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LMH1226: CDR Reset and Release Requirement

Kawai
Guru 19775 points
Part Number: LMH1226
Other Parts Discussed in Thread: LMH1219, LMH1218

Hi Team,

I see below NOTE in the LMH1226 datasheet. Could you please help us understand the CDR Reset and Release requirement ?

Q1). I understood that LMH1226 will start CTLE adaptation and CDR lock process when it loses input signal (i.e. loses lock).  What would happen when the input video data is actively changed with a routing switcher ? My thought was that since the device has false lock protection function, it would start new adaptation process even changed between SMPTE data rates.

Q2). If the data rate is changed from SMPTE data rate to Ethernet, I believe the device would lose lock and starts a new adaptation/lock process automatically. So, the device would not require CDR Reset and Release. What do you think ?

Q3). Do you really need this CDR Reset and Release process every time after input data rate is changed actively (No loss of signal) ? Customer requreiment is to be used stand alone.

Q4). How long would it take for a deivce to detect "loss of signal" when input signal is disconnected ?

Best Regards,

Kawai

  • 0
    TI__Guru* 92320 points
    Hi Kawai-San,

    Q1). When data rate changes - even without loss of signal - CDR loses lock and restarts lock process - as you noted.

    Q2). Please bear in mind, to enable Ethernet or 10GbE, either through pin or SMBus we have to enable 10GbE - this changes rate/sub-rate table setting to enable the device to lock to 10GbE. If we don't enable 10GbE through pin or SMBus then device is not going to lock since it is optioned for SMPTE.

    Q3). Not really. CDR reset and release is not needed after each rate change. We require loss of signal because this could speed up lock process(more details below).

    Q4). Loss of signal can be detected in less than 100uS.

    In summary, when data rate is changed and there is no loss of signal, it takes about 1mS for device to recognize loss of lock. and start lock process. On the other hand, if there is loss of signal it takes under 100uS for the state machine to see this and start lock process. So loss of signal reduces overall lock time.

    Regards,,,nasser
  • 0 in reply to Nasser Mohammadi
    Guru 19775 points

    Hi Nasser-san,

    Thank you for your comments.

    Q5). In conclusion, can we understand that the device does not need CDR Reset and Release when input video format / data rate changes between SMPTE data rate or between 10G/1G Ethernet ?

    Q6). If rate setting is changed through IN_OUT_SEL pin (no SMBus/SPI connection), do you need additional step to achieve CDR Reset and Release process ?  

           When input signal changes from SMPTE to Ethernet (or on the other way), device is required to change the rate setting.
           If rate setting changed through register, I agree one should issue CDR Reset and Release.

    Best Regards,

    Kawai

  • 0 in reply to Kawai
    TI__Guru* 92320 points
    Hi Kawai-San,

    Q5). To re-lock there is no need for CDR reset and release.

    Q6). For LMH1219 or LMH1226, IN_OUT_SEL automatically engages reset since we are going from SMPTE to 10GbE & rate/sub-rate needs to be changed internally so there is no need for reset.

    Regards,,,nasser
  • 0 in reply to Nasser Mohammadi
    Guru 19775 points

    Hi Nasser-san,

    I am clear now.

    Just for clarification, when would be the case that you need the CDR Reset and Release which is describe in this NOTE ?

    If it isn't needed, how about deleting this description from the datasheet since it is confusing ?

    Best Regards,
    Kawai

  • 0 in reply to Kawai
    TI__Guru* 92320 points
    For fast lock time, then loss of signal would speed up the lock time process.

    Fast lock time, is one of our main advantages and this is why we put this in data sheet. If you notice, starting from LMH1218 we mention fast lock time as one of our advantage versus competition.

    Regards,,nasser
  • 0 in reply to Nasser Mohammadi
    Guru 19775 points
    Hi Nasser-san,

    I understood that CDR Reset and Release could be used to fasten lock time, probably for about a few hundred us.

    Best Regards,
    Kawai