UART参考设计，功能兼容NS16450, Lattice提

UART参考设计，功能兼容NS16450, Lattice提供VHDL代码

-- Copyright (c) 2001 by Lattice Semiconductor Corporation
-- --------------------------------------------------------------------
--
-- Permission:
--
--   Lattice Semiconductor grants permission to use this code for use
--   in synthesis for any Lattice programmable logic product.  Other
--   use of this code, including the selling or duplication of any
--   portion is strictly prohibited.
--
-- Disclaimer:
--
--   This VHDL or Verilog source code is intended as a design reference
--   which illustrates how these types of functions can be implemented.
--   It is the user's responsibility to verify their design for
--   consistency and functionality through the use of formal
--   verification methods.  Lattice Semiconductor provides no warranty
--   regarding the use or functionality of this code.
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