【常用電路】線性回報移位寄存器（LFSR）

讀華為技術文檔《FIFO經驗談》看到的這個電路：

　　FIFO的讀寫位址産生比較簡單，當讀使能有效時，在時鐘作用下，讀位址加1；當寫使能有效時，寫位址加1。

1 /************************************************************\
 2 * *
 3 * Generation of Read and Write address pointers. They use *
 4 * LFSR counters, which are very fast. Because of the *
 5 * nature of LFSR, one address is sacrificed. *
 6 * *
 7 \************************************************************/
 8 wire read_linearfeedback, write_linearfeedback;
 9 
10 assign read_linearfeedback = ! (read_addr[8] ^ read_addr[4]);
11 assign write_linearfeedback = ! (write_addr[8] ^ write_addr[4]);
12 
13 always @(posedge clock or posedge fifo_gsr)
14     if (fifo_gsr)     read_addr <= 9'h0;
15     else if (read_allow)
16         read_addr <= { read_addr[7], read_addr[6], read_addr[5],
17                             read_addr[4], read_addr[3], read_addr[2],
18                             read_addr[1], read_addr[0], read_linearfeedback };
19 
20 always @(posedge clock or posedge fifo_gsr)
21     if (fifo_gsr)     write_addr <= 9'h0
22     else if (write_allow)
23         write_addr <= { write_addr[7], write_addr[6], write_addr[5],
24                             write_addr[4], write_addr[3], write_addr[2],
25                             write_addr[1], write_addr[0], write_linearfeedback };