i'm beginner in verilog and digital circuits, and i have one doubt in the code below. In this code i made a state machine that saves values in a "reg" into another module in verilog. I made this code just to explain my doubt:
//STATE MACHINE
module RegTest(clk,enable,reset, readData1_out);
parameter State1 = 0;
parameter State2 = 1;
parameter State3 = 2;
parameter State4 = 3;
parameter State5 = 4;
parameter State6 = 5;
parameter State7 = 6;
parameter State8 = 7;
parameter State9 = 8;
parameter State10 = 9;
parameter Beg = 10;
input clk, enable, reset;
output readData1_out;
wire clk,enable, reset;
reg[5:0] State;
reg writeEn ;
reg [15:0] writeData;
wire [15:0] readData1;
wire writeEn_out = writeEn;
RegFile registrador_component (
.dataIn(writeData),
.dataOut(readData1),
.clock(clk),
.writeEnable(writeEn)
);
defparam
registrador_component.WIDTH = 16;
always @(posedge clk or posedge reset) begin
if (reset)
begin
State = Beg;
end else begin
case (State)
Beg:
begin
State = State1;
end
State1:
begin
writeEn = 1 ;
writeData = 10;
State = State2;
end
State2:
begin
writeEn = 0 ;
State = State3;
end
State3:
begin
writeEn = 1;
writeData = readData1 + 10;
State = State4;
end
State4:
begin
writeEn = 0 ;
State = State5;
end
State5:
begin
writeEn = 1 ;
writeData = readData1 + 10;
State = State6;
end
State6:
begin
writeEn = 0 ;
State = State7;
end
State7:
begin
writeEn = 1 ;
writeData = readData1 + 10;
State = State8;
end
State8:
begin
writeEn = 0 ;
State = State9;
end
endcase
end
end
endmodule
//Example of a register file
module RegFile(clock, writeEnable, dataIn, dataOut);
parameter WIDTH = 16;
input clock, writeEnable;
input [WIDTH-1 : 0] dataIn;
output [WIDTH-1 : 0] dataOut;
wire [WIDTH-1 : 0] dataOut;
reg [WIDTH-1 : 0] wha;
assign dataOut = wha;
always@( posedge clock)
begin
if (writeEnable)
wha = dataIn;
end
endmodule
My doubt is, why do I need to wait 1 cycle to get the value that is stored in RegFile? Why can't I skip the State2 for example?
You do in fact have 2 clock cycles of delay in the code you wrote above. It would help if you simulated this so you can see it for yourself, but I'll describe it.
On the first cycle, WriteEnable goes high. It takes 1 full clock cycle to be valid to other parts of the logic. So after the first clock cycle is done, WriteEnable will be available for use elsewhere.
On the second cycle, your regFile module can "see" the fact that WriteEnable went high previously. It then will put dataIn into wha signal, which gets passed to dataOut. So after the second clock cycle is done, dataOut will be available for use elsewhere. (It gets used on the third clock cycle in State 3).
This is why you need to go to state 2. State 2 allows for the 1 extra clock cycle needed for RegFile to register the output data.
Clock cycle delay is an extremely important concept to understand, so it's good that you're putting in the time when you are still new to fully grasp it.
For more information see this tutorial here, it explains registers and clock cycle delay and will be useful for you. Clock Cycle Delays and Registered Logic