fixptbestprec

This command returns the maximum precision available for the real-world value 4/3 using a signed, 8-bit number.

out = fixptbestprec(4/3,8,1)

out =
   0.015625

Alternatively, you can specify the fixed-point data type:

out = fixptbestprec(4/3,sfix(8))

out =
   0.015625

This value means that the maximum precision available for 4/3 is obtained by placing six bits to the right of the binary point since 2^-6 equals 0.015625:

You can use the maximum precision as the scaling in fixed-point blocks. This enables you to use fixptbestprec to perform a type of autoscaling if you would like to designate a known range of your simulation. For example, if your known range is -13 to 22, and you are using a safety margin of 30%:

knownMax = 22;
knownMin = -13;
localSafetyMargin = 30;
slope = max( fixptbestprec( (1+localSafetyMargin/100)* ...
 [knownMax,knownMin], sfix(16) ) );

The variable slope can then be used in the expression that you specify for the Output data type parameter in a block mask. Be sure to select the Lock output data type setting against changes by the fixed-point tools check box in the same block to prevent the Fixed-Point Tool from overriding the scaling. If you know the range, you can use this technique in place of relying on a model simulation to provide the range to the autoscaling tool, as described in autofixexp (Fixed-Point Designer).