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CHAPTER 17 USB FUNCTION
17.3.1
Flow of Data Transfer
This section describes the flow of data transfer by the USB function.
■ Setup Stage of Control Transfer (Most Standard Commands)
The protocol engine automatically processes almost all standard commands received from the
host to reduce the load on the CPU of the device. The CPU of the device need not perform any
processing of these commands. The protocol engine does not even report reception of these
commands to the CPU of the device.
commands listed below.
Clear_Feature, Get_Configuration, Get_Interface, Get_Status, Set_Address, Set_Configuration,
Set_Feature, and Set_Interface
Figure 17.3-1 shows the flow of the setup stage of control transfer (to process most standard
commands).
Figure 17.3-1 Flow of the Setup Stage of Control Transfer (to Process Most Standard Commands)
USB
The flow shown by the figure is explained below.
(1) When a command for the setup stage of control transfer is received from the USB, the
protocol engine asserts the SETUP pin and sets the SETUP bit of the status register.
(2) If a standard command other than Get_Descriptor, Set_Descriptor, and Sync Frame is
received and the setup stage ends, the protocol engine sets the NACKOS bit of the status
register. In this case, the protocol engine performs the processing required for the command
and does not write data to any FIFO buffer for the end point.
Note:
The CPU (application program) need not perform any processing.
If the setup stage command is received normally, an ACK handshake signal is transferred to the
USB. If the setup stage command is not received normally, nothing is transferred to the USB (time-
out occurs).
410
(1)
Protocol
engine
(2)
The protocol engine automatically processes the
FIFO buffers for
end points
Control
and
status
registers
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