MATLAB REAL-TIME WORKSHOP EMBEDDED CODER - S Manuel d'utilisateur télécharger pdf (Page 36)

value. To solve this kind of problem a conversion rule is made to use the entire

number range provided by an integer and to be able to shift this range. The

following equation describes the relation between the real value, x, and its integer

representation, x´:

OxSx +⋅= ´

, where

S is the scaling factor and

O represents an offset

that can be used to shift the integer number range.

Using a scaling factor of 0.5,

S =

−

, and an offset, 25

O gives the following

conversion of the real value into the 8-bit unsigned integer range:

Figure 8: Representing a value with scaling

Using this approach, more than 75% of the 8-bit integer number range is covered

and used. This gives a better resolution of the representation of the real value.

This kind of scaling is called the power-of-two scaling which uses the scaling

factor

S 2=

, where n is an integer type. Another type of scaling can be used

too, called arbitrary scaling where

S is an arbitrary real number.

6.3.1 TargetLink

Fixed-point is supported in all TargetLink blocks, as mentioned earlier. Both

power of two scaling and arbitrary scaling, where the scaling factor is an arbitrary

real number, are supported in TargetLink. Scaling can be done with or without

offset value. The user can easily choose scaling parameters manually in every

block in the model. Blocks can also refer to an already defined scaling objects

from the Data Dictionary, see chapter 5.1.1 TargetLink and Table 4.

The user can also perform automatic scaling in the model. TargetLink supports

two methods for the determination of the range of a variable: by simulation and by

worst case value range propagation. Based on the determined ranges, TargetLink

calculates suitable scaling parameters and automatically enters them in the

respective block dialog. The user must first enter as many known ranges as

possible in the model. If there is a closed loop in the model, at least one block

must have known ranges otherwise worst case value range propagation will stop.

Every block has a protection option against scaling. The user can choose to leave

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MATLAB REAL-TIME WORKSHOP EMBEDDED CODER - S Manuel d'utilisateur Page 36