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TMDS181: Retimer Mode with lower data rates

Part Number: TMDS181

Dear Team,

we received following question for our customer regarding the TMDS in retimer mode for lower data rates.

The customer is having an issue with a source that seems to generate quite an amount of jitter on its TMDS signals, and this can be greatly reduced when in Retimer mode.

The jitter seems to be so bad at a tmds clock of 111.38 MHz that their FPGA transceivers can not stay locked on this signal. So although the TMDS181 should switch over to retimer mode at 1.0Gbps (= 100 MHz tmds clock) it doesn’ always do so (probably because 111.38MHz is too close to 100MHz?).

When they manually enable the retimer mode, the jitter is greatly reduced and the transceivers can lock.

The question is: can they put the TMDS181 in Retimer mode for all the frequencies, even below 1Gbps?

In the datasheet at the bottom of page 34 there is stated: “When changing crossover point, need to toggle PD_EN or toggle external HPD_SNK” how should we interpret this?

Thanks and best regards
Martin

  • Hi Martin,

    Toggling PD_EN or HPD_SNK is required to update the crossover point, changes are applied when HPD_SNK goes high or PD_EN moves to Normal operation.

    So, if you want to set retimer mode across full data rate range, you still may need to override analog lock, this is to avoid unlocking in small resolutions.
    Write 0x01 to register 0xFF. This will select Page 1
    Write 0x23 to register 0x00. Sets A_LOCK_OVR
    Write 0x00 to register 0xFF. This will select Page 0.

    Regards
  • Hi Moises,

    thanks a lot for the answer.

    Having a follow up question from the customer.

    ---------

    Is the customer's process correct:

    1) Write 2’b11 to bits 1:0 of register 0x0A. This will set the Retimer mode across full range 250 Mbps to 6 Gbps
    2) Toggle PD_EN or HPD_SNK to update the crossover point (is this the same as writing 1’b1 to APPLY_RXTX_CHANGES?)
    3) Write 0x01 to register 0xFF. To select Page 1
    4) Write 0x23 to register 0x00. To set A_LOCK_OVR
    5) Write 0x00 to register 0xFF. To select Page 0

    Steps 3-5 are a bit confusing, since I can’t find anything about this in the TMDS181 datasheet.

    Do I have to run the whole sequence once after startup or do I need to do this every time the TMDS frequency changes?

    ---------

    Thanks and best regards
    Martin
  • Hi Martin,

    The sequence looks good, you just have to apply it once after power up.
    Toggle PD_EN or HPD_SNK is similar as writing 1’b1 to APPLY_RXTX_CHANGES but is not the same they apply different configurations.

    Steps 3-5 are not listed in datasheet, as you can you step 3 changes register page to page 1, these are hidden register used for debug. you just have to know the sequence, move to page one, set A_LOCK_OVR, then return to page 0 to avoid write undesired registers.

    Regards