🛠 Micro-HDMI : exploiter les limites d'affichage de la BBB

Objectives

Connect a display to the micro-HDMI connector of the BeagleBone Black (TDA19988-class framer driven in mainline by the DRM tda998x driver) while understanding its REAL limitations: maximum mode 1280×1024, 1920×1080 only at 24 Hz with EDID, stereo audio over the same cable. And the pitfall of pin P9_22, which carries the incoming 24.576 MHz audio clock into the SoC.

Hardware

• 1 BeagleBone Black
• 1 micro-HDMI to HDMI cable (the board's connector is fragile: pull straight, no leverage)
• 1 monitor accepting 1080p24 or 1280×1024 (many TVs reject 24 Hz: plan for a PC monitor)

Complete code

# 1. Le connecteur est-il vu par DRM ?
ls /sys/class/drm/
cat /sys/class/drm/card0-HDMI-N-1/status        # connected

# 2. Les modes publiés par l'EDID du moniteur (attendre 1280x1024 et 1080p24)
cat /sys/class/drm/card0-HDMI-N-1/modes | head

# 3. Console framebuffer + visionneuse d'image légère
sudo apt install fbv
sudo fbv /usr/share/backgrounds/*.png

# 4. Session graphique légère (512 MB de RAM : pas de GNOME !)
sudo apt install weston
weston-simple-egl

# 5. Audio HDMI : vérifier que la carte son du framer est là
aplay -l | grep -i hdmi

Expected verification

The DRM status changes to connected, the mode list contains 1280x1024 and 1920x1080 at 24 Hz (no 60 Hz at 1080p: this is hardware). fbv displays the image on the screen, weston-simple-egl runs an EGL triangle.

Common errors

• Demanding 1080p60: the BBB cannot do it (framer limitation, documented in the SRM); TVs that reject 24 Hz result in a black screen — test on a PC monitor.
• Reusing P9_22 as GPIO: it is the 24.576 MHz clock input for HDMI audio into the SoC; the SRM is clear, reusing it as GPIO requires cutting the clock, thus killing HDMI audio.
• Poor quality micro-HDMI cable or forced insertion: the connector is the mechanical weak point of the board.
• Installing a full desktop: with 512 MB of RAM and a 200 MHz SGX530 GPU, the intended use is headless or lightweight weston.
• Looking for sound without EDID: HDMI audio depends on negotiation with the monitor; without valid modes, there is no sound card.