A logic option that specifies the clock input used as a frequency reference for a DQS I/O pin. The clock is the pin that drives the DDIO circuitry for the dedicated DDR SDRAM interface.

This option is useful for indicating the I/O pin that acts as a reference clock for the dedicated DDR SDRAM interface on the target device.

This option is ignored if it is applied to anything other than the DQS I/O pin and should refer to the name of the pin in the input design. This option is available for Stratix^™ and Stratix^™ GX devices.