Another question:
Could someone please explain the following:
This does not simulate, just as expected - if clocks (clock2x and mcu_wr_n) are asynchronows to each other, sooned or later the value of mcu_wr_n happens to be unknown (edge violates setup and hold times) at the edge of clock2x. In real life this will not be a problem, because if edge is not detected on one clock edge of clock2x, it will be detected on next edge anyway. So it is usable for simulation if synchronows clocks are used to simulate:
reg [1:0] mcu_wrreq_s;
wire mcu_wren = mcu_wrreq_s == 2'b01; //synchonous mcu write request
always @ (posedge mcu_wr_n) //register mcu write data and address
begin
if (~mcu_cs_n)
begin
mcu_addrwr <= mcu_addr;
mcu_datawr <= mcu_data;
mcu_b0wr <= mcu_b0_n;
mcu_b1wr <= mcu_b1_n;
end
end
always @ (posedge clock2x) //register mcu write request
begin
mcu_wrreq_s[1] <= mcu_wrreq_s[0];
mcu_wrreq_s[0] <= mcu_wr_n;
end
But this code simulates properly, no matter if mcu_wr_n clock is synchronous or asynchronows to clock2x:
reg [1:0] mcu_wrreq_s;
wire mcu_wren = mcu_wrreq_s == 2'b01; //synchronous mcu write request
always @ (posedge mcu_wr_n) //register mcu write data and address
begin
if (~mcu_cs_n)
begin
mcu_addrwr <= mcu_addr;
mcu_datawr <= mcu_data;
mcu_b0wr <= mcu_b0_n;
mcu_b1wr <= mcu_b1_n;
end
end
always @ (posedge clock2x) //mcu write request is registered here
begin
mcu_wrreq_s[1] <= mcu_wrreq_s[0];
if (mcu_wrreq_s[0] != mcu_wr_n) mcu_wrreq_s[0] <= ~mcu_wrreq_s[0]
end
This basically does the same thing, but instead of registering mcu_wr_n signal, it inverts mcu_wrreq_s[0] if it is different to mcu_wr_n. However, mcu_wr_n can be undefined at that exact moment, but still it simulates without giving "X" state to mcu_wrreq_s[0].
My only explanation is that when mcu_wr_n violates the setup/hold times from clock2x's point of view, the simulator treats mcu_wr_n as "X" state, but comparing it to mcu_wrreq_s[0] register, it threats it as a value, different than 0 or 1, so it inverts the mcu_wrreq_s anyway, despite the violation of setup/hold requirements.
Am I thinking right? Also, I think in real life both will work equally well. Am I right?
