Important note about the ADSU8 ....


Posted by IamAnon , May 01,1999,22:34 Post Reply  Forum

You may remember my postings regarding the ADSU8, and where to get a
schematic (the push option of the right-click menu), and how
the ADSU8 functions. What I posted previously, in regards to
the schematic, and how it works, were correct: It adds when its ADD input is high, and it subtracts when it is low.

However, used in this manner, the output will be the result of
an unsigned addition or subtraction. This is bad.

If any of you have been using it as described in previous postings
(postings 516 & 525) based on the information in them, all I can
say is "Oops. Sorry."

And, that you can still get the correct results out of it by
putting an inverter between the Mux output and the ALU input,
and ignoring the schematic of the ADSU8, when you want to do an
add, let the ADD input be low, and when you want to do a subtract,
let the ADD input be high, and assert CI (carry in).

This should give you the correct behavior. If you want (I realize
time is slipping away), you can check this by making a new project,
just adding the ADSU8:1, an INV8:1, and an input for ADD, tied to
CI as well as ADD, an input for the INV8:1, and an output for
the output, and for OFL if you test using boundary conditions.

I did this, and got the correct results. I hope that nobody has
been having problems due to a configuration error like this, but
if you have, again, "Oops. Sorry."