Setting Chunk Size and Latency

[Ben Plotnick <bnadav@xxxxxxxxxxxxxx>]

Hi all,

This email is related to the "Minimum Chunk Size and Latency" thread by Daryl Lee about a month back.  I am using an FT232R chip in an application which requires low latency but doesn't really care much about the load on the line (very quick packets of varying small sizes).  I am in linux and want to use the VCP driver with termios to configure the UART - the program is already written in such a way.

Without configuring the chip at all I see a ~16 ms delay on a read command (I assume this is related to the timeout), which is out of spec for my application.  When I go to lower the chunk size and latency using libftdi, I can set it and read it back, but then when I try to switch back to VCP mode I get a /dev/ttyUSB0 not found error.  I assume that libftdi disables the VCP driver or something?

I then cycled the power on the chip and was able to access ttyUSB0 again, but the configuration reverted to the defaults.  And I am guessing that the configuration information is not stored on the EEPROM.  So my question boils down to this (assuming that I haven't totally misread the documentation)...

1) How would I configure the chip and then proceed to use VCP?

2) After I am able to change the latency and chunk size, can I simply drop them down very low or, as Daryl saw, is there an optimal value for varying sized packets. (I suppose I can figure this out empirically)

Thank you,
-Ben

-- 
Ben Plotnick
University of Pennsylvania
bnadav@xxxxxxxxxxxxxx

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