How to measure driver impedance of output drivers in OCD calibration for DDR2

Hi,

I am implementing Off-chip driver calibration in the DDR2 memory controller design on Altera Cyclone III EP3C120 board and am struck in the drive mode. Drive mode is used to measure the DDR-SDRAM output driver impedance.

The DDR2 specs have mentioned to use any external device or the memory controller to measure the voltage level of the output signals (DQ, DQS, and /DQS) and

determine whether or not the pull-up/pull-down resistance is at the target value.

Is there any way this can be achieved sensing the DDR I/O cells on the board and implement it in the RTL?

Any related input is highly appreciated.

Thanks in advance.