fpga4fun.comwhere FPGAs are fun

Crossing clock domains 1 - Signal

A signal to another clock domain

Let'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.
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(
    input clkA,   // we actually don't need clkA in that example, but it is here for completeness as we'll need it in further examples
    input SignalIn_clkA,
    input clkB,
    output SignalOut_clkB

// We use a two-stages shift-register to synchronize SignalIn_clkA to the clkB clock domain
reg [1:0] SyncA_clkB;
always @(posedge clkB) SyncA_clkB[0] <= SignalIn_clkA;   // notice that we use clkB
always @(posedge clkB) SyncA_clkB[1] <= SyncA_clkB[0];   // notice that we use clkB

assign SignalOut_clkB = SyncA_clkB[1];  // new signal synchronized to (=ready to be used in) clkB domain

The 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: