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frequency selection for _clkfreq — Parallax Forums

frequency selection for _clkfreq

I'll post this at top level, as others may wonder this, too.
Video resolution and framerate and audio (ADC) topics have been lively lately, and there have been many references and example code that mention specific _clkfreq settings that seem to be required to make an approach work.

oddly? I'd like to do both things at the same time and some code (Chip's reading 8 ADC) wants _clkfreq = 324_000_000/2 while discussions of higher video resolutions suggested an assortment of frequencies > 300MHz.

Am I expected to adjust the code of one Spin object to handle the _clkfreq desired by the more complicated Spin object? How, otherwise to I choose?


  • cgraceycgracey Posts: 14,112

    That 324_000_000/2 in my code was just me trying out my code at different speeds.

    The P2 is designed to run at any sufficient speed, while NCO's, or numerically controlled oscillators make precise timing for certain applications.

    In the case of HDMI,however, the main clock frequency does dictate the pixel rate. In every other case, there is effective decoupling through use of NCO-based timing.

  • RaymanRayman Posts: 13,711
    edited 2024-01-21 17:10

    I think VGA at 1080p is another exception because of the ~150 MHz dot clock. You really want to run that around 297 MHz (300 can work too, in most cases...).

  • evanhevanh Posts: 15,043
    edited 2024-01-21 17:45

    Contrary to many examples, even when setting for whole multiples, video modes generally don't require a specific dot frequency. Both blanking and scan/refresh rates are flexible within ranges. How flexible those ranges are is a little vague however.

    So, the audio can dictate if you desire it.

    PS: Having said that, VGA analogue signalling for LCDs is less flexible than the equivalent modes over DVI/HDMI signalling. This is due to the display's scan converter having to guess the resolution from the frequency of the sync's of the analogue signalling. Oddball resolutions are not recognised over VGA cables. A far cry from the days of CRT monitors.

  • @cgracey , @Rayman , @evanh OK, thank you. I'm clear on that now.

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