跨时钟域处理-flag

转载自http://www.fpga4fun.com/CrossClockDomain2.html

A flag to another clock domain

If the signal that needs to cross the clock domains is just a pulse (i.e. it lasts just one clock cycle), we call it a "flag". The previous design usually doesn't work (the flag might be missed, or be seen for too long, depending on the ratio of the clocks used).

We still want to use a synchronizer, but one that works for flags.

The trick is to transform the flags into level changes, and then use the two flip-flops technique.

module Flag_CrossDomain(    input clkA,    input FlagIn_clkA,     input clkB,    output FlagOut_clkB);// this changes level when the FlagIn_clkA is seen in clkAreg FlagToggle_clkA;always @(posedge clkA) FlagToggle_clkA <= FlagToggle_clkA ^ FlagIn_clkA;// which can then be sync-ed to clkBreg [2:0] SyncA_clkB;always @(posedge clkB) SyncA_clkB <= {SyncA_clkB[1:0], FlagToggle_clkA};// and recreate the flag in clkBassign FlagOut_clkB = (SyncA_clkB[2] ^ SyncA_clkB[1]);endmodule

Now if you want the clkA domain to receive an acknowledgment (that clkB received the flag), just add a busy signal.

module FlagAck_CrossDomain(    input clkA,    input FlagIn_clkA,    output Busy_clkA,    input clkB,    output FlagOut_clkB);reg FlagToggle_clkA;always @(posedge clkA) FlagToggle_clkA <= FlagToggle_clkA ^ (FlagIn_clkA & ~Busy_clkA);reg [2:0] SyncA_clkB;always @(posedge clkB) SyncA_clkB <= {SyncA_clkB[1:0], FlagToggle_clkA};reg [1:0] SyncB_clkA;always @(posedge clkA) SyncB_clkA <= {SyncB_clkA[0], SyncA_clkB[2]};assign FlagOut_clkB = (SyncA_clkB[2] ^ SyncA_clkB[1]);assign Busy_clkA = FlagToggle_clkA ^ SyncB_clkA[1];endmodule