NTSC Video Using the 68HC908QY4


Posted on Feb 5, 2014

When configured as an input there is a more subtle problem. If the input has its pullup enabled it could affect the circuitry that is expecting to see an A/D input. But if the pullupis not enabled, then when its A/D input is not selected it will be a digital input that is either left floating or it will be driven by a signal that could be somewhere between 0` and 1`.


NTSC Video Using the 68HC908QY4
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Neither is a good idea. (I chose to not enable pull-ups. ) As I mentioned, since 1993 all TVs having a screen size of at least 13" manufactured for sale in the U. S. have been required to have a Closed Caption Decoder. TVs with smaller screens are not required to have the decoder (and I have not seen a small-screen TV that had one. ) The standards for Closed Captioning can be found in 47CFR15. 119 which can be downloaded by going to: and scrolling down to 15. 119 where you can download it in text or PDF formats. The technical standards for Closed Captioning are in 47CFR73. 682 (a) (22) which can be downloaded by going to and scrolling down to 73. 682 where you can download it in text or PDF formats. (22)(i) Line 21, in each field, may be used for the transmission of a program-related data signal which, when decoded, provides a visual depiction of information simultaneously being presented on the aural channel (captions). Line 21, field 2 may be used for transmission of a program-related data signal which, when decoded, identifies a rating level associated with the current program. Such data signals shall conform to the format described in figure 17 of Sec. 73. 699 of this chapter, and may be transmitted during all periods of regular operation. On a space available basis, line 21 field 2 may also be used for text-mode data and extended data service information. The part that is relevant to us is where it mentions figure 17...




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