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Using TS3DV642 to switch HDMI ports?

Evan Edstrom
Prodigy 30 points
Other Parts Discussed in Thread: TS3DV642

I wanted to see if I could use the TS3DV642 part to switch TMDS lines between ports.

I.E. a 3-port device that switches two inputs to one output port. There is a similar part from Fairchild, the FSHDMI08 part, which shows an example of what I want to do. However, that part doesn't have as high of bandwidth. I will include a picture of the intended application.

Can I do this with the TI part? Do I need the decoupling caps in series with the TMDS lines? If so, what side should they be on, I understand putting the caps on both sides can allow the part to see negative voltages which would be bad. Or can I simply route each connector to the respective TMDS pins without any caps / pull-ups / etc. Please advise. Thanks!

  • 0
    TI__Mastermind 45575 points
    Evan,

    The TS3DV642 is designed to switch differential high speed signals from a common connection to two separate connections as you have depicted above. The Transition-minimized differential signaling (TMDS) signals are simply differential signals that can be passed through the TS3DV642 as long as they remain within the recommended operating conditions. As you have mentioned putting series capacitors on both sides of the TS3DV642 signal path will center the differential signals 0 V and violate the recommended operating conditions as it will see a negative voltage swing.

    To my knowledge the HDMI spec does not require a series capacitor before the processor as the Display Port specification requires. Let me know if you would like me to move this thread to the consumer and computing interface forum who would have more information on how to interface and terminate HDMI signals according to the specification.

    Thank you,
    Adam
  • 0
    TI__Expert 6335 points

    Evan,

    I have some background supporting our HDMI and DP re-drivers. From what I remember, HDMI signaling is achieved using Current Mode Logic and therefore AC caps should NOT be used.

    The HDMI sink (e.g. TV) has a pullup resistor 'R_T' (nominally 50 ohms) to a termination voltage A_VCC (nominally 3.30V).

    The HDMI source (e.g. Graphics Card) includes a current source 'I_snk'.

    To transmit a High (on D0+ for example), the HDMI source does not pull any current, so the voltage on the channel is A_VCC.

    To transmit a Low (on D0- for example), the HDMI source will pull current I_Snk, so the voltage on the channel drops to A_VCC - R_T*I_Snk.

    The single-ended swing (D0+ or D0-) is expected to be from A_VCC down to A_VCC - R_T*I_Snk. The differential swing (D0+ - D0-) is expected to be from R_T*I_Snk down to -R_T*I_Snk.

    Adding an AC cap prevents the source from pulling down the channel to Low when needed (i.e. The sink would always see A_VCC) and is therefore not advised.

    As Adam mentioned, you can always consult the Consumer and Computing Forum to be absolutely sure.

    Best Regards,
    Nirav