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Joined: Fri May 22, 2015 12:06 pm

Need help on HDMI timings. HDMI->VGA->RGsB LCD(w. datasheet)

Tue Jun 09, 2015 10:56 am

since some days i am trying to get an old automotive display in my car working with the raspberry pi. The controller needs a RGB signal with a composite synchronisation on the green channel. The Resolution is a tiny 400x240 pixels, so i have to go with manual hdmi timings to drive it. I have also build a little circuit to combine the separate sync signals and overlay them on the green channel. From here:

At this point i am getting something of a kind of a picture. It seems it is synced in vertical and horizontal because it does not roll through the screen. The horizontal lines are somehow sequentially shifted, so the image looks squewed and distorted and the colours are not right.
I am quite happy till now that i got it that far but it is very tiresome to test different timings. I feel like just fooling around with no real clue. I even have the LCD Datasheet , but i dont understand how to read it or to translate the given data to my hdmi timings. I see that it´s typical operating frequncy is 8MHz and that fits somehow the resolution ( 400 + sync) x (240 + sync) x 60 Hz = ~ somewhere around 8MHz :) (depends on the sync duration). But i don´t understand much more.

Now before i spend some more days like a monkey trying various numbers i want to ask for some help on reading the LCD datasheet.
Maybe i would do more monkey work because thats how i usaually get away on problems, but now i am tired and i am not even sure
my little sync-on-green circuit is working properly. So it would be awesome to be at least sure on the hdmi timings before i experiment further.
I am even considering to buy a USB oscilloscope to compare the original car signals to my own ones but i never used an oscilloscope before.

The timings i was using last time, where i actually was able to see kind of something of a picture were:

#h_active_pixels = 400
#h_sync_polarity = 0
#h_front_porch = 8
#h_sync_pulse = 9
#h_back_porch = 90
# * = is 507 pixels per line
#v_active_lines = 240
#v_sync_polarity = 0
#v_front_porch = 2
#v_sync_pulse = 20
#v_back_porch = 10
# * = is 272 lines per frame
#v_sync_offset_a = 0
#v_sync_offset_b = 0
#pixel_rep = 0
#frame_rate = 60
#interlaces = 0
#pixel_freq = 8274240
#aspect_ratio = 6

Hope anyone can give me some hints on this.
If any info is missing here, please tell me.
I am desperate :) Thanks

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