Re: FullAdder imp.

Posted by cadet1620 on Nov 23, 2012; 2:22pm
URL: http://nand2tetris-questions-and-answers-forum.52.s1.nabble.com/FullAdder-imp-tp4025675p4025677.html

[Edit: fix typo.]

ysh443 wrote

Hi
im trying to implement the FuallAdder usind two H.A.
when adding the C bit using a H.A. , i conclude that the carry_out
is not important so i want to leave it unwired

but the Hardware simulator is crying that its the carry output
shoul be connected

how can i cause it be unconnected?

regards

You do need to do something with the carry outputs from both half adders to make a correct full adder. Consider adding a=0, b=1 and c=1.

The first half adder produces sum₁=1, carry₁=0.
The second half adder adds sum₁ and c producing sum₂=0, carry₂=1.

sum₂ is the sum output from the full adder. carry₁ and carry₂ need to be combined in some fashion to produce the full adder's carry output.

It is OK to leave out the connection to an unused output in a part line. For instance the full adder for bit 15 in your Add16 won't have its carry connected so you can write:     FullAdder(a=a[15], b=b[15], c=c14, sum=out[15]);

--Mark