Re: [Discuss-gnuradio] fusb_nblock and fusb_block_size

[Eric Blossom]

On Wed, Nov 29, 2006 at 01:46:14PM -0800, Thomas Schmid wrote:
> Hi all,
> 
> What do the parameters fusb_nblock and fusb_block_size exactly do in
> how data is transmitted over the USB to the USRP? I assume that
> fusb_nblock indicates how many blocks we have in each USB packet, and
> fusb_block_size indicates how big one block is.

fusb_block_size is the size in bytes of the maximum transfer that we
will ask the kernel to make to/from user-space.  fusb_nblocks is the
maximum number of transfers (of maximum size fusb_block_size) that we
can have in flight at any given time.

Take a look at fusb_linux.{h,cc} for the details.

> My question is, if there is less data available on the USRP (i.e. not
> fusb_nblock*fusb_block_size), does it still get sent over the USRP to
> the computer?

Yes.  The USRP packages data into 512 byte USB packets and sends them
as soon as it can.  That's 128 complex samples (16-bit I & Q).

> How is the smallest amount of data necessary in the USRP defined such
> that a packet is sent over the USB to the computer? Is it
> fusb_block_size?

It's 512 bytes.  It's set in the FX2 firmware.

Eric