TMS320F28377S: USB Boot Mode

Hi

I'm testing USB boot with 28377s

I saw ' TMS320F28075: USB Boot Mode ' thread...

and I do exactly same as what Stanly Admas did..

USB Boot & Flash Firmware .. everything works well..

and I apply below code to my Custom RTOS.

just run 'Usb_Boot code'  in a task 

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USB_Boot(0x0C5A);
asm(" LB 0x00");

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and I saw below message..

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Manufacturer: Texas Instruments
Product ID: TMS320F28x7x USB Boot Loader
Serial number: 2

Sending 14206 bytes of data from file f2837xD_fw_upgrade_example\F2837xD_usb_fla
sh_kernels_cpu01_origin.dat... done!
14206 out of 14206 bytes sent
Success!
Manufacturer: Texas Instruments
Product ID: TMS320F28x7x USB Boot Loader
Serial number: 2

Sending 190134 bytes of data from file Application.dat...
Error sending bulk transfer: 0x001f
0 out of 190134 bytes sent
USB operation failed!

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any suggestion??

What does Error code(0x001f)  mean ?

thanks in advance..

Moonsun