USB Transactions
Last modified by Microchip on 2023/11/10 11:19
Universal Serial Bus (USB) transactions are initiated by the host and intended to complete a communications event. In addition to a synchronizing preamble, transactions can have up to three packets:
- Token: Sent by the host and describes the type of transaction.
- Data: Depending on the transaction type, this is optional. This is the data payload that is sent by the host for OUT transactions and by the device for IN transactions.
- Handshake: Status of the communications request.
Token Packets | ||
|---|---|---|
| OUT | 0001b | Beginning of OUT transaction |
| IN | 1001b | Start of an IN transaction |
| SETUP | 0101b | Signals beginning of a Control Transaction |
| SOF | 1101b | Start of Frame packet |
Data Packets | ||
| DATA0 | 0010b | Clears the toggle bit in the data packet |
| DATA1 | 1011b | Sets the toggle bit in the data packet |
Handshake | ||
| ACK | 0110b | Transmission successfully completed |
| NACK | 1010b | Transmission unsuccessful |
| STALL | 1110b | Delayed response, retry needed |
If the device responds to the host request and the associated Cyclic Redundancy Check (CRC) is correct, the entire communications event is conducted within one transaction. If the device responds with a NACK or a STALL, depending upon the application requirements, the host will retry the transaction on the next scheduled frame. After several unsuccessful retries, the Host may suspend the device with a control command.
Sample USB Transactions
