CAGE User's Guide

Lookup Table, Fixed Point

The advantage of using fixed-point precision is the reduction in computation needed for such numbers. However, it restricts the numbers available to the user.

For example, consider using a lookup table for the physical quantity spark advance for maximum brake torque (MBT spark). Typically, the range of values of MBT spark is 0 to 50 degrees. This is the physical data. The ECU can only store bytes of information and you want to restrict the hardware store to a range of 0 to 8, with at most one decimal place stored.

To adjust the fixed-point precision of the lookup table:

1. Select the Physical Data edit box and enter the range of the physical data. In the preceding example, enter 0 50.
2. Select the Hardware Data and enter the range to store. In the preceding example, enter 0 8.
3. Determine the range of values that you want to have in the table. In the preceding example, enter 0 50.

To edit the size of the precision, choose from

• Byte (8 bits)
• Word (16 bits)
• Long (32 bits)
• Custom

In the preceding example, the hardware is restricted to 8 bytes and to one decimal place.

4. Choose whether you want the numbers to be signed (negative and positive) or unsigned (nonnegative) by clicking the radio buttons.

Polynomial Ratio, Fixed Point

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