126.96.36.199.3. Timing Closure: Output Strobe Setup and Hold Delay Constraints
The Output Strobe Setup Delay Constraint and Output Strobe Hold Delay Constraint ensure that the data output from the FPGA to the external device meets the setup and hold requirements of the external device. The value of these constraints are calculated from various timing parameters such as setup and hold timing of the external device, board trace delay and clock skew.
The following figure shows the considerations required to determine the Output Strobe Setup Delay Constraint and Output Strobe Hold Delay Constraint values. These constraints are depending on the clock and data traces, and setup and hold requirements of the external device. With system-centric delays, you can obtain the setup and hold requirements, clock delay, and data trace delay values for the external device through the device data sheet.
The following is the derivation for Output Strobe Setup Delay Constraint and Output Strobe Hold Delay Constraint:
Output strobe setup delay constraint = Maximum board skew + maximum tSU
Output strobe hold delay constraint = Minimum board skew + minimum tH
where maximum board skew = maximum data trace - minimum clock trace
minimum board skew = minimum data trace - maximum clock trace
maximum tSU = clock setup time
minimum tH = clock hold time
- Input clock frequency = 100 MHz
- Board skew estimation = ± 0.03 ns
- Maximum tSU = 0.75 ns
- Minimum tH = 0.75 ns
Output Strobe Setup Delay Constraint = 0.03 + 0.75= 0.78 ns
Output Strobe Hold Delay Constraint = -0.03 - 0.75 = -0.78 ns