Hey there, let's break down your questions one by one based on the OTG setup you've described:

The connection follows these key steps tailored to your setup:

• Mode Trigger via ID Pin: As soon as your device plugs into the phone's Micro USB port, the phone first checks the ID pin's voltage. Since you've grounded the ID pin, the phone detects a low level and switches itself to OTG Host mode—this means it starts supplying 5V power to the peripheral via the VBUS pin.
• Device Speed Detection: Next, the phone's USB Host controller scans the D- and D+ lines. Your setup pulls D- high (though we'll talk about the correct way to do this later), which tells the Host this is a Low-Speed USB device (per USB specs, Low-Speed uses D- pull-up, while Full/High-Speed uses D+).
• Basic Enumeration (No Need to Dig Into Details): The Host then sends a reset signal and initiates basic USB enumeration. Since you don't care about the communication details, you just need to know that by this point, the system has recognized a device is connected.

Let's split this into two parts:

• ID Pin Setup: Correct: Per Micro USB OTG specs, grounding the ID pin on the peripheral is exactly how you trigger the phone to switch to Host mode. This part is spot-on.
• D- Pin Setup: Incorrect: Pulling D- directly to 5V is not the right way to mark a Low-Speed device. The standard approach is to use a 1.5kΩ pull-up resistor to connect D- to VBUS (the 5V supply from the phone). Directly shorting D- to 5V risks damaging the phone's USB port (most phone USB IO pins are 3.3V tolerant, so 5V would cause overvoltage) and violates USB signal standards. Swap that direct connection for a 1.5kΩ resistor and you'll be compliant.

You don't need to mess with detecting the ID pin's falling edge or D-'s rising edge directly—Android handles all the low-level hardware detection for you. Here's what to do:

• Listen for System USB Broadcasts: Android sends the android.hardware.usb.action.USB_DEVICE_ATTACHED broadcast whenever a USB device is connected. Your app can listen for this in two ways:
  • Static Registration: Declare a BroadcastReceiver in your AndroidManifest.xml and specify it should listen for this action.
  • Dynamic Registration: Register the receiver via code when your app starts or moves to the foreground—this is more flexible if you only want notifications when the app is active.
• Request USB Permissions: Add the <uses-permission android:name="android.permission.USB_PERMISSION" /> permission to your manifest. You'll also need to request access to the USB device in your broadcast receiver so your app can interact with it (and trigger the notification).
• Trigger the Notification: As soon as your app receives the android.hardware.usb.action.USB_DEVICE_ATTACHED broadcast, use Android's notification API to pop up your desired alert.

To answer your specific doubt: The ID pin's falling edge triggers the phone's mode switch, and D-'s level identifies the device speed, but these low-level signals are handled by the system kernel. Your app can't directly monitor hardware pin changes—listening for the system broadcast is the reliable, supported way to get immediate notifications.

内容的提问来源于stack exchange，提问作者lukeluck