MatLab 2012b sum block with signed/unsigned numbers

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If I have a sum block that's taking 2 constants in, one signed the other unsigned, and I add them together I do not get the result that I expect in the picture below: OutDataTypeStr for the sum block is Inherit: Inherit via internal rule, expected is 475.
I noticed that MatLab created a new data type, sfx32_E1, and that the number was 1 off from what I expected it to be. When I changed the sum block to output as an int32, I got the result that I expected it to be.
So I was wondering why the results were different and what that data type that's outting in the first picture means. I'm going to assume that this is the same for the other math operation blocks in MatLab? Thanks!
  1 Comment
Lucas
Lucas on 8 Aug 2014
One bump to see if anyone knows what is going on here.

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Accepted Answer

Yu Jiang
Yu Jiang on 8 Aug 2014
Edited: Yu Jiang on 8 Aug 2014
Before the model is executed, SIMULINK will try to figure out what is the output datatype of the Add Block. Since the Display is not datatype-dependent, and the two input data types are different, SIMULINK seems to be "confused" and ends up with generating the new datatype sfix32_E1.
  2 Comments
Yu Jiang
Yu Jiang on 11 Aug 2014
Hi Lucas
I am not sure how exactly the internal rule works and why it is one less. You can find its description here.
From my point of view, this is a side effect when SIMULINK tries to use the internal rule without knowing what the expect output datatype should be. In this case, the difference happens to be one, but it may be something else in other cases.
-Yu

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