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在 USB（通用串行总线）通信中，术语 ACK、NAK 和 STALL 指的是用于控制数据流和错误处理的握手数据包。下面是对每个术语的详细解释：

1. ACK（确认）：

   • ACK 数据包由接收方发送给发送方，以表明前一个数据包已成功接收且没有错误。当设备从主机成功接收到一个数据包时，它会回应一个 ACK 数据包以确认收到。这使主机知道它可以继续发送下一个数据包。

2. NAK（负确认）： NAK 数据包由主机发送。

   • 接收方发送 NAK 数据包，表示暂时无法处理数据。这可能发生在设备的缓冲区已满或尚未准备好接收数据的情况下。例如，如果设备接收到一个数据包，但当时无法处理，它就会响应一个 NAK 数据包。这就提示主机稍后重试发送数据包。

3. STALL:

   • STALL 数据包表示出现错误或设备无法完成当前传输。这通常用于表示端点遇到了无法恢复的情况。例如，如果设备无法理解命令或无法完成请求的操作，它可能会回应一个 STALL 数据包。这时，主机必须采取纠正措施，如重置端点或执行错误恢复程序。

总之：

• ACK 表示成功收到并准备好接收下一个数据包。
• NAK 表示暂时无法处理数据包，提示发送方稍后重试。
• STALL 表示出现错误或无法进行所请求的操作，需要主机进行错误处理。

这些握手数据包对于维持 USB 主机和设备之间的可靠通信和确保数据完整性至关重要。

In USB (Universal Serial Bus) communication, the terms ACK, NAK, and STALL refer to handshake packets used for controlling data flow and error handling. Here is a detailed explanation of each:

1. ACK (Acknowledgment):

   • The ACK packet is sent by the receiver to the sender to indicate that the previous packet was received successfully and without errors. When a device successfully receives a data packet from the host, it responds with an ACK packet to confirm receipt. This allows the host to know that it can proceed with sending the next packet.

2. NAK (Negative Acknowledgment):

   • The NAK packet is sent by the receiver to indicate that it is temporarily unable to process the data. This could happen if the device’s buffer is full or if it is not ready to receive data. For instance, if a device receives a data packet but cannot handle it at that moment, it responds with a NAK packet. This signals the host to retry sending the data packet later.

3. STALL:

   • The STALL packet indicates an error condition or that the device is unable to complete the current transfer. This is often used to signify that the endpoint has encountered a condition it cannot recover from. For example, if a device cannot understand a command or cannot complete a requested action, it might respond with a STALL packet. The host then has to take corrective actions, such as resetting the endpoint or performing an error recovery procedure.

In summary:

• ACK indicates successful receipt and readiness for the next packet.
• NAK indicates temporary unavailability to process the packet, prompting the sender to retry later.
• STALL indicates an error or inability to proceed with the requested operation, requiring error handling by the host.

These handshake packets are essential for maintaining reliable communication and ensuring data integrity between the USB host and devices.