steffensq
Posts: 6
Joined: Thu Oct 22, 2015 4:07 pm

HDMI Hot Plugging

Wed Oct 24, 2018 8:22 pm

Hi,

I'm creating a HDMI to TTL daughter board for a LCM.
The LCM in question has a custom resolution (480x640)

Config is set as follows:

framebuffer_width=480
framebuffer_height=640

hdmi_group=2
hdmi_mode=87

hdmi_cvt= 480 640 60 1 0 0 0

hdmi_force_hotplug=1

When plugging into a monitor directly, before connecting to the daughter board, I see "Input Not Supported" as expected.
I can then remove the HDMI cable from the monitor and connect to the daughter board and see a stable image.

Otherwise, connecting to the daughter board directly and performing a power cycle, the Pi3+ does not reach the desktop. It just continues to cycle through the boot process.

Changing hdmi_force_hotplug does not seem to have any effect.

My question is, may this be related to either the GPU allocated memory or the hot-plug trigger for the HDMI interface?

Thank you,
Steffen

aBUGSworstnightmare
Posts: 1491
Joined: Tue Jun 30, 2015 1:35 pm

Re: HDMI Hot Plugging

Thu Oct 25, 2018 5:12 am

I'm pretty sure you don't have an EDID EEPROM connected to HDMI_SCL and HDMI_SDA signals, right?
But a common pitfall is that people forget to add the pull-ups for these signals; they were needed anyhow.

If you don't have pull-ups on the HDMI I2C interface add 1k8Ohm to 5V.
As this is a custom board, why not add a 100kOhms pull-up to 5V to HDMI_Hotplug signal (no19 on HDMI connector)?

Note: I always recommend to use HDMI_timing line, rather than what you have. HDMI_timings will allows you to fully adjust display timing as per spec. Details on the timing are here https://www.raspberrypi.org/documentati ... /README.md; procedure is the same for HDMI and DPI.

steffensq
Posts: 6
Joined: Thu Oct 22, 2015 4:07 pm

Re: HDMI Hot Plugging

Thu Oct 25, 2018 3:18 pm

I'm pretty sure you don't have an EDID EEPROM connected to HDMI_SCL and HDMI_SDA signals, right?
But a common pitfall is that people forget to add the pull-ups for these signals; they were needed anyhow.
Correct, I typically only work with one resolution at a time so was under the impression this is not required.
If you don't have pull-ups on the HDMI I2C interface add 1k8Ohm to 5V.
As this is a custom board, why not add a 100kOhms pull-up to 5V to HDMI_Hotplug signal (no19 on HDMI connector)?
I originally had a 10kOhm pull-up between pins 18 (5V) & 19 (Hot Plug Detect).
I've removed the pull-up to see if there would be any effect and it appears the symptoms are identical. I intend on re-testing to confirm.
Note: I always recommend to use HDMI_timing line, rather than what you have. HDMI_timings will allows you to fully adjust display timing as per spec. Details on the timing are here https://www.raspberrypi.org/documentati ... /README.md; procedure is the same for HDMI and DPI.
I used hdmi_timing previously and did not see expected results. I have the spec and calculated the timing, entered in the values accordingly and all I see is a blank screen. Plugging into a monitor shows "No Input Detected" which leads me to believe the HDMI is disabled if the timing is incompatible.

hdmi_cvt is the only custom mode that shows an image. An alternative option was using the CEA or DMT VGA options and using the display_hdmi_rotate to get the correct resolution, but this was not precise due to overscan that I was having trouble correcting. I would much prefer to use the hdmi_timing command and plan to look into it further once I resolve the hot plug issue.

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