See

zcrc explained to me how to define display timing values:

your screen's datasheet should give you the values for the front porch / back porch and clock frequency to use
lildurdenYesterday at 9:55 PM
Datasheet gives me values to use for 720x1280
At least vertical and horizontal synchronization are wrong on it.
zcrcYesterday at 10:09 PM
ah
the panel is 640x480, right?
lildurdenYesterday at 10:09 PM
Yes
zcrcYesterday at 10:09 PM
Ok. So let me explain the theory
You want to solve this equation:
htotal * vtotal * bytes_per_pixel * 60 == (432 MHz / X)
X being an integer
is it a 16-bit or 24/32-bit panel?
with htotal = hdisplay + h_front_porch + hsync + h_back_porch
and vtotal = vdisplay + v_front_porch + vsync + v_back_porch
lildurdenYesterday at 10:13 PM
24bit
zcrcYesterday at 10:14 PM
ok
So that means:
(hdisplay + h_front_porch + hsync + h_back_porch) * (vdisplay + v_front_porch + v_back_porch) == 432M / (4 * 60 * X)
you know hdisplay and vdisplay, these are 640 / 480
you just need to find values that work for the other ones
Now, a possible solution for this equation would be htotal = 800, vtotal = 600
because 800 * 600 == 432.000.000 / (4 * 60 * 15)
X = 15 and it's an integer, so that works
That means: 640 + h_front_porch + hsync + h_back_porch == 800
and 480 + v_front_porch + vsync + v_back_porch == 600
lildurdenYesterday at 10:21 PM
Ok I think the values I choose are close  to this
zcrcYesterday at 10:21 PM
so:
h_front_porch + hsync + h_back_porch == 160
v_front_porch + vsync + v_back_porch == 120
The actual values you pick don't matter that much, as long as this equation is respected. The panel itself will support a range of values for the front/back porches and h/v sync
if you use values that are too small / too big then it may not work correctly, but in general the panels don't care that much
If you wonder why that equation should be respected: this gives you a perfect 60.00 Hz frame rate
lildurdenYesterday at 10:24 PM
ok that's clear!
in fact my values are 4 times too big to solve the equation
Can I vary X?
zcrcYesterday at 10:28 PM
yes
lildurdenYesterday at 10:31 PM
great, thank you very much, I'll work on it.
another question, last one, how do I choose .clock parameter?
zcrcYesterday at 10:58 PM
800*600*60
so 28,8 MHz aka 28800 kHz
so .clock = 28800
lildurdenYesterday at 11:06 PM
Great, I wait compilation ends then will be able to try



Final values