I've understood that the behaviour is correct when I make a simple adder:

enter image description here

But why does carry on light up just because I switch on the + operation for my 4-bit system?


The spec I try to follow has all the details but I can't really understand why the system in effect is like:

1 + 0 = 1

2 + 0 = 1 carry 1 (carry in lights up)

  • 2
    \$\begingroup\$ You'll need to tell us more about your system for us to have any hope of answering you. What does it mean for "carry on" to "light up"? What kind of system is your "4-bit system"? \$\endgroup\$
    – The Photon
    Dec 30 '12 at 19:40
  • \$\begingroup\$ Thank you for the comment. I can add the complete details. I think it works but I just don't understand. I download the system to an Altera DE2 board. I'm adding more info to the question so that we can inspect the system. I wanted to make something like a primitive microprocessor. \$\endgroup\$ Dec 30 '12 at 20:28
  • 2
    \$\begingroup\$ You might want to clarify what "carry on" is... there is no "carry on" in your schematic, there's only carry in and carry out. \$\endgroup\$
    – Shamtam
    Dec 30 '12 at 20:54
  • \$\begingroup\$ Thanks for the investigation. I mean that carry in IS on, by lightning up when I download the complete system to my Altera DE2 board. I can test +1 counting up but I can't control + the way I understand from the designs. I isolated the problem to the adder and found that I don't understand why carr in turns on its light on the board just because I switch on the code for operation +. I verified this in the summer and it worked so I suspect that I'm just misunderstanding and forgetting how I wired this. \$\endgroup\$ Dec 30 '12 at 23:07
  • 1
    \$\begingroup\$ We can't explain (although DrFried has a pretty good guess) why carry in is on if you don't show us what's connected to carry in. Do you have a schematic for the relevant part of the board you're using? \$\endgroup\$
    – The Photon
    Dec 31 '12 at 16:21

The question doesn't contain near enough detail, so I'm mostly making assumptions here...

Carry in is an input. I suspect you stop driving the carry-in input (so it "floats") when you switch to addition mode.

There is a good discussion of this issue here.

  • \$\begingroup\$ Thank you for the answer. I think my design is correct and that I just don't understand the behavior. I've linked to the spec for which I use an Altera DE2 education board. \$\endgroup\$ Dec 31 '12 at 13:40

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