Silicon Laboratories Stepper Machine Specifications download pdf (Page 10)

Transactions

 A transaction allows a set of operations to be grouped in such a way that

they either all succeed or all fail; further, the operations in the set appear

from outside the transaction to occur simultaneously. If the transaction

is unsuccessful then the host/device ignores any data that was received.

Transaction Types

SETUP:

Specifies a control transfer.

Setup transactions are always targeted to Endpoint 0 and are bi-directional (IN and OUT endpoint).

Has token and handshake phases with an optional data phase.

All USB devices must support setup transactions.

DATA:

The host is requesting to send(receive) data to(from) an endpoint.

IN – Responsible for sending data from the endpoint to the host.

OUT – Responsible for sending data from the host to the endpoint.

STATUS:

During control transfers the STATUS transaction uses the IN or OUT data phase to convey

success or failure of a transaction.

Take a moment to read the text in this slide as it conveys the key message regarding

transactions. A transaction is a combination of packets. For example, the Setup

transaction contains the Token packet we saw from the previous slide. With the

Token packet the host is transmitting the PID, the address and the endpoint number

of the device it wants to communicate with. The next packet it sends is the Data

Packet which tells the device that it wants to get/set some information from/to the

device. Next there is a handshake packet to signify a success or failure of the

transaction. So to review, the transaction is made up of multiple packets. In the

example provided, the setup transaction had 3 packets associated with it: token, data

and handshake.

There are three transaction types as listed in the slide. The Setup, Data and Status

transactions. Setup is used during the configuration process after a device reset.

The Data phase is used to transfer data to/from the device as requested by the host.

Taking the example we just gave with the Setup transaction it could be followed by

a Data transaction where the device can receive data that will set its address or it

could send data that tells the host what its vendor ID is. The Status transaction

identifies to the USB at a higher level than the handshake packet that a complete

transfer (multiple structured transactions) was successful. We will look at transfers

next.

After the device has been powered, it must not respond to any bus transactions until

it has received a reset from the bus. After receiving a reset, the device is then

addressable at the default address. The default address is address 0 and is termed

the Control Pipe.

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Silicon Laboratories Stepper Machine Specifications Page 10