APB4 Slave MUX的设计

　　在设计一个复用器的时候，可以使用时序逻辑去做，即通过if或者case语句去比较选择信号的值；也可以通过组合逻辑去做，即真值表的形式。一般来讲，使用组合逻辑去做APB的Slave MUX比较好，因为APB的速度也不是很快，同时也可以少一个clock cycle。

代码：时序逻辑下的MUX设计方法

always @(*) begin
        if(DECODE4BIT == 0)
            PRDATA = PRDATA0
        else if(DECODE4BIT == 1)
            PRDATA = PRDATA1
        else if(DECODE4BIT == 2)
            PRDATA = PRDATA2
        // 后面略掉    
 end

always @(*) begin
        case(DECODE4BIT)
           0: PRDATA = PRDATA0
           1: PRDATA = PRDATA1
           2: PRDATA = PRDATA2
        // 后面略掉    
    end

代码：组合逻辑下PREADY输出信号

  assign PREADY  = ~PSEL |
                   ( dec[ 0]  & (PREADY0  | ~en[ 0]) ) |
                   ( dec[ 1]  & (PREADY1  | ~en[ 1]) ) |
                   ( dec[ 2]  & (PREADY2  | ~en[ 2]) ) |
                   ( dec[ 3]  & (PREADY3  | ~en[ 3]) ) |
                   ( dec[ 4]  & (PREADY4  | ~en[ 4]) ) |
                   ( dec[ 5]  & (PREADY5  | ~en[ 5]) ) |
                   ( dec[ 6]  & (PREADY6  | ~en[ 6]) ) |
                   ( dec[ 7]  & (PREADY7  | ~en[ 7]) ) |
                   ( dec[ 8]  & (PREADY8  | ~en[ 8]) ) |
                   ( dec[ 9]  & (PREADY9  | ~en[ 9]) ) |
                   ( dec[10]  & (PREADY10 | ~en[10]) ) |
                   ( dec[11]  & (PREADY11 | ~en[11]) ) |
                   ( dec[12]  & (PREADY12 | ~en[12]) ) |
                   ( dec[13]  & (PREADY13 | ~en[13]) ) |
                   ( dec[14]  & (PREADY14 | ~en[14]) ) |
                   ( dec[15]  & (PREADY15 | ~en[15]) );

　　以下是组合逻辑的参考设计。16个slaves，每个slave都会输出pready，prdata，和perror信号。此复用器需要根据选择信号，选择相应slave的pready，prdata，perror信号，送给AHB2APB bridge。

代码：组合逻辑下16个Slave的译码和使能信号的产生

// 根据参数parameter PORTx_ENABLE为每个端口产生一个使能信号 
wire [15:0] en  = { (PORT15_ENABLE == 1), (PORT14_ENABLE == 1),
                      (PORT13_ENABLE == 1), (PORT12_ENABLE == 1),
                      (PORT11_ENABLE == 1), (PORT10_ENABLE == 1),
                      (PORT9_ENABLE  == 1), (PORT8_ENABLE  == 1),
                      (PORT7_ENABLE  == 1), (PORT6_ENABLE  == 1),
                      (PORT5_ENABLE  == 1), (PORT4_ENABLE  == 1),
                      (PORT3_ENABLE  == 1), (PORT2_ENABLE  == 1),
                      (PORT1_ENABLE  == 1), (PORT0_ENABLE  == 1) };
// 在根据DECODE4BIR为每一个端口产生译码信号，只有当DECODE4BIT等于对应端口值的时候，相应的译码信号才为高；
// DECODE4BIT[3:0] 为MUX模块的输入信号，如果有16个Slave，那么DECODE4BIT输入可以定义为[3:0]的位宽
wire [15:0] dec = { (DECODE4BIT == 4'd15), (DECODE4BIT == 4'd14),
                      (DECODE4BIT == 4'd13), (DECODE4BIT == 4'd12),
                      (DECODE4BIT == 4'd11), (DECODE4BIT == 4'd10),
                      (DECODE4BIT == 4'd9 ), (DECODE4BIT == 4'd8 ),
                      (DECODE4BIT == 4'd7 ), (DECODE4BIT == 4'd6 ),
                      (DECODE4BIT == 4'd5 ), (DECODE4BIT == 4'd4 ),
                      (DECODE4BIT == 4'd3 ), (DECODE4BIT == 4'd2 ),
                      (DECODE4BIT == 4'd1 ), (DECODE4BIT == 4'd0 ) };

  assign PSEL0   = PSEL & dec[ 0] & en[ 0];
  assign PSEL1   = PSEL & dec[ 1] & en[ 1];
  assign PSEL2   = PSEL & dec[ 2] & en[ 2];
  assign PSEL3   = PSEL & dec[ 3] & en[ 3];
  assign PSEL4   = PSEL & dec[ 4] & en[ 4];
  assign PSEL5   = PSEL & dec[ 5] & en[ 5];
  assign PSEL6   = PSEL & dec[ 6] & en[ 6];
  assign PSEL7   = PSEL & dec[ 7] & en[ 7];
  assign PSEL8   = PSEL & dec[ 8] & en[ 8];
  assign PSEL9   = PSEL & dec[ 9] & en[ 9];
  assign PSEL10  = PSEL & dec[10] & en[10];
  assign PSEL11  = PSEL & dec[11] & en[11];
  assign PSEL12  = PSEL & dec[12] & en[12];
  assign PSEL13  = PSEL & dec[13] & en[13];
  assign PSEL14  = PSEL & dec[14] & en[14];
  assign PSEL15  = PSEL & dec[15] & en[15];

代码：组合逻辑下PSLVERR输出信号

  assign PSLVERR = ( PSEL0  & PSLVERR0  ) |
                   ( PSEL1  & PSLVERR1  ) |
                   ( PSEL2  & PSLVERR2  ) |
                   ( PSEL3  & PSLVERR3  ) |
                   ( PSEL4  & PSLVERR4  ) |
                   ( PSEL5  & PSLVERR5  ) |
                   ( PSEL6  & PSLVERR6  ) |
                   ( PSEL7  & PSLVERR7  ) |
                   ( PSEL8  & PSLVERR8  ) |
                   ( PSEL9  & PSLVERR9  ) |
                   ( PSEL10 & PSLVERR10 ) |
                   ( PSEL11 & PSLVERR11 ) |
                   ( PSEL12 & PSLVERR12 ) |
                   ( PSEL13 & PSLVERR13 ) |
                   ( PSEL14 & PSLVERR14 ) |
                   ( PSEL15 & PSLVERR15 );

　　根据下述三个原则，设计PREADY信号。PREADY的default值应该为1，即使PSEL为0的时候；当PSEL为1的时候，根据译码信号dec选择相应的PREADYx信号；当端口没有使能的时候en[x] == 0， 对应的PREADYx信号不会被选择

代码：组合逻辑下PRDATA输出信号

  assign PRDATA  = ( {32{PSEL0 }} & PRDATA0  ) |
                   ( {32{PSEL1 }} & PRDATA1  ) |
                   ( {32{PSEL2 }} & PRDATA2  ) |
                   ( {32{PSEL3 }} & PRDATA3  ) |
                   ( {32{PSEL4 }} & PRDATA4  ) |
                   ( {32{PSEL5 }} & PRDATA5  ) |
                   ( {32{PSEL6 }} & PRDATA6  ) |
                   ( {32{PSEL7 }} & PRDATA7  ) |
                   ( {32{PSEL8 }} & PRDATA8  ) |
                   ( {32{PSEL9 }} & PRDATA9  ) |
                   ( {32{PSEL10}} & PRDATA10 ) |
                   ( {32{PSEL11}} & PRDATA11 ) |
                   ( {32{PSEL12}} & PRDATA12 ) |
                   ( {32{PSEL13}} & PRDATA13 ) |
                   ( {32{PSEL14}} & PRDATA14 ) |
                   ( {32{PSEL15}} & PRDATA15 );