio pir = new PeripheralManagerService().openGpio("BCM4")
} catch (IOException e) {
// not thrown in the case of an empty pin
}
However even if the pin is empty I can still connect to it (which technically makes sense, as gpio is just binary on or off). There doesn't seem to be any api, and I can't legitimately think of logically how you can differentiate between a pin that has a peripheral sensor connected and one that is "empty".
Therefore at the moment, there is no way for me to assert programmatically that my sensors and circuit is setup correctly.
Any one have any ideas? Is it even possible from a electronics point of view?
Reference docs:
https://developer.android.com/things/sdk/pio/gpio.html

Answer 1

There are lots of ways to do "presence detection" electrically, but nothing that you will find intrinsically in the SoC. You wouldn’t normally ask a GPIO pin if something is attached—it would have no way to tell you that.
Extra GPIO pins are often used to detect if a peripheral is attached to a connector. The plug for some sensor could include a “detect” line that is shorted to ground and pulls the GPIO low when the sensor is attached, for example. USB and SDIO do something similar with some dedicated circuitry in the interface.
You could also build more elaborate detection circuits using things like current sensing, but they would inevitably have to put out a binary signal that you capture through a dedicated GPIO.
This is easier to achieve for serial peripherals, since you can usually send a basic command and verify that you get a response.

Answer 2

Detection using solely the input line can be tough. First, you'd want to narrow the scope of the problem. Treat as not-present the condition of a sensor not being connected, the sensor being connected but not responding, or the sensor responding in an uncharacteristic manner.
So, if it is a digital sensor, then communicating with the sensor may be enough to tell if it is present or not (especially if checksums or parity bits are involved).
Some analog sensors also have specific specs on how it behaves when triggered. You can utilize deviation from those specs to determine if the sensor is not present.
If you have a digital sensor w/o any error checking on it's output, where you clock out data (so all 0s or all 1s is valid) or it's just a binary 1 or 0 for output, then you'd need external help. Same for most analog sensors.
This external help would be something where you put the system in a known controlled state, press a button, and it then checks the sensors for output within a specific range. To be absolutely sure, you'd want at least two different states, to ensure your digital or analog inputs didn't happen to be stuck at the correct state for your test.
Just about any other method would be external to the system. Using additional IO to "detect" a sensor could help increase confidence the sensor is there, but you could get false positives where all you've learned is that "something" is there - not necessarily the sensor you expect.