HDL Coding Styles
A well defined hierarchy helps with floorplanning, and can greatly assist in achieving timing closure later. Critical paths in a level of hierarchy that are grouped together can often simplify floorplanning.
For SSI devices, you might need to manually assign logic to the individual SLR components.
•Defining the hierarchy so that all the logic can be assigned to a common SLR makes such assignments easier to create.
•Defining the hierarchy so that all the logic can be assigned to a common SLR allows place and route to complete faster.
•Defining the hierarchy so that all the logic can be assigned to a common SLR facilitates design analysis.
Functional and Timing Debugging
Timing debugging is easier when critical timing paths are localized to areas of the logical design.
The Xilinx tools allow designs to have gate-level timing netlists written out by hierarchy. This can facilitate and simplify the timing and functional debugging of a large FPGA design.
Pipelining
Two major factors can limit timing closure:
•Poor placement leads to routes that are too long to satisfy timing constraints.
•There are too many logic levels to allow the design to operate at the designed speed.
Benefits of pipelining can include the following:
•Pipelining might be the only way to reduce logic levels enough to meet timing requirements.
•Planning pipelining sooner (rather than later) can greatly simplify timing closure.
Adding pipelining to certain paths can propagate latency differences across the circuit. One seemingly small change can turn into a major redesign of portions of the code.
•Identifying pipelining opportunities early in the design can:
•Facilitate timing closure.
•Reduce implementation runtime (because of easier-to-solve timing problems).
•Reduce device power consumption (because of reduced switching of logic)
Managing Fanout Non-Clock Nets
Managing high fanout nets is easier early in the design process. What constitutes a high fanout net is often determined by performance requirements and the construction of the paths.
Observe nets with loads measured in the thousands early to understand their possible impact on the design.
Large FPGA Methodology Guide |
www.xilinx.com |
37 |
UG872 (v14.3) October 16, 2012
Chapter 4: System Level Design
Mitigating High Fanout Nets
When a high fanout net is identified, mitigation techniques include:
•Reduce Loads
•Use BUFG and BUFH
•Manage Replication in the Tools
Reduce Loads
Reduce loads to portions of the design that do not require it.
•For high fanout control signals, determine whether all coded portions of the design require that net to function properly. Reducing the load demand can greatly improve timing.
•For data paths, determine whether restricting the logic can reduce fanout.
Use BUFG and BUFH
Use BUFG and BUFH components to drive the net.
For more information, see Using Clock Buffers for Non-Clock Nets in Chapter 5, Clocking.
Manage Replication in the Tools
Manage replication in the tools.
Most synthesis tools can automatically or manually control replication of register or logic to help minimize the impact of large fanout signals.
This capability can greatly improve timing for high fanout nets, often without modifying any RTL code.
38 |
www.xilinx.com |
UG872 (v14.3) October 16, 2012 |