Silicon Laboratories Stepper Machine Specifications download pdf (Page 11)

What is a Transfer?

 The transfer is the process of

making a communications request

with an endpoint. Transfers

determine aspects of the

communications flow such as:

 Data format imposed by the USB

 Direction of communication flow

 Packet size constraints

 Bus access constraints

 Latency constraints

 Required data sequences

 Error Handling

 A transfer has one or more

transactions which then has one,

two or three packets

•Transfers are divided into transactions.

•Transactions are made up of packets.

•The host controls transfers by allocating transactions

to a frame.

•Transfers may span multiple frames.

Transfer

Transaction

Token Packet

Data Packet

Handshake

Packet

Handshake

Packet

Token Packet

Data Packet

Handshake

Packet

Handshake

Packet

…

We previously discussed how transactions are formed using multiple packets. Well

transfers are formed using multiple transactions. You can see from the slide that

hierarchy of the Transfer-Transaction-Packet relationship. The transfer is the

highest level followed by the transaction and then at the lowest level is the packet.

The red denotes the top level or Transfer, the yellow denotes the Transaction and

the green represents the actual packets. Both the transfer and the transaction are

logical implementations whereas the packets denote what you actually transmit

across the bus. A transfer is comprised of one or more transactions. Our example

from the previous slide showed three packets forming a single transaction. Lets

take that a step further. The example we gave is actually one transaction of what is

called a Control transfer that is formed by a total of three transactions. That first

transaction made up the Setup phase of the transfer. This is the phase that told the

device that the host is getting or sending configuration information and what that

information is going to be. A Data transaction then follows whereby the host either

sends the data it wants the device to use in order to configure certain parameters

(like its USB address) or it receives configuration data from the device that tells

how the device is set up to communicate. We would then have another transaction

to complete what is called the Status phase. This is a transaction that validates the

complete transfer (all three transactions).

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Silicon Laboratories Stepper Machine Specifications Page 11

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