Introduction

Xilinx FPGAs provide the benefits of custom CMOS VLSI and allow you to avoid the initial cost, time delay, and risk of conventional masked gate array devices. In addition to the logic in the CLBs and IOBs, the XC4000 family, XC5200 family, and Spartan family FPGAs contain system-oriented features such as the following.

• Global low-skew clock or signal distribution network
  
  
• Wide edge decoders (XC4000 family only)
  
  
• On-chip RAM and ROM (XC4000 family and Spartan)
  
  
• IEEE 1149.1 - compatible boundary scan logic support
  
  
• Flexible I/O with Adjustable Slew-rate Control and Pull-up/Pull-down Resistors
  
  
• 12-mA sink current per output and 24-mA sink per output pair
  
  
• Dedicated high-speed carry-propagation circuit
  
  

You can use these device characteristics to improve resource utilization and enhance the speed of critical paths in your HDL designs. The examples in this chapter are provided to help you incorporate these system features into your HDL designs.