AcroClock — Project History

Milestone log for the AcroClock project. Records key decisions, structural changes, and completed phases.

2026-06-28

Deferred clock project revived. Working title AcroClock (AcroSched-based clock; name to be confirmed).
Goal: a 24-hour desk clock on the AcroSched 1.7 cooperative scheduler, with DS3231 timekeeping, large green seven-segment display, ambient-light brightness control, four alarms, and an audible buzzer.

requirements.md created — initial 13 requirements (REQ-001..013) and a task backlog across 10 phases (Phase 0 — Foundation through Phase 9 — Documentation). Draft v0.1.0.
HISTORY.md created (this file).
CHANGELOG.md created.

RTC — DS3231SN (TCXO, I2C, ±2 ppm, backup cell, internal temperature sensor reused for on-demand temperature display).
Display — four single-digit, common-cathode, green seven-segment indicators E11801-J-UG4-8-W (45.72 mm), static drive (no multiplexing).
Colon — two discrete green LEDs between hours and minutes (the indicators expose a decimal point, not a colon); blinks at 1 Hz as a live "seconds" cue.
Ambient light — BH1750FVI over I2C (replaces the originally considered photoresistor; more stable, digital, on the same bus).
Settings storage — 24C01A I2C EEPROM (128 bytes), per-record CRC with fall-back to defaults on invalid CRC (lesson from a past silent bad-configuration bug).
Audible signal — BMT-1203UX buzzer with built-in oscillator (GPIO + transistor switch + flyback; simple cadence, no melodies).
Input — rotary encoder 3315Y-025-016L (no integral switch) read via a hardware timer in encoder mode, plus two buttons (Mode, Set/Enter).
MCU — STM32F103C8T6 (Cortex-M3, 72 MHz, LQFP48). Chosen over the earlier mis-pasted STM32F100R8T6B; AcroSched F103 port already exists.
Scheduler — AcroSched 1.7 with the optional software-timer and IPC modules enabled.

Static display, no software multiplex. Single-digit indicators driven statically; display state changes only when a digit changes.
Self-made segment driver from discrete logic: a chain of 74HC595 shift registers over SPI, followed by a high-side source driver (common-cathode) and per-segment resistors. Avoids the cost of integrated scan drivers (MAX7219 / HT16K33) and keeps the LQFP48 pin budget comfortable.
Brightness via /OE PWM on the shift registers (global dimming) driven from BH1750 readings with averaging + hysteresis; not by varying segment current.
All four alarms in firmware (DS3231 hardware alarms — only two — not relied upon).
24-hour format fixed → no AM/PM indication needed.
Date format — ISO 8601 (YYYY-MM-DD) in file headers, matching AcroSched.

Project name — AcroClock confirmed (working title kept).
Leading-zero blanking — the hours tens digit is blanked when zero ( 9:05, not 09:05).
Alarm repeat — 7-bit weekday mask per alarm (default daily); matching evaluated on minute change to avoid repeat triggers within the same minute.
Segment voltage — measured (2026-06-29). Bench test: segment current limits to 20 mA at ~6 V (three dies in series). 5 V is insufficient, 12 V unnecessary → segment rail ~6.5–7 V. Full static indication draws ~0.76 A (0.64 A display + 2×20 mA colon + 4×20 mA alarm LEDs), cut by /OE PWM dimming. The earlier "9–12 V" figure was retracted as unfounded.

Static vs. dynamic display drive — owner re-evaluating to reduce part count and average current; the higher segment rail is needed either way. This decision gates the driver topology.
Whole-device 18650 backup — deferred; needs charging, protection, and mains/battery source switching. The DS3231 already retains time on its own cell during mains loss.

Added a CMSIS-based project solution to the repository to support standardized build and target configuration for STM32F103 (CMSIS device and toolchain integration).

Added the AcroSched library version 2.2.0 to the workspace and updated integration notes to reference the newer scheduler release (replacing the prior 1.7 reference).