notes/vga.txt


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Notes on GPIO Writing
------------------------------------------------------------------------------
Classic digitalWrite() gives ~500ns pulse time (2MHz)

gpio_write_bit() is about 360ns (2.78MHz)

Writing to GPIO?_BASE is about 60ns (16.6MHz -> 18MHz)

PWM write 0x0001 is about 14ns (72MHz) with prescaler as 0 (!)

VGA Timing
------------------------------------------------------------------------------
1/25.125MHz = 39.72ns (640x480 pixel clock)

Crude 640x480 directions:
    From www.epanorama.net/documents/pc/vga_timing.html
    480 lines
    31.77 us horizontal line length -> 2287.44 clock cycles -> 2287
    3.77 us sync period -> 271 clocks -> 271
    1.89 us front porch? -> 136 clocks -> 136
    25.17 us video -> 1812.24 clocks -> 1812
   
    So... 
    2287 reload 
    271  1: Hsync high
    407  2: Video on
    2219 3: Video off
    2287 4: Hsync low

    Vertically, it's 
    480 lines active video
    11 lines front porch
    2 lines Vsync (low)
    31 lines back porch