Cypress Semiconductor FX2LP Technical Information Seite 72

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EZ-USB FX2 Technical Reference Manual
Page 2-18 EZ-USB FX2 Technical Reference Manual v2.1
The firmware handles
Set Descriptor
requests by clearing the HSNAK bit (
by writing 1 to it
), then
reading descriptor data directly from the
EP0B
UF buffer. The FX2 keeps track of the number of
byes transferred from the host into
EP0B
UF, and compares this number with the length field in
bytes 6 and 7. When the proper number of bytes has been transferred, the FX2 automatically
responds to the STATUS phase, which is the third and final stage of the CONTROL transfer.
The firmware controls the flow of data in the Data Stage of a Control Transfer. After the firmware
processes each OUT packet, it writes any value into the endpoint’s byte count register to re-arm
the endpoint.
Table 2-17. Set Descriptor-String
Byte Field Value Meaning Firmware Response
0 bmRequestType 0x00 IN, Device
Read string descriptor data over
1 bRequest
0x07
“Get_Descriptor”
EP0BUF.
2 wValueL 0x00 String Number
3 wValueH 0x03 Descriptor Type: String
4 wIndexL 0x00 (Language ID L)
5 wIndexH 0x00 (Language ID H)
6 wLengthL LenL
7 wLengthH LenH
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