FreeCalypso DUART28 is our official adapter for connecting from a USB host (such as an ordinary PC or laptop) to the dual UART interface on our Calypso development boards. This DUART28 adapter works with both FCDEV3B and Caramel2 development boards.
FC DUART28 is based around the very mature FT2232D USB to dual UART adapter chip from FTDI, but it differs from a generic FT2232x breakout board in that it is specifically designed for FreeCalypso:
DUART28 puts out 2.8 V logic levels instead of 3.3 V on all of its UART outputs, to avoid overstressing Calypso UART and GPIO inputs with higher voltages. These uncommon logic voltage levels are produced by passing all UART outputs through on-board LVC buffers, powered from an on-board 2.8 V LDO regulator.
2.2 kOhm series resistors are inserted after each UART output driver (LVC buffer), limiting the current that will flow from these UART outputs into the connected Calypso device when the latter has no power supply or is in its normal VRPC switched-off state.
Another set of LVC buffers is inserted before FT2232D UART inputs to prevent large currents in the other partial power-down scenario, when the Calypso device is up and running, the DUART28 adapter is connected, but there is no USB host connected and thus no USB power.
DUART28 has two dedicated open drain buffers for driving PWON and RESET on Calypso boards, driven by otherwise unused FT2232D Channel B RTS and DTR outputs.
Because of its specialized nature, DUART28 is not intended to be a product in its own right, instead it is an accessory to be supplied with our FCDEV3B and Caramel2 development kits.