[Open-graphics] USB interface on the logic/bus-analyzer

[Andreas R]

Hi all,

I have some experience with the FTDI USB controllers and I thought I'd share
what I know.

FTDI's USB-to-RS232 controller is quite simple to use: It looks and works
like a regular serial port from both sides. No surprises there. (Ok, one big
surprise, there's a Windows driver bug to watch out for., but it's another
discussion.)

The major difference however, is the long latency: 1 to 10ms, or more,
depending on system load. Flow control is necessary, and the data must be
packetized if you want any kind of performance.

I have not used it myself, but parallel transfer is potentially even simpler
than serial, as it uses a basic FIFO interface. It looks like this:

D0-D7 (inout): Data bus.
TXE# (output): When low, data can be written to the FIFO.
RXF# (output): When low, data can be read from the FIFO.
WR# (input): Writes data to the FIFO on negedge.
RD# (input): New data byte can be read on D0-D7 after negedge.
Data output enabled while low.

Pin descriptions and timing diagrams are on page 10 and 11, here:
http://www.ftdichip.com/Documents/DataSheets/DS_FT245R_v105.pdf

Finally, someone asked if it is possible to target a specific USB device.
The answer is yes: You assign a serial number that is stored in the USB IC.
The driver enumerates all attached FTDI USB devices so the application can
pick the right one from the list.

I hope that clears some things up.

Cheers,
Andreas
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