Subject: MPSSE Send Immediate not working?
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.
libftdi - see http://www.intra2net.com/en/developer/libftdi for details.
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