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SN65CML100: Interfacing with a single ended LVCMOS at 2.5V and 3.3V

Adam Dupuis17
Prodigy 150 points
Part Number: SN65CML100

Hello,

I read through "E2E SN65CML100: Using VBB for single-ended interface"

1. Please confirm that it is enough to simply have a divider using vbb and providing 1.4V at the Pin B, also any recommendations on resistor values?  23.3K and 10k ok? 

2. I've tried to simulate this in Hyperlynx but I ended up having to add a 50 Ohm pulldown on Pin A to remove major noise on the top and bottom of the square wave signal.  Is this an ibis model issue?

3. does the voltage divider threshold of 1.4V need to change for a 2.5V signal?

thanks,

Adam

  • 0
    TI__Mastermind 43795 points

    Hi Adam,

    1. 10K and 23.3K would work to divide the VBB voltage and get 1.4V.  The values will result in a current < 400uA so it is okay to connect them to VBB.

    2. The inputs to the SN65CML100 are high impedance, they do require external termination.  Without the termination the reflections you are seeing are normal for single-ended signals.

    3. It depends on the single ended signal which is driving input A.  Usually a 1.4V threshold voltage applied to input B will work for 2.5V or 3.3V single ended signals which typically have a VIL = 0.8V and a VIH = 2.0V

    Regards,

    Lee

  • 0 in reply to Lee Sledjeski
    Prodigy 150 points
    Thanks Lee,

    1. The datasheet shows for a single ended LVPECL a 50 Ohm termination to VTT, with VTT defined as VCC-2V. Would I just use a R divider to get my termination level or do I need to use the VBB for this?

    2. Hyperlynx for some reason is inverting my signal when I apply a 3.3V LVCMOS signal to PIN 2 (A). If I move it to pin B it looks fine.
    Note I have a divider on the opposite pin using a hyperlynx builtin supply as no VBB pin exists on hyperlynx model of the SN65CML100.
  • 0 in reply to Adam Dupuis17
    TI__Mastermind 43795 points

    Hi Adam,

    1. VBB cannot be used to develop a termination voltage for LVPECL or any other interface.  It is a reference voltage for the input only.  If you are driving LVPECL signals you can either terminate to VCC-2V (VTT).  VTT must be capable to sink current - the VBB connection cannot do this, it is limited to 400uA or less.  If you are driving the signal with a LVCMOS driver, I would recommend a series termination instead of 50 ohms to GND to save power and ensure a full swing at the CML100 input.

    2. I am not sure why this would happen.  Input pin A is the "true" input or "+" input so there should be no inversion.  Using a built-in supply voltage to provide a reference to the other input is good.

    Lee