libftdi Archives

Subject: MPSSE Send Immediate not working?

From: "Fewell, Edward" <efewell@xxxxxx>
To: "libftdi@xxxxxxxxxxxxxxxxxxxxxxx" <libftdi@xxxxxxxxxxxxxxxxxxxxxxx>
Date: Thu, 19 Aug 2010 10:05:03 -0500

I have an application working with the FT2232H to do JTAG emulation.  We append an MPSSE Send Immediate command to every outgoing transaction so that data scanned in is returned immediately, and we leave the latency timer set to 255 to avoid splitting return data on large scans.  This works well on Windows using FTDI’s driver, but doesn’t seem to be working at all on Linux with libftdi.  Running with latency set to 255 using libftdi results in horribly slow performance despite the Send Immedate commands.  If I set the latency to 1 ms, then I get throughput roughly half the performance I get when running on Windows.  If I set the latency to 0 ms, then I get close (but not quite) the same throughput as on Windows.  Because I use a Send Immediate for every transaction, I didn’t expect to see any performance degradation when setting the latency timer to a high value.  It looks like the Send Immediate command is working, but I don’t get why that would be when the other MPSSE commands are behaving fine.


Edward Fewell


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