Android usb host: asynchronous interrupt transfer

Android USB Host: Asynchronous Interrupt Transfer

This article explores asynchronous interrupt transfers in the context of Android USB Host mode. It covers the fundamentals, advantages, implementation steps, and relevant considerations.

Understanding Interrupt Transfers

Interrupt transfers are a USB communication mechanism designed for low-latency, event-driven data exchange. They are particularly suitable for:

• Receiving status updates or notifications from peripheral devices.
• Handling real-time data like sensor readings or button presses.

Key Characteristics

• Asynchronous: Devices can initiate data transfers without the host explicitly requesting them.
• Low Latency: Designed for quick communication, often used for time-sensitive data.
• Small Data Packets: Typically limited to a few bytes, suitable for status updates or control commands.

Asynchronous Interrupt Transfers in Android

Implementation Steps

1. Device Descriptor Analysis: Analyze the USB device descriptor for interrupt endpoints. Identify the endpoint address, maximum packet size, and polling interval.
2. USB Request Creation: Craft a USB request to initiate the asynchronous transfer using the UsbRequest class in Android.
3. Endpoint Configuration: Configure the interrupt endpoint using the UsbEndpoint class.
4. Queue the Request: Add the USB request to the request queue associated with the interrupt endpoint.
5. Data Reception: When data arrives, Android’s USB stack will automatically trigger the registered callback function, providing access to the received data.

Code Example

import android.hardware.usb.UsbDeviceConnection;
import android.hardware.usb.UsbEndpoint;
import android.hardware.usb.UsbRequest;

// ...

UsbEndpoint interruptEndpoint = ...; // Get the interrupt endpoint
UsbRequest request = new UsbRequest();
request.initialize(connection, interruptEndpoint);
boolean success = request.queue(buffer, buffer.length);

// Callback function
private final UsbRequest.Callback callback = new UsbRequest.Callback() {
    @Override
    public void onCompleted(UsbRequest request, int len) {
        if (len > 0) {
            // Process the received data from buffer
        }
    }
};

// Register the callback
request.setClientData(callback);

Comparison Table: Interrupt vs. Bulk Transfers

Considerations

• Endpoint Address: Interrupt endpoints are typically assigned odd endpoint addresses (e.g., 0x81, 0x83).
• Polling Interval: Determine an appropriate polling interval for the specific device and data rate.
• Data Buffer Size: Choose a buffer size that can accommodate the maximum data packet expected from the peripheral.

Conclusion

Asynchronous interrupt transfers are a powerful technique for handling event-driven, low-latency communication in Android USB Host mode. By understanding their characteristics and implementing the appropriate steps, developers can effectively interact with peripheral devices and leverage real-time data transfer capabilities.