noprune VHDL Synthesis Attribute

A VHDL synthesis attribute that prevents the Intel^® Quartus^® Prime software from removing a register that does not directly or indirectly feed a top-level output or bidir pin, such as a fanout-free register. This attribute differs from the preserve attribute, which only prevents a register from being reduced to a constant or merged with a duplicate register. Standard synthesis optimizations remove nodes that do not directly or indirectly feed a top-level output pin or bidir pin.

You may wish to retain a fanout-free register in many cases. For example, if you are compiling a preliminary design that will eventually include fanout logic for the register. You may also intend to use the register for debugging in the Intel^® Quartus^® Prime Simulator or Signal Tap Logic Analyzer. Finally, you may want to route from the register to a Signal Probe Pin after compilation.

Internally, the Intel^® Quartus^® Prime software uses the Preserve Fan-out Free Register Node logic option to implement a noprune attribute. Thus, the attribute has the same device support limitations as the logic option.

To use the noprune synthesis attribute in VHDL, you must first declare the attribute in the local scope or import its declaration from the altera_syn_attributes package in the altera library. You can then use an attribute specification to associate the attribute with a signal or variable that infers a register in your design. For example, in the following code, the attribute declaration declares the noprune attribute as a boolean object in the local scope, and the attribute specification associates the attribute with the signal reg1:

signal reg1: stdlogic; attribute noprune: boolean; attribute noprune of reg1: signal is true;