芯片设计之CDC异步电路分析(五)

1CDC常见错误

1.1Reconvergence

1.1.1single_source_reconvergence

结构：同一个信号源头，两个同步处理器。这里提一下，有两个CDC分析工具的参数配置：

1.1.2案列1：divergence_depths为0

// divergence point
always @ (posedge tx_clk)
    ctrl <= ci0 | ci1 ;
 
// two_dff synchronizer
always @ (posedge rx_clk) begin: two_dff
    reg temp;
    temp <= ctrl;
    two_dff_sync <= temp;
end
 
// shift_reg synchronizer
always @ (posedge rx_clk) begin: shift_reg
    shift_reg_sync <= {shift_reg_sync[0], ctrl};
end
 
// reconvergence point
always @ (posedge rx_clk)
    dout <= two_dff_sync ^ shift_reg_sync[1];
 

电路如下：divergence_depth为0

CDC报告如下：

1.2Redundant

案例1：

// two_dff synchronizer of tx_sig
always @ (posedge rx_clk) begin: two_dff
    reg s0 , s1;
    s0 <= tx_sig; // 1st flop
    s1 <= s0; // 2nd flop
end


// two_dff synchronizer of tx_sig
always @ (posedge rx_clk) begin: shift_reg
    reg [1:0] sh_reg;
    sh_reg <= {sh_reg[0], tx_sig};
end

1.3multi_sync_mux_select (DMUX)

MUX的sel端fan-in信号超过一组同步器，不推荐。通常MUX的sel端只能有一组同步器。

案例1：

always @(posedge rx_clk) begin
    reg s1_sel1, s2_sel1;
    reg [1:0] s_sel2;
    
    s1_sel1 <= tx_sel1;
    s2_sel1 <= s1_sel1;
    
    s_sel2 <= {s_sel2[0], tx_sel2};
    
    if (s_sel2[1] | s2_sel1)
        rx_data <= tx_data;
end

电路如下：

1.4combo_logic

1.4.1错误案列1

always @ (posedge rx_clk) begin
    s1 <= tx_sig & din;
    s2 <= s1;
end

当然还有如下这种错误，除非additional logic全部是静态变量。

1.5async_reset_no_sync（异步复位、同步撤离）

1.5.1案列1

// Reset triggered by tx_clk
always @(posedge tx_clk)
tx_sig <= rst;
 
// Unsynchronized reset used in
// Rx domain
always @(posedge rx_clk,negedge tx_sig)
if (!tx_sig) rx_sig <= 1’b0;
else rx_sig <= din;

1.5.2错误案列2

// Reset triggered by tx_clk
always @(posedge tx_clk)
tx_sig <= rst;
// Improperly synchronized reset used
// in Rx domain
always @(posedge rx_clk,negedge tx_sig)
if (!tx_sig) rx_reset <= 1’b0;
else rx_reset <= 1’b1;

正确的结构如下：

1.6dff_sync_gated_clk

案列1，与门做时钟gating有毛刺，需要clock gating cell。

// gated clock expression
assign gclk = rx_clk & clk_en;
always @(posedge gclk)
    sync1 <= tx_sig; // 1st DFF
always @(posedge rx_clk)
    sync2 <= sync1; // 2nd DFF

1.7fanin_different_clks

同步器的输入由两个异步时钟域的组合逻辑构成，如下图所示：（还有combo logic）

值得注意的是，如果sig_a或者sig_b中有一个信号是stable静态变量，那么上图结构的电路就不会被报fanin_different_clks或者combo_logic错误。

假设有sig_a、sig_b、sig_c三个信号及以上的fan_in呢？抛开静态变量后，

若所有信号都是同一个时钟域，CDC错误类型就是combo_logic；

若所有信号来自至少2个时钟域，CDC错误类型就是fanin_different_clks；

举个例子，如下图：

上图电路仍会报fanin_different_clks，但是电路确实是设计者的意图，我们只需要将TEST时钟域的test_sel设置为常数0即可。

1.7.1案列1

always @ (posedge tx1_clk)
    tx1_sig <= in1;
always @ (posedge tx2_clk)
    tx2_sig <= in2;
always @ (posedge rx_clk) begin
    sync0 <= tx1_sig | tx2_sig;
    sync1 <= sync0;
end

审核编辑：刘清