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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, which can more easily cross clock domains.

module Flag_CrossDomain(
    input clkA,
    input FlagIn_clkA,   // this is a one-clock pulse from the clkA domain
    input clkB,
    output FlagOut_clkB   // from which we generate a one-clock pulse in clkB domain
);

reg FlagToggle_clkA;
always @(posedge clkA) FlagToggle_clkA <= FlagToggle_clkA ^ FlagIn_clkA;  // when flag is asserted, this signal toggles (clkA domain)

reg [2:0] SyncA_clkB;
always @(posedge clkB) SyncA_clkB <= {SyncA_clkB[1:0], FlagToggle_clkA};  // now we cross the clock domains

assign FlagOut_clkB = (SyncA_clkB[2] ^ SyncA_clkB[1]);  // and create the clkB flag
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