Crossing clock domains - Flag

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(
    clkA, FlagIn_clkA, 
    clkB, FlagOut_clkB);

// clkA domain signals
input clkA, FlagIn_clkA;

// clkB domain signals
input clkB;
output FlagOut_clkB;

reg FlagToggle_clkA;
reg [2:0] SyncA_clkB;

// this changes level when a flag is seen
always @(posedge clkA) if(FlagIn_clkA) FlagToggle_clkA <= ~FlagToggle_clkA;

// which can then be sync-ed to clkB
always @(posedge clkB) SyncA_clkB <= {SyncA_clkB[1:0], FlagToggle_clkA};

// and recreate the flag from the level change
assign 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(
    clkA, FlagIn_clkA, Busy_clkA, 
    clkB, FlagOut_clkB);

// clkA domain signals
input clkA, FlagIn_clkA;
output Busy_clkA;

// clkB domain signals
input clkB;
output FlagOut_clkB;

reg FlagToggle_clkA;
reg [2:0] SyncA_clkB;
reg [1:0] SyncB_clkA;

always @(posedge clkA) if(FlagIn_clkA & ~Busy_clkA) FlagToggle_clkA <= ~FlagToggle_clkA;
always @(posedge clkB) SyncA_clkB <= {SyncA_clkB[1:0], FlagToggle_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

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This page was last updated on July 04 2010.