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docs: add DS3231 RTC to Hardware section (BUG-18)

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Mathieu Moran
2026-08-15 18:46:51 -04:00
parent 7a9be9d63f
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PLAN.md
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@@ -29,6 +29,13 @@ pick up the project without needing prior chat history.
- **Power**: Hard-wired 5V/GND directly to GPIO header (red to Pin 2, black to Pin 6), bypassing - **Power**: Hard-wired 5V/GND directly to GPIO header (red to Pin 2, black to Pin 6), bypassing
the worn micro-USB connector. `vcgencmd get_throttled` returns `0x0` — no undervoltage or the worn micro-USB connector. `vcgencmd get_throttled` returns `0x0` — no undervoltage or
throttling. Pi runs at full 1.2 GHz (was ~600 MHz downclocked before the fix). throttling. Pi runs at full 1.2 GHz (was ~600 MHz downclocked before the fix).
- **RTC**: **DS3231** module (BUG-18, installed 2026-08-14) on the free I²C bus `i2c-1` — SDA
pin 3, SCL pin 5, VCC pin 1 (3.3V, *not* 5V), GND pin 6. ZS-042 board's charge diode removed
for the non-rechargeable CR2032 backup cell. Enabled via `dtparam=i2c_arm=on` +
`dtoverlay=i2c-rtc,ds3231` in `/boot/firmware/config.txt`, `fake-hwclock` disabled, `i2c-dev`
in `/etc/modules-load.d/i2c-dev.conf`. Probes at `0x68` (driver → `UU` in i2cdetect), 24C32
EEPROM at `0x57`; power-cycle test boots with the correct time (was `2000-01-01`). SIM868
HAT's CR1220 also installed and its RTC set via `AT+CCLK` as a secondary backup.
- **Car**: 2013 Toyota Yaris. - **Car**: 2013 Toyota Yaris.
### SIM868 HAT bring-up notes (important gotchas) ### SIM868 HAT bring-up notes (important gotchas)