State machine fail to hold default value

[Altera_Forum]

Hi, all 

 Here's example of my state machine coding in verilog:      --- Quote Start ---   always @(posedge clkADC or posedge RST)  begin  if (RST)  state <= s0;  else   case (state)  s0: if (START) state <= s1; else state <= s0;  s1: ns <=s2;  s2: ns <=s3;  s3: ns <= s0;  endcase  end     always@(state, DataIn)  begin  case (state)  s1:begin  fifo_wreq <= 1;  Data <= DataIn;   end   s2:begin  Data <= DataIn;  end  s3:begin  Data <= DataIn;  fifo_wreq <= 1;  end  default:begin  Data <= 12'b000000000000;   fifo_wreq <= 0;  end  endcase  end    --- Quote End ---    fifo_wreq at s2 suppose to be default value, which is 0. However, I found out that it remain 1 at state machine s2. May I know any mistake i do so far?

[Altera_Forum]

You didnt assign it to 0 in s2. In the second always block, you need to assign all values in all states. 

S1 will occur before s2, therefore fifo_wreq has already been set to 1 in the previous state. You created a latch (which is not a good thing) but letting it remember it's value between states.

[Altera_Forum]

Hi, Tricky 

 Thanks for reply. For my understanding, if I didn't assign fifo_wreq to 0 in s2, suppose fifo_wreq value is followed by the default value as stated 0. Am I right?  May I know how to prevent the latch happen?

[Altera_Forum]

--- Quote Start ---  Hi, Tricky Thanks for reply. For my understanding, if I didn't assign fifo_wreq to 0 in s2, suppose fifo_wreq value is followed by the default value as stated 0. Am I right?   --- Quote End ---    No, once you assign a value to a variable, it stays at that value until it is assigned again     --- Quote Start ---  May I know how to prevent the latch happen?  --- Quote End ---    Ensure all variables are assigned in all cases in non-clocked always blocks. They should preferably be assigned with non-blocking assignments..

[Altera_Forum]

--- Quote Start ---  No, once you assign a value to a variable, it stays at that value until it is assigned again     Ensure all variables are assigned in all cases in non-clocked always blocks. They should preferably be assigned with non-blocking assignments..  --- Quote End ---    Variables in a non-clocked always block should be assigned with blocking assignments. Clocked always blocks should use non-blocking assignments.