IO blocks have registers: for input, for output and for OutputEnable
signal - IREG, OREG and TREG respectively.
Each of the registers has one implicit non-switched wire, which one
depends on the type of register (IREG has a Q wire, OREG has a D wire).
Although the registers can be activated independently of each other they
share the CLK, ClockEnable and LocalSetReset wires and this places
restrictions on the possible combinations of register types in a single
IO.
Register placement in IO blocks is enabled by specifying the command
line keys --vopt ireg_in_iob, --vopt oreg_in_iob, or --vopt ioreg_in_iob.
It should be noted that specifying these keys leads to attempts to place
registers in IO blocks, but no errors are generated in case of failure.
Signed-off-by: YRabbit <rabbit@yrabbit.cyou>
b82cf51aea