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6.5 Data Transmit Function
This section explains USB Controller's data transmit function.
6.5.1 Overview of transmit processing
USB Controller divides the data segments in system memory, into USB packets, then transmits them to the Host
PC. The V
4120A sets the size of USB packet in the MAXP field of the EP0 Control Register, the EP1-2 Control
R
Register, the EP3-4 Control Register, and the EP5-6 Control Register (in the example shown below, a value of 64
bytes has been set).
USB
Packet
Packet
64 Bytes
64 Bytes
Last USB packet in a data segment will be smaller than the value set in the MAXP field (40 bytes in the example
shown above). As a result, the Host PC can identify the boundary between data segments. When the last USB packet
size is equal to the value in the MAXP field, a zero-length USB packet will be transmitted after the last item of the
divided data in transmit SZLP Mode. In transmit NZLP Mode, a zero-length USB packet is not transmitted. For an
explanation of the transmit modes, see Section 6.5.3 Data transmit modes.
After all the data segments have been transferred, USB Controller writes the "transmit indication" which has
transmit status information into the MailBox in system memory.
For an explanation of the transmit indication, see Section 6.5.6 Tx indication.
Data segments transmission time at a given EndPoint can be scheduled upon the issue of the corresponding
transmit command. The current transmit status can be determined by reading the contents of the USB Tx EndPoint
Status Register (Address: 1000_1048H).
For an explanation of how to issue the transmit command, see Section 6.5.4 V
transmitting.
6.5.2 Tx buffer configuration
The V
4120A creates a Tx buffer in system memory, then informs USB Controller to transmit data to the Host PC.
R
The configuration of the Tx buffer is as shown below.
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CHAPTER 6 USB CONTROLLER
Figure 6-4. Division of Data into USB Packets
D ata S egm ent
USB
USB
Packet
64 Bytes
Preliminary User's Manual S15543EJ1V0UM
USB
USB
Packet
Packet
64 Bytes 40 Bytes
4120A processing at data
R
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