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Raspberry Pi2 Screen issues

This summary captures the "perfect storm" of hardware and software conflicts you are facing. It is designed to be shared with developers or used as a reference for future troubleshooting.

1. Hardware Inventory

  • Computer: Raspberry Pi 2 Model B (BCM2836 SoC).

  • Display: KeDei 3.5" SPI LCD, Version 3.0 (2015/12/01).

    • Critical Fact: This is a "Legacy" board that uses three 74HC595 shift registers to mimic a parallel interface. It is NOT a standard SPI display.

  • Interface: GPIO Pins (SPI Bus 0).

2. Software Environment

  • OS: Raspberry Pi OS "Trixie" (Debian 13) 32-bit with Desktop.

  • Kernel: 6.x (Modern Linux Kernel).

  • Python: 3.13.

  • Drivers: Transitioned from the old sysfs GPIO interface to the modern Character Device (gpiod) and lgpio architecture.

3. The Core Problem: "The Legacy Gap"

The fundamental issue is that 2015 hardware is trying to communicate with a 2025 operating system.

  • GPIO Allocation Conflict: In modern Debian/Trixie, the kernel "claims" pins via the Device Tree. When you try to use RPi.GPIO or luma.lcd, the OS throws an lgpio.error: 'GPIO not allocated' because it views the Python script as an intruder trying to hijack system-reserved SPI pins.

  • Timing Mismatch: The 2015 shift registers require specific, slow timing (latencies) that were easily achieved on old kernels (like Jessie). Modern 64-bit/32-bit kernels use high-speed DMA (Direct Memory Access) for SPI, which sends data faster than the 74HC595 chips can process, resulting in a permanent "White Screen."

  • Driver Deprecation: The original KeDei drivers were binary-blobs compiled for Kernel 3.x/4.x. These are physically incompatible with the Kernel 6.x architecture used in Trixie.

4. Technical Facts for Future Reference

FactDetail
Confirmed Working OSRaspbian Jessie (Legacy Kernel 4.x).
Fail SymptomPersistent White Screen (Backlight on, logic off).
Key Error Messagelgpio.error: 'GPIO not allocated' or ImportError for legacy bitbang classes.
IncompatibilityModern vc4-kms-v3d graphics driver conflicts with SPI GPIO access.
Bus Speed LimitShift registers typically fail above 1MHz to 2MHz on modern Pi hardware.
Pin Mapping (BCM)DC=9, RST=22, MOSI=10, SCK=11, CS=8.

If the "Zero Feed-in" automation is the priority, these are the only viable ways to view your data:

  1. The Web Dashboard: Since you are on a Pi 2, running a lightweight Flask web server allows you to see the status on your phone or laptop without using any GPIO pins for display.

  2. HDMI Status Screen: Since you have the "Desktop" version of Trixie, you can auto-launch a full-screen Python/Tkinter window on your HDMI monitor to show the power stats.

  3. Hardware Upgrade: Replace the 2015 KeDei with a modern I2C OLED (SSD1306) or a Waveshare SPI display that has native driver support in the 2025 Linux kernel.

Would you like me to help you set up the "Auto-Launching HDMI Dashboard" since you already have the Raspberry Pi Desktop environment running?

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