Cyclone V SE Pulldowns on QSPI I/F. Why?

Hi Ian,
 

The 200 Ω pull-down resistors are not a general Cyclone V SE QSPI interface requirement. They appear to be a board-specific choice, likely to keep the connector-exposed QSPI signals at a defined low state and improve signal behavior when the external interface is disconnected.
 

For a standard QSPI connection, the Cyclone V Pin Connection Guidelines specify the following for QSPI_CLK: use a series termination resistor near the SoC device-50 Ω for a single-memory topology, or 25 Ω for other topologies. The guideline does not specify 200 Ω pull-down resistors on QSPI_CLK or QSPI_IO[0:3].

Reference: Cyclone V Device Family Pin Connection Guidelines (PCG-01014-3.2).
 

Because 200 Ω is a relatively strong load, we do not recommend adding these resistors to a new design unless they are required by the specific board topology and have been validated.

Best regards,
Fakhrul