What does the following Verilog code realize

Question

I was learning verilog hardware description language. I was confused a bit with the blocking and non-blocking statements. Can someone tell me what the following verilog codes realize and simple hints how

always@(posedge clk)
begin
 q1=d;
 q2<=q1;
 q3=q2;
end
always@(posedge clk)
begin
 q1<=d;
 q2<=q1;
 q3=q2;
end

These are two different codes implemented separately. I think FF comes into the picture because of edge detection. But I don't understand what it realizes as it has a intermix of blocking and non-blocking statements

Answer 1

Let's start with d=0 q1=1 q2=2 q3=3.

always@(posedge clk)
begin
 q1=d;

q1 immediately gets the value of d=0.

 q2<=q1;

Non-blocking assignment: save the current value of q1 (which is currently the same as d=0)

 q3=q2;

q3 gets the unchanged value of q2, =2

end

At this point the nonblocking assignment happens and q2 gets the value 0.

I'm assuming the next always block is in a different module; putting it in the same module will just cause further confusion as you're overwriting the same variables again.

Start with d=0 q1=1 q2=2 q3=3 again.

always@(posedge clk)
begin
 q1<=d;
 q2<=q1;

Queue assignment of q1 to 0 and q2 to 1. These happen at the end of the block.

 q3=q2;

Set q3 to the current value of q2, ie 2.

end

Finally set q1=0 and q2=1.

Generally it's bad style to mix = and <= in the same block, and generally for synthesis you should always use <=. '=' can be helpful in testbenches where it behaves more like a conventional imperative programming language.

• sai kiran grandhi: Actually this was a question asked by Freescale Semiconductor company under recruitment process but using its equivalent VHDL. Can you help me by giving the circuit that the above two codes realize
• Stephen Collings: Can you clarify? You're applying for a job, and someone asked you this question? So you'll be turning this answer in to the interviewer?
• sai kiran grandhi: In the process of interview I was posed with this question and I was not able to crack it. Now I was trying to overcome my mistakes