异步通讯接口URAT2(VHDL代码)

UART参考设计，功能兼容NS16450, Lattice提供

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-- Copyright (c) 2001 by Lattice Semiconductor Corporation
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-- Permission:
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-- Disclaimer:
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--   This VHDL or Verilog source code is intended as a design reference
--   which illustrates how these types of functions can be implemented.
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