The little board to be built in this project is an adapter for connecting an
OMAP BeagleBoard-xM to a FreeCalypso device, specifically connecting Calypso's
MCSI to OMAP's McBSP (primary objective) and also connecting a 4-wire UART
(data + hw flow control) between the two devices - BB expansion interface
provides such a UART, and it costs us very little extra effort to connect it
along with MCSI.

The present adapter will plug into the expansion header on the bottom side of
the BeagleBoard, implement logic voltage level translation between OMAP's 1.8 V
and Calypso's 2.8 V using the 1.8 V and 5 V power supplies provided by the
BeagleBoard, and bring 2.8 V UART and MCSI interfaces out on headers, intended
to be connected to a FreeCalypso board with jumper wires.

The logic voltage level translation function will be accomplished using two
74AVC4T774PW ICs, one for the UART and one for MCSI.  As a somewhat arbitrary
pick made for greater ease of PCB layout, the A side of each translating
transceiver IC will be the 2.8 V side and the B side will be the 1.8 V side.
Each of the 4 1-bit channels in the 74AVC4T774PW has its own direction control
pin, allowing us to neatly use one of these ICs for each 4-wire interface
despite mixed signal directions.  MCSI_CLK and MCSI_FSYNCH are assumed to be
outputs from the Calypso like they are when driven by the DSP ROM code in the
silicon, and they will be voltage-translated in the Calypso to OMAP direction;
the other direction is NOT supported.