CODEWITHSUNDEEP
androidusbaccessory
Nathan
Nathan

Reputation: 21

android can not receive a data of 512bytes in usb Accessory mode

when android communicate with pc via usb accessory mode, the android can not receive a data if pc send 512bytes data to android. but, there are no problem if over(or below) than 512bytes data transfer. And if android receive other byte data after receive 512bytes data then incoming both missing data and other byte data(512bytes + other data).

my read code on thread is below.

@Override
public void run() {
    byte[] readbuffer = new byte[16384];
    int bytesRead = 0;

    try {
        while ((bytesRead = mInputStream.read(readbuffer, 0, readbuffer.length)) != -1) {
            //my code here afrer read.
            ....
            mHandler.sendMessage(msg);
        }
    } catch (IOException e) {
        e.printStackTrace();
    }
}

this is happened not only 512bytes but some other specific lengths(512bytes, 1024bytes, 2048 bytes...). is this android accessory mode bug?

anybody know this issue?

Upvotes: 2

Views: 575

Answers (1)

J Godoy
J Godoy

Reputation: 21

It is not a bug with AOA but your sender not finishing the USB transaction. Unlike USB control transfers, bulk transfer does not transmit the data size, so for a bulk transfer to finish one of these conditions must be satisfied:

  1. The amount of data received is the amount of data requested.
  2. The size of the data is less than maximum buffer size.
  3. A Zero-length package is received.

For high speed mode, the maximum buffer size is 512 bytes, so if you send 0-511 bytes, condition 2 is satisfied. In case data is 513-1023 length, it will be split in two packages 512 bytes + 1-511 bytes, so again, the last package satisfies the 2nd condition.

In case you send exactly 512 bytes, the receiver does not know either you have finish the transaction or there is remaining data (in an additional package) so it keeps waiting and freezes. So, for lengths multiple of buffer size (512 on high speed and 64 in full-speed) you need to send an additional zero length package for finishing the USB transfer.

Upvotes: 2

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