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tvp5151 channel B startup issues

Keith Williams71
Prodigy 70 points
Other Parts Discussed in Thread: TVP5151

Hi,

We’re seeing an issue on one of our products which uses the TI TVP5151 video capture device.

 

If we connect a video source to input channel B on the IC and nothing on channel A, when we power cycle our product (SW set to look at channel B) we get a completely white image for the display. Looking at the registers in the device it shows no lock for channel B even though there’s a video source connected.

 

If we repeat the test but this time with video sources on both channel A and B and have software power up looking at B again it works fine.

Powering up with SW looking at channel A always works.

 

Do you know of any sync/locking issues with channel B when there’s no video on channel A? or is there any errata on this device?

  • 0
    TI__Intellectual 1700 points
    We have external clamp diodes on the video input signals to protect the TVP5151 against overvoltage on those pins. Removing our clamp diodes fixed our video startup issues.

    Question:

    Do we need the external clamp protection or does the TVP5151 already have some protection built in?

    Thanks, Carlo.
  • 0 in reply to Carlo Marsano
    TI__Mastermind 43875 points
    Carlo,
    The external protection requirements are entirely up to the customer's requirements.

    There is some degree of ESD protection provided directly on the device and this is indicated in the TVP datasheet. If a higher level of protection is needed then external protection will need to be added in order to meet those higher requirements.

    Having said this though external ESD protection should not cause any issues at all. All I can think is that the external protection was applied incorrectly thus causing the input signal to be crippled.

    I suggest measuring the signal AT THE TVP INPUT with an oscilloscope both with and without the ESD protection to see if you can determine why the ESD protection is causing a difference.

    BR,
    Steve
  • 0 in reply to Steve Clynes
    Prodigy 70 points
    Hi Steve,
    The diodes are BAT54SW and are used purely to protect the input from overvoltage, not ESD. I've since changed them to BAV99 and the problem has gone away, but we were wondering in we could get rid of them completely? i.e. does the TVP5151 already have built-in protection for overvoltage on the video-in pins?
    thanks,
    Keith.
  • 0 in reply to Keith Williams71
    TI__Mastermind 43875 points
    A regular diode is not the correct thing to use here since the forward voltage is too low. These will corrupt the signal.

    A better solution is to use a zener at the resistor divider input rather than the TVP input. This will allow a higher value zener (say 3V to 4V) and introduce less distortion.

    Over voltage is not usually a big issue since the input is AC coupled.

    Typical ESD protection diodes usually give sufficient protection from DC over voltage and ESD too.

    BR,
    Steve
  • 0 in reply to Steve Clynes
    TI__Intellectual 1895 points
    Hello,

    In regards to your question on channel A and B selection, this device only decodes one composite or one S-Video input at a time, however a composite input can be connected to AIP1A and AIP1B. An S-Video could also be applied as indicated in Table 3-12. For this case with 2 composite inputs, the decoder LOCKS (horizontal and sub-carrier) TO THE SELECTED INPUT. For proper internal decode/processing and to output correct H and V timing, the correct input must be selected. The device does not auto select if only one input is applied. Please see section 3.21.1 of datasheet " Video input source selection #1 register" If you are using channel B only then you must change the address of this register to "10". This register is set to default channel A at 00.

    Thank you
    Faizul Bacchus