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Hi, is it possible that the usb_cts and usb_rts nets in the pipistrello.ucf constraint file are mixed up? In my opinion, the following should be correct: #NET "usb_cts" LOC = "A9" | IOSTANDARD = LVTTL | DRIVE = 8 | SLEW = FAST ; // SWITCHED WITH RTS -mn #NET "usb_rts" LOC = "C10" | IOSTANDARD = LVTTL | PULLUP; // SWITCHED WITH CTS -mn Schematic: RTS# = pin 40 on U7 = D2 = C10 on FPGA CTS# = pin 41 on U7 = D3 = A9 on FPGA In my understanding (please correct if wrong) CTS serves double duty on the FTDI chip: - Setting 1'b1 (which mean "not clear to send") blocks incoming traffic from USB. - Any level change kicks off the current 62+2 byte package over USB without waiting for the latency timer. I did an experiment with existing code where triggering a pulse on CTS decreased the roundtrip time (for two "rounds") from 4.00 to 3.75 ms, a small but measurable improvement.