CODEWITHSUNDEEP
usb
quantum231
quantum231

Reputation: 2593

What does USB transfer need status phase?

Basically after every IN, OUT or SETUP transaction we have an ACK/NAK packet at the end of the transaction. If a handshake packet is part of every transfer as it comes after the data packet which is preceded by token packet, then why do we need a status stage? This seems to be present in the control transfer.

Upvotes: 0

Views: 613

Answers (1)

ralf htp
ralf htp

Reputation: 9422

In the protocol endpoints are in a status: ACTIVE, HALT, STALL,... in the status phase this status is determined (GET_STATUS(0x00) request (http://www.beyondlogic.org/usbnutshell/usb6.shtml) )

The status phase check is a bit like a CRC checksum over the entire request not over each single packet.

http://www.beyondlogic.org/usbnutshell/usb4.shtml:

"

Status Stage reports the status of the overall request and this once again varies due to direction of transfer. Status reporting is always performed by the function.

IN: If the host sent IN token(s) during the data stage to receive data, then the host must acknowledge the successful recept of this data. This is done by the host sending an OUT token followed by a zero length data packet. The function can now report its status in the handshaking stage. An ACK indicates the function has completed the command is now ready to accept another command. If an error occurred during the processing of this command, then the function will issue a STALL. However if the function is still processing, it returns a NAK indicating to the host to repeat the status stage later.

OUT: If the host sent OUT token(s) during the data stage to transmit data, the function will acknowledge the successful recept of data by sending a zero length packet in response to an IN token. However if an error occurred, it should issue a STALL or if it is still busy processing data, it should issue a NAK asking the host to retry the status phase later.

"

or see http://wiki.osdev.org/Universal_Serial_Bus

" Finally, a STATUS transaction from the function to the host indicates whether the [control] transfer was successful. "

Upvotes: 1

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