We are currently experiencing some issues with one of our custom board DDR3 interface, we suspect an issue with Vref as no pulld down resistors are populated (fortunately the pads are present and can be populated)
For our design, we looked at the KCU105 board schematics as a reference to implement our DDR3 interface.
If I understood well from the documentation (pg150-ultrascale-memory-ip.pdf and ug571-ultrascale-selectio.pdf) :
- DDR4 must use internal Vref
- when using internal Vref settings the corresponding Vref pin must pulled down to ground with a 500 Ohm or a 1 kOhm resistor
I would like to know why the resistor R686 and R695 on the schematics (see picture attached) seem not populated DNP (do not populate), which seems in contradiction with the comment on the schematics.
Did I miss something ?
Is the pull down resistor on Vref really necessary for DDR3 and DDR4 interface when using internal Vref ?