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Sungjin's avatar
Sungjin
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8 months ago
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Assistance Required: Persistent Oscillation Issues on MAX10 LVDS Receiver Inputs

Hello, I'm currently working with an Intel MAX10 device configured with a 10-lane LVDS interface. However, I’ve been struggling with persistent oscillations occurring at the LVDS receiver inputs. ...
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  • FvM's avatar
    FvM
    8 months ago

    Hi,
    I believe the answer is implementation dependent. Adding a DC bias to enforce a 0 or 1 level for floating LVDS inputs would reduce performance, e.g. increase minimal signal level and add an asymmetrical time shift of input signal, reducing effective skew margin. Both effects reduce maximal cable length. Common mode bias is required in AC coupled input circuit, but differential bias is rarely applicable for high speed differential signaling.

    When receiving an open LVDS input signal with 10b8b decoder, we still get a certain rate of false correct frames. An additional time filter detects stable input signal.

Sungjin's avatar
Sungjin
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8 months ago

Thank you, Frank.

It was my mistake to assume that the MAX 10 device includes an internal fail-safe function.

I apologize for the incorrect information.

That said, I still have a practical issue to resolve.

According to the MAX 10 documentation, floating LVDS inputs require biasing,

but aside from the standard 100-ohm termination resistor, no specific bias resistor values are given.

As a rule of thumb, a <10kΩ – 100Ω – 10kΩ> biasing network can often be used to provide fail-safe behavior in many cases.

And using pull-up or pull-down resistors below 10kΩ is generally not recommended, as it may result in excessive current flow.

So, In our setup, this network creates only about a 10 mV voltage difference between the positive and negative LVDS inputs,

which may not be sufficient to establish a clear logic state.

Conclusion : When designing a practical fail-safe circuit, we must consider the risk of malfunction, even after fail-safe biasing.

Is this resonable conclusion? Please correct me if I'm misunderstood.