USB has four different packet types. Token packets indicate the type of transaction to follow, data packets contain the payload, handshake packets are used for acknowledging data or reporting errors and start of frame packets indicate the start of a new frame.
- Token Packets：There are three types of token packets,
-
- In - Informs the USB device that the host wishes to read information.
- Out - Informs the USB device that the host wishes to send information.
- Setup - Used to begin control transfers.
Token Packets must conform to the following format,
| Sync | PID | ADDR | ENDP | CRC5 | EOP |
- Data Packets：There are two types of data packets each capable of transmitting up to 1024 bytes of data.
- Data0
- Data1
High Speed mode defines another two data PIDs, DATA2 and MDATA.
Data packets have the following format,
| Sync | PID | Data | CRC16 | EOP |
- Maximum data payload size for low-speed devices is 8 bytes.
- Maximum data payload size for full-speed devices is 1023 bytes.
- Maximum data payload size for high-speed devices is 1024 bytes.
- Data must be sent in multiples of bytes.
- Handshake Packets：There are three type of handshake packets which consist simply of the PID
- ACK - Acknowledgment that the packet has been successfully received.
- NAK - Reports that the device temporary cannot send or received data. Also used during interrupt transactions to inform the host there is no data to send.
- STALL - The device finds its in a state that it requires intervention from the host.
Handshake Packets have the following format,
| Sync | PID | EOP |
- Start of Frame Packets：The SOF packet consisting of an 11-bit frame number is sent by the host every 1ms ± 500ns on a full speed bus or every 125 µs ± 0.0625 µs on a high speed bus.
| Sync | PID | Frame Number | CRC5 | EOP |