[Discuss-gnuradio] The effect of FUSB_BLOCK_SIZE and FUSB_NBLOCKS

[Yong J. Chang]

Hi all,

Now I'm working on measuring Round Trip Time (
FPGA(RXFIFO)->USB->PC->USB->FPGA(TXFIFO) ). 
Since I'm using 128kbps bitrate and 10 samples per symbol, sample rate is
1.28Msps. 
And I do not give any options about FUSB_BLOCK_SIZE and FUSB_NBLOCKS. So
they might be 0.
In this case, the result is following:

Minimum RTT : 1.96ms
Maximum RTT : 5.12ms

As Thomas Schmid's paper, this result is so reasonable. However, if I use
FUSB_BLOCK_SIZE=2048 and FUSB_NBLOCKS=8 such like paper mentioned, it starts
to be messed up.

RTT becomes larger and variation is also broaden. Also it gives me lots of
overrun message. 

What is the effect of these two parameters? As I know, in USB bulk transfer
mode the USB packet size is fixed as 512 bytes. So these two parameters will
determine the packet size transfered between Linux kernel and user space. It
does not affect physical USB transaction. Am I correct? If yes, why does it
make overrun error? 

Please give some clues. Thanks in advance!!!


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