What does the small circles in figure 5.9 mean?

[manu]

As per figure 5.9 the ALU output is fed back to Dregister  as it's in. But, when I code this in CPU.hdl, the simulator gives a "circle in pin connections" error.

What am I missing here? I noticed the small circles in the diagram. for eg, there is one between the ALU out and Dregister in. What do they mean?

Manu.

[manu]

I found some explanation in the forum that if we give an uncloacked feedback we will get this "This chip has a circle in its parts connections" error.

But, if we take the example of giving the ALU output as feed back to Dregister input, to avoid this error, if I cloack the feedback, then there will be two clocks in the loop which I think will cause problems with sychronizing.

Th eDregister is a clocked device. so If I clock the feedback to the D register, any feedback value will have to wait for two clock signals to reach the ALU input again.

What am I missing?

[cadet1620]

manu wrote

I noticed the small circles in the diagram. for eg, there is one between the ALU out and Dregister in. What do they mean?

The small gray circles simple indicate that the wires are connected.

I found some explanation in the forum that if we give an uncloacked feedback we will get this "This chip has a circle in its parts connections" error.

But, if we take the example of giving the ALU output as feed back to Dregister input, to avoid this error, if I cloack the feedback, then there will be two clocks in the loop which I think will cause problems with sychronizing.

Th eDregister is a clocked device. so If I clock the feedback to the D register, any feedback value will have to wait for two clock signals to reach the ALU input again.

What am I missing?

You shouldn't need to add anything specific to the feedback loops in the CPU. The only two loops are through the D- and A-registers which, as you say, are already clocked.  If you have your own HDL chips other than CPU in your projects/05 directory, try deleting them so that you will guarantee that you are using the built-in versions of all chips.

If you can't find your feedback problem, feel free to e-mail me your CPU.hdl and I'll take a look at it for you.

--Mark

[manu]

Hello Mark,

I have sent you my CPU.hdl file.

 Please let me know why I am getting "This pin has circle in its parts connections" error.

 In one of your comments in another thread, I had seen that if a feedback is not clocked this error will happen.Here you said "You shouldn't need to add anything specific to the feedback loops in the CPU. The only two loops are through the D- and A-registers which, as you say, are already clocked." Could you please explain. I have noticed that when I clock the feedback using a register, the error disappears.

Manu.

[manu]

Hello Mark,

I found out and corrected the bug in my code.

Now its working fine.

Thank you. :)