Are series resistors required on ddr address line?

[gingerologist]

I have a STM32MP157D-DK1 board. I downloaded design files and surprisingly found there were 56ohm series resistors on ddr address lines. Though series resistors are normally found on data and address lines for ddr1 or ddr2, I have never seen them used for ddr3 mem chips. Are those resistors necessary for STM32MP1, or they are just optional, for the best possible signal integrity?

Those resistors occupies a large board area, which is a pain for compact design.

[PatrickF]

Hi,

There is no 'series' resistors. The 56 ohms resistors seen on STM32MP157D-DK1 (which is same PCB than STM32MP157F-DK2) are termination resistors (to a VTT voltage).

As you can see from AN5031 and AN5122, in case of 16-bit DDR3L (i.e. single memory package, which mean point to point address lines), the termination resistors are optional.

STM32MP157F-DK2 board PCB was done in an early product stage and we keep termination to lower risks.

Now, we have seen from our customers that STM32MP15x DDR3L timings are quite robust if wires are kept short and rules from AN5122 are fulfill.

Anyway, In case of minor signal integrity issues linked to signal reflection, the output impedance could be increased to 48 ohms using DDRPHYC ZPROG[3:0] settings.

See also https://community.st.com/s/article/FAQ-STM32MP1-What-ST-offers-to-make-a-PCB-design-that-ensures-signal-integrity

Regards,