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AGENTS.md — PiClock

This document is LLM-generated.

Guidance for AI agents (and humans) working in this repo. Read this before making changes.

What this is

An always-on digital bedroom clock built from a Raspberry Pi Zero W (or Zero 2 W) driving an Adafruit 1.2" 4-Digit 7-Segment Display with I2C Backpack (HT16K33). It lives in a 3D-printed housing. Design goals, in priority order:

  1. Never needs to be reset. It boots, finds the network, and shows the correct local time with no human interaction — ever.
  2. Always correct across DST. Spring-forward / fall-back transitions happen automatically with no code change and no manual adjustment.
  3. Survives power loss. Unplug it, plug it back in; it recovers on its own.
  4. Quiet and legible in a dark bedroom (sensible default brightness, non-blinking colon).

How "never reset / always-correct DST" actually works

The Pi has no battery-backed RTC. We do not add one. Correctness comes from:

  • NTP over Wi-Fi keeps the system clock accurate (systemd-timesyncd, on by default in Raspberry Pi OS). No network at boot → time is wrong until Wi-Fi associates, then it self-corrects. The clock software should tolerate the brief "time not yet synced" window rather than displaying a bogus time (see Software design).
  • The IANA tz database handles DST. We set the timezone once on the Pi (sudo timedatectl set-timezone America/New_York or wherever it lives) and read local time in Python. The system timezone database supplies DST rules automatically; OS package updates keep the tz rules current. Never hardcode a UTC offset. Never compute DST ourselves.

So: NTP for the instant, tzdata for the offset. That combination is the whole reliability story — keep it intact.

Hardware

  • Board: Raspberry Pi Zero W or Zero 2 W, running Raspberry Pi OS Lite (headless, no desktop). One code path serves both; see the architecture note under Software design re: the arch-selected zeroconf pin. The Zero W is armv6 (32-bit OS only); the Zero 2 W is armv7l/aarch64 (32- or 64-bit OS).
  • Display: Adafruit 1.2" 4-digit 7-segment + I2C backpack, HT16K33 controller.
    • I2C default address 0x70 (solderable to 0x71–0x77 if changed — check the board).
    • We drive the HT16K33 directly over I2C with smbus2 — no Blinka/Adafruit stack. The segment font and the digit/colon RAM offsets live in piclock/display.py, which is the canonical reference; that mapping is the thing to get right.
  • Bus: I2C is enabled on the Pi (sudo raspi-config nonint do_i2c 0); it exposes /dev/i2c-1. Verify the display address with i2cdetect -y 1 (expect 0x70).
  • Detailed wiring/build instructions live in HARDWARE_RUNBOOK.md.

Software design

Default to 24-hour time. The code is split so the logic is testable off-Pi and the only hardware-touching part is isolated:

  • piclock/clock.py — pure time → DisplayState (4-char text + colon + sync dot). No I/O. Shows ---- until the clock's first-ever sync; after that, losing NTP keeps the (free-running, still approximately right) time on the display and only the sync dot goes dark — the dashes never come back mid-run. The colon is steady by default (this is a bedside clock — no movement at night); an optional 1 Hz blink is config-gated and derived from the wall clock, not a tick counter, so it can't drift.
  • piclock/display.py — the HT16K33 smbus2 driver, plus a pure build_buffer() (font/colon/indicator dots → 16-byte RAM; the NTP sync indicator is the backpack's lower-left dot and the Wi-Fi indicator the upper-left dot, both sharing the colon's RAM byte) and a NullDisplay for off-Pi runs. The driver skips redundant I2C writes when nothing changed. The hardware is wrapped in ResilientDisplay: if the display is absent or a connector is bumped, show() never raises — it logs once and retries the connection every ~10 s, so the service keeps running (no crash-loop) and the correct time appears the instant the display reconnects.
  • piclock/timesync.py — is_ntp_synced() via timedatectl show -p NTPSynchronized. Any error → treated as not-synced (pre-first-sync that means dashes — never a bad time). The loop polls it every 5 s until the first sync, then every 60 s. Because NTPSynchronized is the kernel's maxerror estimate (it lags an NTP outage by ~9 h), the sync dot additionally requires freshness: last_sync_age() stats /run/systemd/timesync/synchronized (touched by timesyncd on every successful sync) and the dot goes dark once the last success is older than sync_fresh_seconds (default 4500 s ≈ 2× timesyncd's max poll interval). No stamp file (dev machine, non-timesyncd system) → freshness is assumed, falling back to NTPSynchronized alone. Sync loss never brings the dashes back — that stays keyed on is_ntp_synced()/ever_synced.
  • piclock/wifi.py — is_wifi_up() reads /sys/class/net/<iface>/operstate (an in-process ~140 µs file read, no subprocess, so the loop polls it every tick). It feeds the upper-left indicator dot: lit while the link is up, dark when the link drops or the interface disappears (e.g. firmware crash). Any read error → down. PICLOCK_WIFI_DOT / PICLOCK_WIFI_IFACE configure it. Both indicator dots share a polarity switch, PICLOCK_TROUBLE_DOTS (Config.trouble_dots, default off): off = status lights (lit while nominal); on = trouble lights (lit only when off-nominal, so a healthy clock shows just the time). Applied in app.py via _dot_lit().
  • piclock/config.py — Config dataclass with PICLOCK_* env overrides (brightness, schedule on/off + times + wake/night brightness, 24h, blink, sync dot + freshness window, wifi dot + interface, I2C bus/address, tick, and PICLOCK_DISPLAY=null for dev). Validates its inputs at construction, including that the schedule edges are in daily order.
  • piclock/schedule.py — pure day/night schedule logic: wall-clock time → phase (off / wake / night). No I/O, unit-tested like clock.py.
  • piclock/ctl.py — optional CLI control: a Unix datagram socket (PICLOCK_CTL_SOCKET; the unit sets /run/piclock/ctl.sock via RuntimeDirectory=) receives one-line commands from scripts/piclockctl (on, off, brightness 0-15; the client's bright/dim are aliases for levels 15/0). A pure parse() validates them; a daemon thread queues them for the loop, which applies them to DisplayControl and mirrors them to HomeKit — same override semantics as Siri. The socket is mode 0660, group clockctl (install.sh creates the group and adds clocker to it so the app can chgrp its own socket; humans join the group to use piclockctl). Startup is fail-soft like HomeKit.
  • piclock/app.py — the loop, wiring the above together; SIGTERM/SIGINT blank the display and exit cleanly. When the schedule is enabled (PICLOCK_SCHEDULE=on) the loop evaluates schedule.phase() each tick and applies each phase change via schedule.edge() — that edge-triggering is what lets a HomeKit override stick until the schedule's next on/off edge (the dim edge only adjusts brightness, so an evening "off" stays off overnight), and since the first tick counts as an edge, a reboot or deploy mid-window comes up in the correct state with no external reconciliation.
  • piclock/control.py + piclock/homekit.py — display on/off + brightness control. DisplayControl is a tiny lock-guarded object the loop reads each tick (blank when "off"); it's always present. It is driven by the optional in-app day/night schedule (default edges: 08:00 blanks, 17:00 shows at the wake brightness — default full — and 20:00 dims to the night brightness — default the dimmest lit level; times tunable via PICLOCK_OFF_AT / PICLOCK_ON_AT / PICLOCK_DIM_AT, levels via PICLOCK_WAKE_BRIGHTNESS / PICLOCK_NIGHT_BRIGHTNESS). The schedule is switched on by a systemd drop-in (systemd/piclock-schedule.conf, installed by install.sh --schedule) — there are no schedule timers or helper units. Also optionally, by HomeKit. HomeKit (PICLOCK_HOMEKIT=on) runs a HAP-python Lightbulb accessory in a daemon thread that mutates the same DisplayControl, and schedule-driven changes are pushed back to the Home app via HomeKit.notify_state so the tile stays in sync (scheduled changes also force brightness to the wake level at the on edge and the night level at the dim edge). pyhap is imported lazily so the core runs without the extra deps (separate requirements-homekit.txt; the zeroconf pin is selected by CPU architecture via PEP 508 markers — 0.39.4 on armv6/Zero W, last pure-Python release with an armv6 wheel; the current release on Zero 2 W and up — so pip picks the right one with no install flag). Both features are opt-in via scripts/install.sh flags. See SOFTWARE_RUNBOOK.md §4–5.

Guarantees worth preserving: the display layer must fail soft on I2C errors (a missing or flaky display logs + reconnects, never crashes the service); the clock must keep showing ---- until is_ntp_synced() is first true — but once it has synced, a later sync loss must not bring the dashes back (show the free-running time, sync dot dark); and HomeKit and control-socket startup are non-fatal — if either can't start (missing deps, unwritable state dir, missing socket dir, …) the app logs and runs the clock without it, since the 7-seg's job must never depend on an optional feature. Config validates its inputs at construction (positive tick, known backend, 0-15 brightness levels, ordered schedule edges) so misconfiguration fails fast instead of busy-looping or silently running the wrong backend.

Tests live in tests/ and cover the pure parts (render, build_buffer) with no hardware. Run with pytest.

Running on the Pi

Managed by systemd (unit at systemd/piclock.service) so it starts at boot and restarts on failure.

  • The service runs as a dedicated clocker system account — no login shell, no sudo, member of the i2c group only (its sole privilege, for /dev/i2c-1). The app needs no elevated privileges: timedatectl show is readable by any user, and I2C access comes from the group. If something ever does need privilege, add a narrow allowlisted entry in sudoers for clocker — never grant it blanket sudo.
  • Deploy target is /opt/piclock, owned by clocker (kept out of /home so the service account can read it). scripts/install.sh (run with sudo) creates the account, rsyncs the code there, builds the venv as clocker, and installs + enables the unit. Idempotent; re-run after pulling code.
  • sudo systemctl {start,stop,restart,status} piclock · journalctl -u piclock -f

Dev environment

  • Runtime deps are pinned in requirements.txt — just smbus2. Dev/test deps in requirements-dev.txt (pytest). Install them however you prefer (a virtualenv is the usual choice, but that's your call).
  • Logic runs on any dev machine (macOS/Linux); use PICLOCK_DISPLAY=null to run the loop without hardware. Treat the Pi as the source of truth for anything touching I2C. A reference Pi is Debian 13 / Python 3.13; the architecture is armv6 on a Zero W, armv7l/aarch64 on a Zero 2 W. On 32-bit Raspberry Pi OS pip resolves prebuilt wheels via piwheels (configured in /etc/pip.conf on the stock image); a 64-bit image resolves manylinux wheels from PyPI.

Deployment / updating

There is no built-in remote-deploy pipeline — deployment is up to you. The supported path is to get the code onto the Pi (clone, git pull, scp, etc.) and run scripts/install.sh with sudo. It is idempotent: re-running it after pulling new code re-syncs /opt/piclock, refreshes the clocker venv, and restarts the service. If you want CI push-to-deploy, an adaptable Gitea Actions template (tag-triggered, least-privilege: a forced-command receiver plus a single allowed sudo apply step) lives in deploy/ — see deploy/README.md. Whatever you build, have it run as an unprivileged account — never give a deploy key blanket root.

Conventions & guardrails

  • Don't reinvent time/DST. Use zoneinfo + system NTP. No manual offset math, no DST tables, no scraping time from the internet in app code.
  • Don't commit secrets (SSH keys, tokens) or local build artifacts (virtualenvs, caches).
  • Keep new runtime deps out of requirements.txt unless genuinely needed — boot time and SD-card space on a Zero W (the most constrained target) are scarce.
  • Match the existing minimal style; this is a small appliance, favor clarity over abstraction.
  • When changing display code, state how you verified it (real hardware vs. logic-only), since most of it can't be exercised off-Pi.