David,
Thanks for your pointers! In the Raspberry Pi's DPI configuration I set
the amount of horizontal pixels to 844px, which at a pixel clock of
20MHz should result in 19.2kHz if I am not mistaken. Sorry, I forgot to
mention this part.
Regardless of how I think I configured it, you are of course right that
it looks like there is something wrong with the horizontal timing. But
the absoltue value of the pixel clock should not play any role here,
should it, assuming that I calculated the correct horizontal timing?
Best wishes
Jens
On 2022-02-20 8:58 p.m., David Forbes wrote:
Jens,
The horizontal line timing is incorrect. This is why the image is
diagonal.
The numbers that you provided do not specify the horizontal line total
time or frequency.
The monitor requires 52 microseconds or 19.2 kHz.
You need to find a way to specify this, perhaps it is the character
count or pixel count and the pixel frequency.
On Sun, Feb 20, 2022, 6:26 PM jb-electronics
<[email protected]> wrote:
Dear all,
Apologies for the off topic question, but I know there are some
CRT experts in our group and I have been banging my head on this
one for a while.
I picked up a cute 6" monochrome CRT module which accepts TTL
video. This is the timing chart:
My goal is to run this thing using the DPI module of the Raspberry
Pi. But for all of that to work I have to specify certain
parameters and pass them along to the DPI module, and here is what
I read off the sheet:
* h sync: 4.615 us
* h front porch: 2.637 us
* h back porch: 2.637 us
* v sync: 0.208 ms
* v front porch: 0.209 ms
* v back porch: 2.917 ms
* active lines: 256
I have verified that at least the vertical signal timing is
implemented correctly, my scope is a tad too slow to resolve the
horizontal timing. But there appears only gibberish on the screen,
highly distorted text. Here is how it looks like:
http://jb-electronics.de/tmp/screen01.jpg
And this is what it should be (composite video output on my Apple
IIc monitor) for the same "scene":
http://jb-electronics.de/tmp/screen02.jpg
Can somebody tell me if I am on the right track here, or if there
is a fundamental mistake somewhere in the numbers I read off? Your
help is very much appreciated.
Best wishes
Jens
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