Issues When Trying to Eliminate Reset in HDL Code - 2020.2 English

UltraFast Design Methodology Guide for Xilinx FPGAs and SoCs

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2020.2 English

When optimizing the code to eliminate reset, commenting out the conditions within the reset declaration does not create the desired structures and instead creates issues. For example, the following figure shows three pipeline stages with asynchronous reset used for each. If you attempt to eliminate the reset condition for two of the pipeline stages by commenting out the code with the reset condition, the asynchronous reset becomes enabled (inverted logic of rst).

Figure 1. Commenting Out Code with Reset Conditions

The optimal way to remove the resets is to create separate sequential logic procedures with one for reset conditions and the other for non-reset conditions, as shown in the following figure.

Figure 2. Separate Procedural Statements for Registers With and Without Reset

Tip: When using a reset, make sure that all registers in the procedural statement are reset.