Во первых, товарищ Евгений_2 - RTFM.

Во вторых вот:

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allow_power_up_dont_care


Power-Up Don't Care logic option

A logic option that causes registers that do not have a Power-Up Level logic option setting to power up with a don't care logic level (X).

If this option is turned off, the register powers up low, as shown in the following illustration:


This option is useful for allowing the Compiler to change the power-up level of a register to minimize the area of the design.

This is a project-wide option. This option can be set in the Assignment Editor (Assignments menu) or the Analysis & Synthesis Settings page or the Fitter Settings page of the Settings dialog box (Assignments menu). This option is available for all Altera devices supported by the Quartus II software.