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CLanc.1
CLanc.1
Associate II
March 24, 2021
Question

DDR3 33 ohm serial resistor in point-to-point topology

  • March 24, 2021
  • 2 replies
  • 966 views

Hi,

AN5122 in paragraph 3.2 recommends to use 33 ohm serial resistor for every A/C of the DDR chip, but the STM32MP1_SeriesDDRmemory_routing_guidelines_examples.zip in STM32MP15XXAA_1DDR3Lx16 example there are NO serial resistor. Can I use this layout example? Is it tested? or it is better to add 33 ohm resistors. In the latter case A/C includes all addresses plus bank addresses, CKE, CASN, CSN, RASN, WEN and ODT?

Thank you for any suggestions.

This topic has been closed for replies.

2 replies

PatrickF
PatrickF
ST Employee
March 24, 2021

Termination resistors are optional when DDR x16 is used. Without termination resistor, memory should be as close as possible from STM32MP1.

If wires are shorts, serie resistors could be omitted in most cases. In case of signal integrity issues, using DDRPHYC ZPROG[3:0] settings, the output impedance could be increased to 48 ohms.

Regards.

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Tesla DeLorean
Tesla DeLorean
Guru
March 24, 2021

TBH my understanding of series resistors in this context is to manage ringing issues in short traces.

Perhaps you can address with with output impedance or slew rate settings.

What I do know, is that if you omit the option on the board it is hard to retro fit later.

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