3. Using A Falling Edge Clock for Data Capturing in Full-rate Transfer Mode
As shown in the Timing Components of a Simplified GPIO Input Path figure, the full-rate transfer mode is represented by signals A and B. In the full-rate mode interface, the two signals bypass the half-rate block. The timing violation on half-cycle transfer from the GPIO Intel® FPGA IP to FPGA core is due to a tighter setup requirement. To rectify this, use a falling edge clock to capture the data because the full cycle window provides a larger setup requirement to fulfill the timing as shown in the top_w1 design example.
Figure 5. Block Diagram of Falling Edge Clock Core Register
The following figure shows the timing waveform of the design example using a falling edge clock. The latch clock now has a full cycle window of 4 ns to capture the input data.
Figure 6. Timing Waveform for Setup Time Using Falling Edge Clock
This solution provides a setup slack of 1.814 ns with no additional latency to the design.