Is the 150 Ohms termination resistor for LPDDR4 clock needed?

[raik_seliger]

The X-815LPD4-CPUCY is the only EVK for the i.MX8M Nano with LPDDR4 support.

In the schematics on page 5 of the SoM, there is a termination resistor with 150 Ohms (R8) placed across the clock lines DRAM_CK_T_A/DRAM_CK_C_A.

The needness of this resistor cannot be found in the CPU reference manual and also no explanation for the value of the resistor is documented in the hardware design guides.

So my question:

- is this resitor really needed (various options for clock termination can be set in the LPDDR4 memory itself, also the drive strength of the clock can be influenced in the CPU

Thanks for help, 

Raik

[raik_seliger]

Hi Weidong,

when we have a look to the IBIS models of LPDDR4 memories, there is the possibility to active various termination resistors - In the case of the EVK, this internal resistance is deactivated, correct?

E.g. with a micron or samsung memory, you can set up none, 40, 48, 60, 80, 120 or 240 ohm termination resistance directly in the memory.

Isn't there the possibility to use internal 120 ohm instead of a discrete 150 ohm resistor? This may reduce track and stub length and can also increase signal performance.

Raik

[weidong_sun]

Hi Raik,

    I discussed your question with i.MX Expert team. He suggested that the resistor had better not be removed.

B.R

weidong

[weidong_sun]

Hi Raik,

   1. it is a terminal resistor.

    2. It's value is got after emulation and optimization.

so the resistor can't be removed.

Hope the information is helpful to you!

Have a nice day!,

B.R,

Weidong