Intel® Quartus® Prime Standard Edition User Guide: Design Compilation

ID 683283
Date 9/24/2018
Document Table of Contents

2.5.3. Consider a Cascaded Reset Structure

Designs typically have a global asynchronous reset signal where a top-level signal feeds all partitions. To minimize skew for the high fan-out signal, the global reset signal is typically placed onto a global routing resource.

In some cases, having one global reset signal can lead to recovery and removal time problems. This issue is not specific to incremental flows; it could be applicable in any large high-speed design. In an incremental flow, the global reset signal creates a timing dependency between the top-level partition and lower-level partitions.

For incremental compilation, it is helpful to minimize the impact of global structures. To isolate each partition, consider adding reset synchronizers. Using cascaded reset structures, the intent is to reduce the inter-partition fan-out of the reset signal, thereby minimizing the effect of the global signal. Reducing the fan-out of the global reset signal also provides more flexibility in routing the cascaded signals, and might help recovery and removal times in some cases.

This recommendation can help in large designs, regardless of whether you are using incremental compilation. However, if one global signal can feed all the logic in its domain and meet recovery and removal times, this recommendation may not be applicable for your design. Minimizing global structures is more relevant for 
high-performance designs where meeting timing on the reset logic can be challenging. Isolating each partition and allowing more flexibility in global routing structures is an additional advantage in incremental flows.

If you add additional reset synchronizers to your design, latency is also added to the reset path, so ensure that this is acceptable in your design. Additionally, parts of the design may come out of the reset state in different clock cycles. You can balance the latency or add hand-shaking logic between partitions, if necessary, to accommodate these differences.

The signal is first synchronized on the chip following good synchronous design practices, meaning that the design asynchronously resets, but synchronously releases from reset to avoid any race conditions or metastability problems. Then, to minimize the impact of global structures, the circuit employs a divide-and-conquer approach for the reset structure. By implementing a cascaded reset structure, the reset paths for each partition are independent. This structure reduces the effect of inter-partition dependency because the inter-partition reset signals can now be treated as false paths for timing analysis. In some cases, the reset signal of the partition can be placed on local lines to reduce the delay added by routing to a global routing line. In other cases, the signal can be routed on a regional or quadrant clock signal.

The figure shows a cascaded reset structure.

Figure 25. Cascaded Reset Structure

This circuit design can help you achieve timing closure and partition independence for your global reset signal. Evaluate the circuit and consider how it works for your design.