Use Global Clock Buffers
Use clock buffers for highest fanout clocks
- Drive low-skew, high-speed long line resources
- Use BUFG primitive to be family-independent
Limit number of clocks to ease placement issues
- XC4000E: 8
- XC4000X : 20
- XC5000: 4
Additional clocks might be routable on long lines
- Otherwise routed on general interconnect
- Slower and higher skew