Libre Computer + X735 V3.0 Module
The GeekWorm-provided X735 scripts are written with the Raspberry Pi in mind, but the X735 V3.0 module works great with the Libre Computer aml-s905x-cc ("Le Potato"). Unfortunately, while they are form compatible, under the hood things vary quite a bit between the two.
Of critical importance is the default values for all the GPIO pins, as well as the GPIO pin numbering. On the Raspberry Pi, GPIO 0 through 8 are connected internally to pull-up by default, while the rest are pull-down; on the S905, GPIO 0-18 are pull-up by default and the rest are pull-down.
Libre Computer provides a great document in Google Sheets that maps all the physical pins of the aml-s905x-cc, and you can then cross-reference the same physical pins on the Pi from any number of sources to determine which physical pin on the new board maps to what logical GPIO pin on Pi.
I've rewritten the fan control for the X735 to make use of the S905 chipset and the Libre-provided dtoverlay, with particular attention to ensuring the PWM generator is hardware-based.
PWM control requires installation of the device tree overlay. The below steps will accomplish this, and are documented in the fan script comments as well.
git clone https://github.com/libre-computer-project/libretech-wiring-tool.git
cd libretech-wiring-tool
make
sudo ./ldto enable pwm-a
sudo ./ldto merge pwm-a
After reading through Geekworm's wiki and experimenting with the board, the X735 V3.0 relies on the default state of pin 38 on the Raspberry Pi to be a 0 (pull down) as it is on the Broadcom chip. Unfortunately, the S905 has a different set of default pull-up/down values for the GPIO pins, and pin 38 is by default a 1 (pull up). In order to make the X735 shutdown script function properly, the S905 pin 38 must default to 0 by attaching it to ground via a 10K resistor (pull down), overriding the internal pull up and allowing the X735 to function as if the board were a Raspberry Pi. Since the software shutdown script works by simply mimicking what a long button press would accomplish on the X735, the same change made for the button (adding a pull-down resistor to pin 38) corrects the software shutdown script as well.
Work in progress.
There's a rather large discrepancy between the documentation provided by Geekworm, the script to shutdown/reboot based on hardware, and the actual behavior of the hardware. The X735 board responds to a button press on pin 38, triggered indirectly by the button press, by either turning off after a period of time following a slow-blink cycle, doing nothing after a fast-blink cycle, or immediately terminating with a long-press. In the first case, we want to shutdown the system, and in the second case, restart. The documentation lists 1-2 second press for a restart, 3-7 for shutdown, and 8+ for immediate termination, while the script is written to restart at 0.6 seconds, and shutdown for anything greater. The actual behavior of the board was observed to be different from both the script and the documentation.
After many cycles of triggering the pin via software with varying delays (in 0.1 second steps), the follwing is the actual behavior of the X735 board:
- Fast blink, no action (restart): up to 1.5 second press
- Slow blink, turn off after delay (shutdown): 1.6-4.0 second press
- Immediate power-off: greater than 4.0 second press