• Product page: https://libre.computer/products/aml-s905x-cc/
• Pin definitions: https://docs.google.com/spreadsheets/d/1U3z0Gb8HUEfCIMkvqzmhMpJfzRqjPXq7mFLC-hvbKlE/edit#gid=0
• Schematic: s905x_6l_170707_v1_0.pdf
• S905X Datasheet: s905x_datasheet_v0.3-20170314-wesion.pdf (There is almost certainly a newer version, but this is the one I see the most floating around on the web)

The UART on the 40pin GPIO header (pins 8 and 10) is not enabled by default. It must be enabled by enabling a device tree overlay that is included with libretech-wiring-tool.

1. Clone, build, and install the libretech-wiring-tool
2. Enable the DTO for UART-A sudo ldto enable uart-a
3. Check dmesg c11084c0.serial: ttyAML6 at MMIO 0xc11084c0 (irq = 49, base_baud = 1500000) is a meson_uart
4. Test with minicom or whatever
5. To persist after reboot, merge: sudo ldto merge uart-a

Somewhat confusingly, both uart-a and uarta exist as device tree overlays. uarta maps to uart-a-rts-cts which uses pins 16 and 18 for hardware flow control.

Specifically, SDA and SCL pins on GPIOs 3 & 4.

1. Clone, build, and install the libretech-wiring-tool as above
2. Install i2c tools: sudo apt install i2c-tools
3. Enable the i2c-ao DTO: sudo ldto enable i2c-ao
4. Check for your devices: i2cdetect -y 1 (DS3231 should be at 0x68)
5. Enable for DS3231 on i2c-ao: sudo ldto enable i2c-ao-ds3231
6. Check dmesg:

   [ 4024.485534] rtc-ds1307 1-0068: SET TIME!
   [ 4024.488196] rtc-ds1307 1-0068: registered as rtc0
   [ 4024.489394] rtc-ds1307 1-0068: setting system clock to 2000-01-01T01:07:25 UTC (946688845)

7. Merge to save: sudo ldto merge i2c-ao ; sudo ldto merge i2c-ao-ds3231