When I synthesize a design with XST, the design does not fit the device, even with an exhaustive fitting mode. However, when I synthesize the design with another synthesis tool, it fits easily into the targeted device.
Different synthesis tools yield different results. Carefully consider selecting or deselecting synthesis options.
Access the synthesis options as follows:
1. Right-click the "Synthesize" process.
2. Select "Properties.... The following three tabs appear:
No single "good" switch will force your design to fit. Although the default options provide the best general results, you might get better results by selecting or deselecting some of the options listed in the tabs.