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AR# 17969

3.1 System Generator for DSP - Why does the black box show the wrong input type when a register is used on the feedback path?

Description

Keywords: black box, no input port, Output type, simulation

Urgency: Standard

General Description:
Why does the black box show the wrong input type when I use a register on the feedback path?

For example:

The output of the black box is a UFIX_20_0, and this is fed to a Register input. The output of the Register(20 bits) is then fed back into the input of the black box. The problem is that the input to the black box is a shown as Boolean, and an error message reports that the data widths are incompatible.

If a Gateway In block is used with a width defined as 20 bits, the input will successfully pass into the black box .

Solution

When you use a register in a feedback loop, especially with a black box, you must use a Cast block to set the data type.

Example:

The output of the black box is a UFIX_20_0, and this is fed to a register input. The output of the Register (20 bits) is then fed to a Cast block that sets it to a UFIX_20_0 and feeds it back into the input of the black box.
AR# 17969
Date Created 09/08/2003
Last Updated 06/24/2004
Status Archive
Type General Article