dom
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 5138
Joined: Wed Aug 17, 2011 7:41 pm
Location: Cambridge

Re: STICKY: HOWTO: Create Custom HDMI modes

Wed Apr 04, 2018 3:43 pm

steinerlein wrote:
Wed Apr 04, 2018 11:10 am
So far so good, the backlight switches on and it doesn't seem to fail right away. However, I am having some issues with displaying an image on the display. It always shows only the topmost line. My suspicion is that the pixel clock frequency is too high for the display. The datasheet specifies 9Mhz typ and 15Mhz max.
You do seem to be hitting a firrmware bug when frequencies below oscillator frequency (19.2MHz) come out as zero.
I have a test firmware you could try that allows your hdmi_timings line to boot with what appears to be the correct frequency:

Code: Select all

$ vcgencmd measure_clock pixel
frequency(29)=9600000
Can you test it?

EDIT: This fix is now included in rpi-update firmware.

steinerlein
Posts: 9
Joined: Thu Mar 21, 2013 4:59 pm

Re: STICKY: HOWTO: Create Custom HDMI modes

Mon Apr 09, 2018 9:07 am

dom wrote:
Wed Apr 04, 2018 3:43 pm
steinerlein wrote:
Wed Apr 04, 2018 11:10 am
So far so good, the backlight switches on and it doesn't seem to fail right away. However, I am having some issues with displaying an image on the display. It always shows only the topmost line. My suspicion is that the pixel clock frequency is too high for the display. The datasheet specifies 9Mhz typ and 15Mhz max.
You do seem to be hitting a firrmware bug when frequencies below oscillator frequency (19.2MHz) come out as zero.
I have a test firmware you could try that allows your hdmi_timings line to boot with what appears to be the correct frequency:

Code: Select all

$ vcgencmd measure_clock pixel
frequency(29)=9600000
Can you test it?

EDIT: This fix is now included in rpi-update firmware.
I didn't give an update last week, but I have gotten the pixel clock clock to be what I want. With these settings in config.txt I can measure 9Mhz on the clock line.
hdmi_ignore_edid=0xa5000080
hdmi_timings=480 0 2 41 43 272 0 2 12 2 0 0 0 60 0 9000000 3
hdmi_group=2
hdmi_mode=87
In hindsight, the display simply didn't turn on with 9.6Mhz pixel clock, but it did with 9Mhz.
I am now experiencing another problem: The display turns on fine and then slowly goes black around the edges, much like vignetting in photography. I am not sure what the cause is.. I'd be thankful for any hints!

steinerlein
Posts: 9
Joined: Thu Mar 21, 2013 4:59 pm

Re: STICKY: HOWTO: Create Custom HDMI modes

Thu Apr 12, 2018 11:05 am

hey dom

I am posting here again, because I am not having much success with the display I am trying to drive.
I'm pretty certain that the timings are not correct yet. Here is what is provided in the datasheet:

Code: Select all

Item			MIN	TYP	MAX	Unit
––––––––––––––––––––––––––––––––––––––––––––––––––––––––
DCLK frequency: 		9	15	Mhz
DCLK Period:		66.7ns	111.1		ns
Horizontal frequency: 		17.14		KHz
Vertical frequency:	50	59.94	70	Hz
H Pulse Width:		2	41	41	DCLK
H Back Porch:		4	43	82	DCLK
H Front Porch:		2	2	82	DCLK
H Blank Period HBP+HFP:	45	45	125	DCLK
H Display Area:		480	480	480	DCLK
H Cycle:		525	525	605	DLCK
V Pulse Width:		1	10	11	HCycle
V Back Porch:		2	12	22	HCycle
V Front Porch:		1	2	227	HCycle
V Blanking Period VBP+VFP:	14		HCycle
V Display Area:		272	272	272	HCycle
V Cycle:		285	286	399	HCycle
––––––––––––––––––––––––––––––––––––––––––––––––––––––––
And these are the timings I am using right now:

Code: Select all

hdmi_timings:480 0 2 41 43 272 0 2 10 12 0 0 0 60 0 9000000 3
Unfortunately, the image quality is very bad:
photo_2018-04-12_13-02-21.jpg
photo_2018-04-12_13-02-21.jpg (135.77 KiB) Viewed 226 times
photo_2018-04-12_13-02-18.jpg
photo_2018-04-12_13-02-18.jpg (123.35 KiB) Viewed 226 times
As you can see, the image has horizontal striping and gets darker from the outside inwards. This darkening effect happens over time and can get "inverted" after a reboot of the pi.

I'd be thankful for thoughts or ideas on how to improve this.

frankievalentine
Posts: 1
Joined: Fri May 04, 2018 5:32 am

Re: Custom HDMI modes

Fri May 04, 2018 5:37 am

shookis wrote:
Sat Apr 01, 2017 10:10 pm
darseneau wrote:
dom wrote: 2560x1080 is not possible. 1920x1200 is the maximum resolution supported.
Is that a firmware limitation at present, as I had supposed, or a hardware limit of the gpu?
I have an LG 2560x1080 display and I was able to get it working using the following:
hdmi_timings=2560 0 48 32 80 1080 0 7 20 12 0 0 0 52 0 159838855 7
hdmi_group=2
hdmi_mode=87
hdmi_drive=2
framebuffer_width=2560
max_framebuffer_width=2560
framebuffer_height=1080
hdmi_pixel_freq_limit=160000000

I know this hdmi_timings from a somewhat painful experience in getting the same display to work with a somewhat older mac mini (mid 2011) which didn't support it out of the box.

Thank you!!!! You saved me with this one shookis. Much appreciated for posting these setttings.

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