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芯片设计之CDC异步电路(五)
芯片设计之CDC异步电路(四)
芯片设计之CDC异步电路(三)
芯片设计之CDC异步电路(二)
芯片设计之CDC异步电路(一)
1 CDC常见错误
===
1.1 Reconvergence
1.1.1 single_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
<pre class="code-snippet__js" data-lang="properties">```
<span class="code-snippet_outer"><span class="code-snippet__meta">//</span> <span class="code-snippet__string">divergence point</span></span>
always @ (posedge tx_clk)
<span class="code-snippet_outer"> <span class="code-snippet__attr">ctrl</span> <span class="code-snippet__string"></span></span>
<span class="code-snippet_outer"><span class="code-snippet__meta">//</span> <span class="code-snippet__string">two_dff synchronizer</span></span>
always @ (posedge rx_clk) begin: two_dff
<span class="code-snippet_outer"> <span class="code-snippet__attr">reg</span> <span class="code-snippet__string">temp;</span></span>
temp
<span class="code-snippet_outer"> <span class="code-snippet__attr">two_dff_sync</span> <span class="code-snippet__string"></span></span>
end
<span class="code-snippet_outer"> </span>
shift_reg synchronizer
<span class="code-snippet_outer"><span class="code-snippet__attr">always</span> <span class="code-snippet__string">@ (posedge rx_clk) begin: shift_reg</span></span>
shift_reg_sync
<span class="code-snippet_outer"><span class="code-snippet__attr">end</span></span>
<span class="code-snippet_outer"><span class="code-snippet__meta">//</span> <span class="code-snippet__string">reconvergence point</span></span>
always @ (posedge rx_clk)
<span class="code-snippet_outer"> <span class="code-snippet__attr">dout</span> <span class="code-snippet__string"></span></span>
电路如下:divergence_depth为0
CDC报告如下:
1.2 Redundant
案例1:
<pre class="code-snippet__js" data-lang="properties">```
<span class="code-snippet_outer"><span class="code-snippet__meta">//</span> <span class="code-snippet__string">two_dff synchronizer of tx_sig</span></span>
always @ (posedge rx_clk) begin: two_dff
<span class="code-snippet_outer"> <span class="code-snippet__attr">reg</span> <span class="code-snippet__string">s0 , s1;</span></span>
s0
<span class="code-snippet_outer"> <span class="code-snippet__attr">s1</span> <span class="code-snippet__string"></span></span>
end
<span class="code-snippet_outer"><br></br></span>
two_dff synchronizer of tx_sig
<span class="code-snippet_outer"><span class="code-snippet__attr">always</span> <span class="code-snippet__string">@ (posedge rx_clk) begin: shift_reg</span></span>
reg [1:0] sh_reg;
<span class="code-snippet_outer"> <span class="code-snippet__meta">sh_reg =<span class="code-snippet__string"> {sh_reg[0], tx_sig};</span></span></span>
end
1.3 multi_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:
<pre class="code-snippet__js" data-lang="properties">```
<span class="code-snippet_outer"><span class="code-snippet__attr">always</span> <span class="code-snippet__string">@(posedge rx_clk) begin</span></span>
reg s1_sel1, s2_sel1;
<span class="code-snippet_outer"> <span class="code-snippet__attr">reg</span> <span class="code-snippet__string">[1:0] s_sel2;</span></span>
<span class="code-snippet_outer"> <span class="code-snippet__attr">s1_sel1</span> <span class="code-snippet__string"></span></span>
s2_sel1
<span class="code-snippet_outer"> </span>
s_sel2
<span class="code-snippet_outer"> </span>
if (s_sel2[1] | s2_sel1)
<span class="code-snippet_outer"><span class="code-snippet__meta"> rx_data =<span class="code-snippet__string"> tx_data;</span></span></span>
end
电路如下:
1.4 combo_logic
1.4.1 错误案列1
<pre class="code-snippet__js" data-lang="properties">```
<span class="code-snippet_outer"><span class="code-snippet__attr">always</span> <span class="code-snippet__string">@ (posedge rx_clk) begin</span></span>
s1
<span class="code-snippet_outer"> <span class="code-snippet__attr">s2</span> <span class="code-snippet__string"></span></span>
end
当然还有如下这种错误,除非additional logic全部是静态变量。
1.5 async_reset_no_sync(异步复位、同步撤离)
1.5.1 案列1
<pre class="code-snippet__js" data-lang="javascript">```
<span class="code-snippet_outer"><span class="code-snippet__comment">// Reset triggered by tx_clk</span></span>
always @(posedge tx_clk)
<span class="code-snippet_outer">tx_sig </span>
<span class="code-snippet_outer"><span class="code-snippet__comment">// Unsynchronized reset used in</span></span>
// Rx domain
<span class="code-snippet_outer">always @(posedge rx_clk,negedge tx_sig)</span>
if (!tx_sig) rx_sig 1’b0;
<span class="code-snippet_outer"><span class="code-snippet__keyword">else</span> rx_sig </span>
![](https://ebaina.oss-cn-hangzhou.aliyuncs.com/wechat-official-crawl/2022-02/164531881517136.jpg)
![](https://ebaina.oss-cn-hangzhou.aliyuncs.com/wechat-official-crawl/2022-02/164531881529256.jpg)
### 1.5.2 错误案列2
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-
-
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``` // Reset triggered by tx_clk `````` always @(posedge tx_clk) `````` tx_sig `````` // 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; ``` ``` ![](https://ebaina.oss-cn-hangzhou.aliyuncs.com/wechat-official-crawl/2022-02/164531881510418.jpg) ![](https://ebaina.oss-cn-hangzhou.aliyuncs.com/wechat-official-crawl/2022-02/164531881538901.jpg) 正确的结构如下: ![](https://ebaina.oss-cn-hangzhou.aliyuncs.com/wechat-official-crawl/2022-02/164531881680946.jpg) 1.6 dff\_sync\_gated\_clk ------------------------- ![](https://ebaina.oss-cn-hangzhou.aliyuncs.com/wechat-official-crawl/2022-02/164531881613301.jpg) 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 `````` always @(posedge rx_clk) `````` sync2 ``` ``` ![](https://ebaina.oss-cn-hangzhou.aliyuncs.com/wechat-official-crawl/2022-02/16453188169720.jpg) ![](https://ebaina.oss-cn-hangzhou.aliyuncs.com/wechat-official-crawl/2022-02/164531881771757.jpg) 1.7 fanin\_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) ![](https://ebaina.oss-cn-hangzhou.aliyuncs.com/wechat-official-crawl/2022-02/164531881729263.jpg) 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: ![](https://ebaina.oss-cn-hangzhou.aliyuncs.com/wechat-official-crawl/2022-02/164531881726308.jpg) 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 `````` always @ (posedge tx2_clk) `````` tx2_sig `````` always @ (posedge rx_clk) begin `````` sync0 `````` sync1 `````` end ``` ``` ![](https://ebaina.oss-cn-hangzhou.aliyuncs.com/wechat-official-crawl/2022-02/164531881829914.jpg) ![](https://ebaina.oss-cn-hangzhou.aliyuncs.com/wechat-official-crawl/2022-02/164531881862108.jpg) 时间不早了,暂写到这里,后续接着完善。 感谢阅读文章,如果文章有用,麻烦点个“在看”或转发分享。 ![](https://ebaina.oss-cn-hangzhou.aliyuncs.com/wechat-official-crawl/2022-02/164531881837708.jpg) 转载:全栈芯片工程师
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