Stateflow

Tips and Tricks for Using Fixed-Point Data in Stateflow

Once you specify fixed-point data (see Specifying Fixed-Point Data in Stateflow), you can use it just as you would any data in Stateflow. However, because of the limitations of fixed-point numbers, it is a good idea to follow these guidelines when using them:

1. Develop and test your application using double- or single-precision floating-point numbers.

1. Using double- or single-precision floating-point numbers does not limit the range or precision of your computations. You need this while you are building your application.

2. Once your application works well, start substituting fixed-point data for double-precision data during the simulation phase, as follows:
1. Set the integer word size for the simulation environment to the integer size of the intended target environment.

• Stateflow uses this integer size in generated code to select result types for your fixed-point operations. See Setting the Integer Word Size for a Target.

2. Add the suffix 'C' to literal numeric constants.

• This suffix casts a literal numeric constant in the type of its context. For example, if x is fixed-point data, the expression y = x/3.2C first converts the numerical constant 3.2 to the fixed-point type of x and then performs the division with a fixed-point result. See Fixed-Point Context-Sensitive Constants for more information.

Note    If you do not use context-sensitive constants with fixed-point types, noninteger numeric constants (for example, constants that have a decimal point) can force fixed-point operations to produce floating-point results.

3. When you simulate, use overflow detection.

1. See Overflow Detection for Fixed-Point Types for instructions on how to set overflow detection in simulation.

4. If you encounter overflow errors in fixed-point data, you can do one of the following to add range to your data.
• Increase the number of bits in the overflowing fixed-point data.

• For example, change the base type for from int16 to int32.

• Increase the range of your fixed-point data by increasing the power of 2 value, .

• For example, you might want to increase from -2 to -1. This decreases the available precision in your fixed-point data.

5. If you encounter problems with model behavior stemming from inadequate precision in your fixed-point data, you can do one of the following to add precision to your data:
• Increase the precision of your fixed-point data by decreasing the value of the power of 2 radix point .

• For example, you might want to decrease from -2 to -3. This decreases the available range in your fixed-point data.

• If you decrease the value of , you might also want to increase the number of bits in the base data type for to prevent overflow.

• For example, change the base type for from int16 to int32.

6. If you cannot avoid overflow for lack of precision, consider using the := assignment operator in place of the = operator for assigning the results of multiplication and division operations.

1. You can use the := operator to increase the range and precision of the result of fixed-point multiplication and division operations at the possible expense of computational efficiency. See Assignment Operator :=.

Specifying Fixed-Point Data in Stateflow

Offline and Online Conversions of Fixed-Point Data