Fixed-Point Conversion Operations :: Using Fixed-Point Data in Stateflow (Stateflow User's Guide)

Stateflow User's Guide

Fixed-Point Conversion Operations

Stateflow converts real numbers into fixed-point data during data initialization and as part of casting operations in the application. These conversions compute a quantized integer, , from a real number input. Stateflow uses offline conversions to initialize data and online conversions for casting operations in the running application. The topics that follow describe each conversion type and give examples of the results.

Offline Conversions for Initialized Data

Offline conversions are performed during code generation, and are designed to maximize accuracy. They round the resulting quantized integer to its nearest integer value. If the conversion overflows, the result saturates the value for .

Offline conversions are performed for the following operations:

Initialization of data (both variables and constants) in the data dictionary
Initialization of constants or variables from the MATLAB workspace

Online Conversions for Casting Operations

Online conversions are performed for casting operations that take place during execution of the application. Designed to maximize computational efficiency, they are faster and more efficient than offline conversions, but less precise. Instead of rounding to its nearest integer, online conversions round to the floor (with the possible exception of division, which might round to 0, depending on the C compiler you have). If the conversion overflows the type converted to, the result is undefined.

Offline and Online Conversion Examples

The following examples show the difference in the results of of offline and online conversions of real numbers to a fixed-point type defined by a 16 bit word size, a slope () equal to 2^-4, and a bias () equal to 0:

Offline Conversion
Online Conversion

3.45
55.2
55
3.4375
55
3.4375

1.0375
16.6
17
1.0625
16
1

2.06
32.96
33
2.0625
32
2

		Offline Conversion	Online Conversion

3.45	55.2	55	3.4375	55	3.4375
1.0375	16.6	17	1.0625	16	1
2.06	32.96	33	2.0625	32	2

In the preceding example,

is the real-world value represented as a fixed-point value.
is the floating-point computation for the quantized integer .
is the rounded value of .
is the approximate real-world value resulting from for each conversion.

Assignment (=, :=) Operations Autoscaling of Stateflow Fixed-Point