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What is on the output pins in TV (composite video) mode? — Parallax Forums

What is on the output pins in TV (composite video) mode?

Cluso99Cluso99 Posts: 18,069
edited 2011-08-31 14:37 in Propeller 1
Are the output pins in TV mode (NTSC and Pal) still purely digital switching between 3V3 and Gnd?

I realise that they are phase shifted, etc. Is the pulse width being used to create an analog voltage?

My question arises because I want to be able to switch them between purely digital I/O pins and TV out pins.

My preference is to use 74LVCxx gates to switch, but maybe I need to use an analog switch IC. If I need to use an analog switch, any suggestions?

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-08-30 14:59
    Are the output pins in TV mode (NTSC and Pal) still purely digital switching between 3V3 and Gnd?

    Yes.
    Is the pulse width being used to create an analog voltage?

    No. The analog voltage is purely a product of the resistor DAC.

    -Phil
  • Cluso99Cluso99 Posts: 18,069
    edited 2011-08-30 16:03
    Thanks Phil. I was hoping you might chime in to confirm what I thought was the case.
  • ericballericball Posts: 774
    edited 2011-08-30 18:14
    Cluso99 wrote: »
    I realise that they are phase shifted, etc. Is the pulse width being used to create an analog voltage?

    The phase shift is just a change in the output on a sub-pixel frequency. No pulse width / duty analog coding unless you're using one of the one-pin drivers. I'm not sure you even need a switch, see http://forums.parallax.com/showthread.php?133688-VGA-TV-DAC-using-resistor-arrays-(and-other-uses)&highlight=resistor+network
  • Cluso99Cluso99 Posts: 18,069
    edited 2011-08-30 18:21
    ericball: Oh yes, I need a switch/gate as the pins are used for other things such as I/O (not VGA). - I am planning to reuse the Eeprom pins ;)
  • TorTor Posts: 2,010
    edited 2011-08-31 02:23
    I have a feeling the most welcome change in P2 is the larger number of pins.. everything else is nice to have, but secondary! :)

    -Tor (following another project which is constantly suffocating from if-there-were-just-a-few-pins-more..)
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2011-08-31 06:30
    I used a bi-lateral switch (4053) on the EEPROM pins so that it would be bi-dirrectional, easily. If they were used for the Vid or VGA circuits their inherent internal resistance (especially at 3.3V) might have to be taken into account within the resitance DACs if accuracy was desired.

    (But then the impirical values do not give a true 75 Ohm feed as has been documented before)
  • Dr_AculaDr_Acula Posts: 5,484
    edited 2011-08-31 07:10
    I've pondered reusing the eeprom pins as well. In fact, I have some boards that have just arrived and I must solder them up and test it works. 4066 gates that disconnect the eeprom after bootup, with a delay that is reset either by a power up or a reset pulse.
  • Cluso99Cluso99 Posts: 18,069
    edited 2011-08-31 14:37
    I gate the SCL pin on the RamBlade to ensure the Eeprom does not get confused and reply to a sequence on the bus.
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