fifo pointers in verilog gray code utilization for synchronius

verilog code for gray counter,synchronous and asynchronous fifo

异步FIFO，利用格雷码作异步FIFO指针减少亚稳态产生，利用同步寄存器放置亚稳态的级联传播。(Asynchronous FIFO, using gray code for asynchronous FIFO pointer to reduce metastable, cascade propagation using synchronous register placed metastable.)