(consumed vars: PDU_TYPE and Length bytes received, 
produced vars: VAR3_ACC enabled only if ctrl, PDU_TYPE, Length and CRC bytes are correct;
sampling of dat_i takes place on the 1st clk tick after VAR3_ACC deasserted)

git-svn-id: http://svn.ohwr.org/cern-fip/trunk/hdl/design@73 7f0067c9-7624-46c7-bd39-3fb5400c0213