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Notes on GPIO Writing
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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 30ns (33MHz) with prescaler as 1 (default)
pwm write 0x0001 is about 14ns (72MHz) with prescaler as 0 (!)
VGA Timing
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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
Currently, setting vs. clearing GPIO registers seems to take a different amount
of time? Or perhaps i'm not analyzing branching correctly. Regardless, if you
SET 100x times then UNSET on one line, then UNSET 100x then SET the next line,
the two changes in color will generally not line up.
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