Solved: Kintex-7 / KC705: MiG DDR3 fails to calibrate

As I implemented a MiG controller for KC705′s on-board SODIMM, the controller failed to calibrate at first. Despite that I’ve copied the instantiation and port connections from the example design. As for the pin placement, this was taken care of by the core itself, by virtue of memctl/memctl/user_design/constraints/memctl.xdc within MiG’s dedicated directory (which I called memctl).

And yet init_calib_complete remained low, indicating calibration had failed.

Actually, I had followed Xilinx’ XTP196 slides, except that I didn’t make an example design — I had my own.

Having ruled out holding the MiG controller in reset or a faulty pinout, it turned out that a constraint needs to be added to the application XDC file, namely

set_property slave_banks {32 34} [get_iobanks 33]

What it says (see UG912), is that I/O banks 32 and 34 should calibrate their on-chip terminations (DCI, Digitally Controller Impedance) based upon the reference resistors connected to the dedicated pins on bank 33. Without this constraints, on-chip termination on banks 32 and 34 doesn’t work, and the signal integrity on the relevant I/Os goes down the toilet. No wonder it didn’t calibrate.

The hint for this is on page 37 of XTP196, “Modifications to Example Design”, which tells us to overwrite the example design created by Vivado with a ZIP file Xilinx supplies. On the following page it lists the changes made, among others “Added DCI Cascade constraints to XDC”.