diff options
Diffstat (limited to 'notes/vga.txt')
| -rw-r--r-- | notes/vga.txt | 26 |
1 files changed, 18 insertions, 8 deletions
diff --git a/notes/vga.txt b/notes/vga.txt index 2230f57..d75281a 100644 --- a/notes/vga.txt +++ b/notes/vga.txt @@ -1,32 +1,42 @@ -classic digitalWrite() gives ~500ns pulse time (2MHz) +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) +Writing to GPIO?_BASE is about 60ns (16.6MHz -> 18MHz) -pwm write 0x0001 is about 30ns (33MHz) +pwm write 0x0001 is about 30ns (33MHz) with prescaler as 1 (default) pwm write 0x0001 is about 14ns (72MHz) with prescaler as 0 (!) -1/25.125MHz = 39.72ns +VGA Timing +------------------------------------------------------------------------------ +1/25.125MHz = 39.72ns (640x480 pixel clock) -crude 640x480 directions: - www.epanorama.net/documents/pc/vga_timing.html +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... + So... 2287 reload 271 1: Hsync high 407 2: Video on 2219 3: Video off 2287 4: Hsync low - vertically, it's + 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. + |