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AM6546: LPDDR4 channel swapping A B

DJ-NG
Genius 9355 points
Part Number: AM6546

Hi champs,

customer need to know if he can  change individual "dual channels" A vs B.

Idea is 2x16bit Dual Rank System 

Channel A = 16bit

Channel B = 16bit

each channel has its own address/control signals.

He wants to connect to two LPDDR4 memories in this way:

Memory IC1 2x16bit Bus

Channel-A -> Memory IC1.Channel-A und IC1.ChipSelect0        A zu A routing

Channel-B -> Memory IC1.Channel-B und IC1.ChipSelect0        B zu B routing

Memory IC2 2x16bit Bus

Channel-A -> Memory IC2.Channel-B und IC1.ChipSelect1        A zu B routing

Channel-B -> Memory IC2.Channel-A und IC1.ChipSelect1        B zu A routing

 

According to layout recommendation from TI this seems not to work:

Channel, Byte, and Bit Swapping

All signals, including data and address/control, must be routed 1 to 1 from the DDR controller to the LPDDR4 memory.

Byte swapping across channels or within a channel is not allowed.

Similarly, data bit swapping across byte lanes or within a byte is also not allowed.

In addition, byte lanes 0 and 1 of the DDR controller must be routed to channel A of the LPDDR4 memory,

and byte lanes 2 and 3 of the DDR controller must be routed to channel B of the LPDDR4 memory.

 

Q: What is background of this statement ? Any ways to realize customers ideas ?

  • 0
    TI__Mastermind 31995 points

    Hi,

    >>customer need to know if he can  change individual "dual channels" A vs B.

    No, please follow the layout guidelines.

    Best regards,
    Kevin

  • 0 in reply to Kevin S
    TI__Genius 9355 points

    Kevin,

    Thanks, but if there is some more explanation, it would help customer.

  • 0 in reply to DJ-NG
    TI__Genius 9355 points

    Hi,

    I added some more explanations from customer.

    His architecture below seems to be important for his design:

    I have a 32bit memory controller (AM65xx) and I want to connect it to a 32bit (2x 16bit dual channel) LPDDR4 memory.

     

    See picture LPDDR4 Memory Dual channel

     

    I want to install two 32bit (dual channel) LPDDR4 RAMs in my design.

    The design recommendation only speaks of a point-to-point connection memory controller to single RAM,

    but what if I want to connect two LPDDR4 memories, one memory to nCS0_A & nCS0_B and the other memory to nCS1_A & nCS1_B.

    Or is it not possible?

     

    Since both channels are independent, the memory controller should not care whether A goes to A or A goes to B.

     

    I don't want to exchange the CA Control signals and the DQ signals, just

    Channel A x16 (CAx_A, DQ_A [0:15) with

    Channel B x16 (CAx_B, DQ_B [0:15]

    Pls check!

  • 0 in reply to DJ-NG
    TI__Guru 50352 points

    DJ,

     

    The question seems to be for “dual rank” (two chip selects) topology where each segment (say DQ[15:0], and similarly DQ[31:16]) is connected to two memories, such as in this diagram (from Micron datasheet):

    There is also hints in the question of channel swapping (which is also not supported, but seems to no longer part of the question).

     

    Whereas this Micron diagram shows 1 package with 4 die, it seems the customer wants to split the 4 die across 2 packages.  Support for Dual Rank (even for 4 dies in one package) has not been validated and is the primary reason we would not recommend/support a dual rank topology.  Going a step further, the PCB layout for supporting two packages would be extremely difficult and it is likely not possible to meet timings at all 3 endpoints (2 memories, 1 SoC) and avoid reflections, etc. 

     

    The documentation is consistent that only a single rank is supported … the TRM states:


     

    And the layout guideline doc (SPRACI2) states:


     

    On the other hand, the TRM does show a mapping from the AC bus to CS1 in the "DDRSS0 AC Bus Mapping" table.  This will be removed in the next revision.


    Regards,

    Kyle


     

  • 0 in reply to kcastille
    Prodigy 20 points

    Dear DJ

    thanks for your replay.

    On the other hand, the TRM does show a mapping from the AC bus to CS1 in the "DDRSS0 AC Bus Mapping" table.  This will be removed in the next Revision.

    Is that true, why?

    Yes I understand

    Note:

    Though LPDDR4 SDRAMs pin out 2 separate channels, independent channel use is not

    supported by this processor. The ADDR_CTRL signals are replicated for load sharing

    purposes and must be connected as shown.

     

    I also don't want to run channel A or B separately, I rather want to run the processor in

    32-bit, single-rank LPDDR4 implementation (No ECC). Reference (AM65x / DRA80xM DDR Board Design and Layout Guidelines, page 24)

    and the Memory Bus Features: Up to 2 ranks (LPDDR4 only) (SPRUID7D–April 2018–Revised June 2019, page 3769)

    complete another LPDDR4 but swap part A and B on the memory so that I can position one memory top and the other bottom.

     

    Schematic as follows

    Best regards

    Michael

  • 0 in reply to Michael Zinke
    TI__Guru 50352 points

    Michael,

    We don't support two discrete LPDDR4 packages connected to one set of Data IOs. 

    Regards,

    Kyle