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usbprotocolstransferdata-transfer

USB - doubts about protocol

I'm currently studying how USB works. I read, that there are transactions, which are build from smaller pieces - packets. I read about all kinds of packets.

I can't understand one thing. As the book says - every transaction consists of 3 packets: token, data and hanshake.

The way I understand my book is depicted in the schema below.

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In my opinion:

I think the first transaction should contain only token IN and data packet, but no hanshake packet (handshake for what?).

I think, that response should only contain ACK hanshake packet (that the data is written properly to the device).

Please, help me understand it in a proper way.

Best regards, Tom.

Solution

  • A transaction is a series of one or more packets.

    A typical IN transaction with no data looks like this:

    • The host sends an IN token.
    • The device sends a NAK handshake packet, which means it doesn't have any data to send.

    A typical IN transaction with data looks like this:

    • The host sends an IN token.
    • The device sends a DATA0 or DATA1 packet with data.
    • The host sends an ACK handshake.

    A typical OUT transaction looks like this:

    • The host sends an OUT token.
    • The host sends a DATA0 or DATA1 packet with data.
    • The device sends a NAK or ACK handshake depending on whether it accepted the data.

    Note that I am just talking about full-speed (12 Mbps) USB 2.0 devices, and things can get a bit more complicated for the higher-speed devices.

    Note that any of these packets could be lost due to noise issues. The USB specification specifically accounts for this, ensuring that packet loss doesn't result in incorrect operation of the device or host.