信号跨越两个时钟域外文资料

信号跨越两个时钟域外文资料
A signal to another clock domainLet's say a signal from clkA domain is needed in clkB domain. It needs to be "synchronized" to clkB domain, so we want to build a "synchronizer" design, which takes a signal from clkA domain, and creates a new signal into clkB domain.

In this first design, we assume that the signal-in changes "slowly" compared to both clkA and clkB clock speeds.
Typically all you need to do is to use two flip-flops to move the signal from clkA to clkB (to learn why, get back to the links).
module Signal_CrossDomain(    clkA, SignalIn,     clkB, SignalOut);// clkA domain signalsinput clkA;input SignalIn;// clkB domain signalsinput clkB;output SignalOut;// Now let's transfer SignalIn into the clkB clock domain// We use a two-stages shift-register to synchronize the signalreg [1:0] SyncA_clkB;always @(posedge clkB) SyncA_clkB[0] <= SignalIn;      // notice that we use clkBalways @(posedge clkB) SyncA_clkB[1] <= SyncA_clkB[0]; // notice that we use clkBassign SignalOut = SyncA_clkB[1];  // new signal synchronized to (=ready to be used in) clkB domainendmoduleThe two flip-flops have the side-effect of delaying the signal.
For example, here are waveforms where you can see the slow moving signal being synchronized (and delayed) to clkB domain by the two flip-flops: