Simulink ADD block keeps returning 255

So I have two inputs 'Pre-State' and 'Pre-Status'. The values going into the ADD block are '1' and '0' respectively, yet for some reason the ADD block keeps returning 255.

The ADD block is setup so that 'Pre-Status' is subtracting 'Pre-State'. So in theory, the output should be -1. Yet it keeps giving 255 for some reason.

[Fangjun Jiang]

Some settings are not right. You have to show it. I guess it is the data type of the output and the saturation flag.

Answer 1

[Andy Bartlett]

My guess is that Sum block's output data type is uint8, and the block is configured to wrap on integer overflows.

So the output would be module 2^8 of the "ideal output result".

idealOutput = -1

idealOutput = -1

actualOutput = mod(idealOutput, 256)

actualOutput = 255

We could also test this theory with Fixed-Point Designer fi objects.

nt = numerictype('uint8')

nt =


          DataTypeMode: Fixed-point: binary point scaling
            Signedness: Unsigned
            WordLength: 8
        FractionLength: 0

fiOutput = fi( idealOutput, nt, 'OverflowAction', 'Wrap', 'RoundingMethod', 'Floor')

fiOutput = 
   255

          DataTypeMode: Fixed-point: binary point scaling
            Signedness: Unsigned
            WordLength: 8
        FractionLength: 0

        RoundingMethod: Floor
        OverflowAction: Wrap
           ProductMode: FullPrecision
               SumMode: FullPrecision