Silicon Laboratories Stepper Machine Specifications download pdf (Page 13)

USB Transfers—A Closer Look

 Control Write Transfers (OUT)—contains Setup, Data (optional), and

handshake transactions

SETUP

PID=1101

DATA0

PID=0011

ACK

PID=0010

OUT

PID=0001

DATA1/0

PID=1011

ACK

PID=0010

NAK

PID=1010

STALL

PID=1110

PID=1001

0-LENGTH

DATA1

PID=1011

ACK

PID=0010

NAK

PID=1010

STALL

PID=1110

Host sends Setup Packet followed by 8 bytes of data for

the request. The device must return an ACK.

Setup Transaction

Data Transaction

Status Transaction

Host sends OUT Packet along with the data for writes.

Device responds with handshake. No response indicates

a data error. DATA1 is first and alternates DATA0 and

DATA1.

Data error

Host sends IN Packet for Status phase. Device responds

with a 0 length packet to indicate the success or failure

of the transfer. Host ACK the status. No response

indicates a data error.

Data error

Host → Device

Device → Host

In the next set of slides we are taking a look at the transactions and packets that

make up the different transfers and this slide shows the Control Transfer that is

sending data to the device. The colored boxes denote which direction the data is

flowing, whether it is from the host or the device. Keep in mind the host initiates all

transfers so the first box will always be yellow. The first transaction is the Setup

which is defined by the setup packet. Next we see the data that is sent in the request

from the host. The data sent in this phase is 8 bytes long and contains info about the

request, for example, if the host wants a device’s configuration or to set the device

address. After the data transaction the device ACKs the transaction. Next is the

data phase. Since this is an OUT control transfer the Data is yellow. The device

can either ACK, NAK or Stall the request. The ACK signifies that the transaction

was successful, the NAK means that the data either was not received correctly or

the device is not ready for the data. A Stall would mean that the device does not

support the requested feature. The Status phase for control transfers is just a 0

length packet in the opposite direction of the data phase. In this diagram the host

was sending the device information (OUT transfer) in the data transaction,

therefore, the in the status phase the host sends an IN token and the device responds

with a 0 length packet. These diagrams and the following flow diagrams like it can

be found in the book from Jan Axelson titled, “USB Complete.”

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Silicon Laboratories Stepper Machine Specifications Page 13

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