Hi, I am into designing of USB device controller. The problem that I am facing is that of the CRC in zero length data packet. There are three querries as such: 1. Do we, as a USB device, send 16 bit CRC when we send a zero length data packet in the status stage of the control write transfer performed by host? 2. If this CRC is to be sent, then what would be those 16 bits? Because, there is no data as such that goes into my CRC calculator. 3. What would be the CRC when host sends a zero length data packet? Please help me out. Regards. Virendra --------------------------------- Do you Yahoo!? New and Improved Yahoo! Mail - 100MB free storage! -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.opencores.org/forums/usb/attachments/20040830/8531a3d8/attachment-0001.htm

Hi Virendra,

Any data packet, irrespective of whether it is sent by host or device, and
again irrespective, whether it is zero/non-zero length packet should bear
the CRC (16-bit) field.


Cuming now to ur question of what would be the crc for the zero length
packet,
as u say, there would be no data on which the crc is to be calculated.
Hence, (i hope u know), the crc registers are in their default values(preset
to 1). And since, crc is normally negated and sent, it will be all zeroes.
This is the raw crc data. however, this data needs to be bitstuffed and nrzi
encoded.

As such, the host and the device have the same crc values when the crc is
calculated on the same packet. This holds gud for the zero length packet
also.

Hope this helps,

Regards,
Gayathri Seshadri.

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Hey Gayathri, Thanx a lot. U've cleared a lot of my doubts rather all because u've answered all the three questions. I m more happy because even I was thinking of the same very thing, that of sending all zeros as 16 bit CRC for zero length data packet and that itself is the fact! As for my case, I need not worry about the bit stuffing and NRZI encoding. There is a standard UTM Interface by Intel that sits between my design and the host which does this bit stuffing and NRZI encoding. Well then thanx again. It is really nice to interact with people like u all. I am learning a lot! Regards. Virendra. gayathri seshadri gaysesh@hotmail.com> wrote: Hi Virendra, Any data packet, irrespective of whether it is sent by host or device, and again irrespective, whether it is zero/non-zero length packet should bear the CRC (16-bit) field. Cuming now to ur question of what would be the crc for the zero length packet, as u say, there would be no data on which the crc is to be calculated. Hence, (i hope u know), the crc registers are in their default values(preset to 1). And since, crc is normally negated and sent, it will be all zeroes. This is the raw crc data. however, this data needs to be bitstuffed and nrzi encoded. As such, the host and the device have the same crc values when the crc is calculated on the same packet. This holds gud for the zero length packet also. Hope this helps, Regards, Gayathri Seshadri. --------------------------------- Do you Yahoo!? Win 1 of 4,000 free domain names from Yahoo! Enter now. -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.opencores.org/forums/usb/attachments/20040831/df0783e9/attachment.htm