MIG can provide calibrated on-die input termination for DDR2 and DDR3 memory interfaces.When this option is selected, the mcb_soft_calibration module calibrates the input termination for the bidirectional memory interface signals dq, ldqs_p/n, udqs_p/n to an external 100 Ohm pull-up resistor on the RZQ pin. It is possible to determine the values that the mcb_soft_calibration module selects for the pull-up (p-term) and the pull-down(n-term) by using the ChipScope Pro toolto read out those values. Reading out the values can provide insight as to whether the calibrated input termination has been performed as expected.
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For ISE 13.2, MIG 3.8, the calibrated input termination algorithm is as follows:
This provides a ~50 Ohm calibrated parallel input termination.
To determine the values for the p-term and n-term that are programmed into thebidirectional memory interface pins, you need to observe the following signals in the mcb_soft_calibration module:
P_Term - p-term value programmed initially into allbidirectional memory interface pins
N_Term- n-term value programmed initially into allbidirectional memory interface pins
P_Term_s -p-term value programmed into all differential bidirectional memory interfacenegative pins
N_Term_w-n-term value programmed into all differential bidirectional memory interfacenegative pins
P_Term_w-p-term value programmed into all differential bidirectional memory interfacepositive pins
N_Term_s -n-term value programmed into all differential bidirectional memory interfacepositive pins
Typical values for these signals depend on the I/O standard and theVCCAUX voltagefor the interface. For a DDR3 interface with a 3.3 Volt VCCAUX setting theP_Term signal should be around 0xF and the N_Term signal should be around 0xB.The other values can becalculated from the P_Term and N_Term values.