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LMK04610: LMK04610 CLKOUTx drive mode vs IBIS options

Paul Buijsman
Prodigy 200 points
Part Number: LMK04610

Hi,

I hope someone can help me with something confusing that I found in LMK04610 datasheet for the CLKOUTx drive mode and the related options for IBIS file clkout mode.

Datasheet chapter 7.10 describes 4 output options: 4 / 6 / 8 mA HSDS and 16 mA HCSL.

Register map description shows 5 options (e.g. register 0x34 OUTCH1_DRIV_MODE): 4 / 6 / 8 mA HSDS and 8 / 16 mA HCSL (respective settings are  0x10 / 0x14 / 0x18 / 0x3B / 0x3F)

IBIS file shows 7 options:

clkout_cml 1.8V digital Output |16mA HCSL/CML configuration Rload 100ohms
clkout_hsds_8m 1.8V digital Output |8mA HSDS configuation Rload 100ohms
clkout_hsds_6m 1.8V digital Output |6mA HSDS configuation Rload 100ohms
clkout_hsds_4m 1.8V digital Output |4mA HSDS configuation Rload 100ohms
clkout_25_8m 1.8V digital Output |8mA HSDS configuation Rload 25ohms
clkout_25_6m 1.8V digital Output |6mA HSDS configuation Rload 25ohms
clkout_25_4m 1.8V digital Output |4mA HSDS configuation Rload 25ohms

* What does Rload 100ohms or Rload 25ohms in the above IBIS file table actually mean? Shouldn't HSDS always be terminated with 100ohms?

* Can someone please explain which IBIS mode belongs to which OUTCHx_DRIV_MODE setting? clkout_cml is obvious, but how about the others?

* Why is HCSL/CML 8mA setting missing in IBIS options?

* In theory 64 different settings (0x00 - 0x3F) are possible for OUTCHx_DRIV_MODE. Only 6 of them are described in the datasheet. What would be the result if I would use a non-described value? Would that e.g. enable the "Rload 25ohms" option that I can't find elsewhere in the datasheet?

Best regards,

Paul

  • 0
    TI__Mastermind 28960 points

    Hello Paul,

    1. Rload 100Ω or Rload 25Ω describes the internal termination used in the driver output stage. I am equally puzzled by why something like a 25Ω Rload mode exists for the HSDS driver, especially since it appears to be the only available settings disclosed for the HSDS driver. I am looking into the origins for these settings, and I will post another response when I have more information.
    2. The HSDS IBIS modes with Rload = 25Ω correspond to the HSDS modes available in the datasheet.
    3. Not sure why HCSL/CML 8mA is missing in the IBIS options. Looking into it.
    4. Please note that settings which are not documented in the datasheet do not have corresponding electrical characteristics, so all settings may not be valid. But generally:
      1. OUTCHx_DRIV_MODE[5:4] sets the output buffer type. 00 = off; 01 = HSDS; 11 = HCSL; 10 = Reserved (unused)
      2. OUTCHx_DRIV_MODE[3:2] sets the driver strength. 00 = 4mA; 01 = 6mA; 10 = 8mA; 11 = 16mA
      3. OUTCHx_DRIV_MODE[1:0] sets the internal termination. 00 = 25Ω; 01 = 50Ω; 10 = 100Ω; 11 = open

      The only modes listed in the datasheet are modes where internal termination is set to 00 (25Ω). So using a non-described value is possible, but at this time discouraged, since the performance characteristics are not specified.

    Regards,

  • 0 in reply to Derek Payne
    Prodigy 200 points
    Hi Derek,

    Thanks for your quick and clear answer.
    Apart from the fact that I think you swapped the definition of OUTCHx_DRIV_MODE[3:2] and OUTCHx_DRIV_MODE[1:0] the settings make much more sense now. At least I know now how to relate our SI analysis results to a specific setting.

    In hindsight a clue could be found in the OSCOUT_DRV_MODE description (register 0x31). But there the encoding of output buffer type differs from the OUTCHx_DRIV_MODE buffer type encoding. So that put me at the wrong foot when I tried to relate OSCOUT setting to OUTCHx setting. Can you confirm that indeed the encoding for output buffer type is different for OSCOUT and OUTCHx (seems a bit unlogical to me)?

    Any extra info on this is appreciated.

    Best regards,
    Paul
  • 0 in reply to Paul Buijsman
    TI__Mastermind 28960 points

    Hi Paul,

    You're absolutely right, I got OUTCHx_DRIV_MODE[3:2] and [1:0] swapped. [3:2] controls output current, [1:0] controls internal termination. I'll edit my response above to help anyone referencing this thread in the future.

    The OSCOUT_DRV_MODE settings also include an LVCMOS buffer type, and the encodings for buffer type are not equivalent between OSCOUT and OUTCH registers. There are other discrepancies according to the datasheet, such as XX11XX encoding HSDS to 8mA but encoding HSCL to 16mA, or all OSCOUT HSDS terminations set to open. The discrepancies are peculiar, but they do exist.

    Updates on previous questions: from a technical standpoint, there is nothing preventing the use of the other valid configurations for OUTCHx_DRIV_MODE. The values presented in TICS Pro and in the datasheet give the best performance for the cases tested in the datasheet, and the exclusion of other terminations looks to be more about release scheduling than about performance differences. The same reasoning seems to apply for the choice of cases in the IBIS model. It could be a worthwhile improvement in the future to add more output formats to the IBIS model, but for now the modes present are what we have available.

    Regards,