Re: 16Mux Problems

Posted by cadet1620 on May 16, 2011; 9:17am
URL: http://nand2tetris-questions-and-answers-forum.52.s1.nabble.com/16Mux-Problems-tp2946612p2947078.html

Zebo wrote

Why is this setup for my Mux16 not working? It says sub bus of internal node may not be used and refers to the x[0], but I really don't understand what that even means and I've been searching up and down the forums and even threads that talk about that same problem just make no sense to me and I've ready over appendix A many times. Also, is there any way to connect all my notsels and sels to an And16, when I tried this earlier it told me the bus widths did not line up which does make sense, but how would expand the width?

The "sub bus of an internal node may not be used" error is an annoying limitation of HDL. All buses must be created at their full width when connected to a part output. You can connect multiple wires and buses to a part output so you can create multiple narrower buses from a wide output, but you cannot connect multiple part outputs to make a bus. Your code would need to read something like

And(a=notsel, b=a[0], out=x0);
And(a=notsel, b=a[1], out=x1);
And(a=notsel, b=a[2], out=x2);
...
Or16(a[0]=x0, a[1]=x1, a[2]=x2,...

Assuming that you have a working Mux.hdl, why are you replicating its low-level implementation 16 times? The layers-of-abstraction thinking that TECS encourages would have you think about how you can use what you've already built to build your Mux16.

The easiest way to make a single signal a bus-wide signal is

Mux16(sel=signal, a=false, b=true, out=signal16);

but that doesn't help much in making a Mux16.

--Mark