2.5.1. Register Power-Up Values
For designs that use preset signals, but the target device does not support presets in the register architecture, synthesis may convert the preset signal to a clear signal, which requires to perform a NOT gate push-back optimization. NOT gate push-back adds an inverter to the input and the output of the register, so that the reset and power-up conditions appear high, and the device operates as expected. In this case, the synthesis tool may issue a message about the power-up condition. The register itself powers up low, but since the register output inverts, the signal that arrives at all destinations is high.
Due to these effects, if you specify a non-zero reset value, the synthesis tool may use the asynchronous clear (aclr) signals available on the registers to implement the high bits with NOT gate push-back. In that case, the registers look as though they power-up to the specified reset value.
When an asynchronous load (aload) signal is available in the device registers, the synthesis tools can implement a reset of 1 or 0 value by using an asynchronous load of 1 or 0. When the synthesis tool uses a load signal, it is not performing NOT gate push-back, so the registers power-up to a 0 logic level. For additional details, refer to the appropriate device family handbook.
Optionally you can force all registers into their appropriate values after reset through an explicit reset signal. This technique allows to reset the device after power-up to restore the proper state.
Synchronizing the device architecture's external or combinational logic before driving the register's asynchronous control ports allows for more stable designs and avoids potential glitches.