CLAUDE.md

This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.

Project

Arduino Mega 2560 clock displaying HH:MM:SS on a 256×64 px SSD1322 OLED via the U8g2 library.

Arduino Pin	SSD1322	Note
47	CS	Chip select, active-low
48	DC	Data / command
49	RES	Reset, active-low
50	SDA	MOSI (software SPI — does NOT match hardware SPI MOSI on pin 51)
51	SCK	Clock (software SPI)

Target MCU: ATmega2560 @ 16 MHz external crystal.

Single-file Arduino sketch (ArduinoClock_SSD1322.ino). No build system beyond the Arduino IDE / CLI.

Timekeeping — Timer 1 CTC interrupt fires at exactly 1 Hz:

Prescaler 256, OCR1A = 62499 → 16 000 000 / 256 / 62500 = 1.000 000 Hz
ISR increments g_h / g_m / g_s (all volatile) and sets g_tick = true
loop() snapshots the three counters atomically with cli()/sei() before redrawing

Display — U8g2 full-framebuffer (_F_) software-SPI constructor:

U8G2_SSD1322_NHD_256X64_F_4W_SW_SPI u8g2(U8G2_R0, /*SCK*/51, /*SDA*/50, /*CS*/47, /*DC*/48, /*RST*/49);

Font: u8g2_font_7Segments_26x42_mn (capital S — lowercase causes a compile error)
String centred with u8g2.getStrWidth() each refresh
Seconds progress bar drawn as a filled box in the bottom 4 rows

Serial time-set — non-blocking, accumulates digits in a static buffer; accepts HHMMSS\n at 9600 baud.

Pins 50/51 do not match the Mega's hardware SPI port (MOSI=51, SCK=52), so the display must always use 4W_SW_SPI, never 4W_HW_SPI.
U8g2 framebuffer for 256×64 mono = 2 048 bytes; the Mega has 8 KB SRAM total — keep allocations small.
The only 7-segment font present in this version of U8g2 is u8g2_font_7Segments_26x42_mn.

Compile and upload via Arduino IDE or Arduino CLI:

arduino-cli compile --fqbn arduino:avr:mega ArduinoClock_SSD1322
arduino-cli upload  --fqbn arduino:avr:mega --port <COMx> ArduinoClock_SSD1322

After every meaningful change: commit and push so there is always a working version to revert to.

git add ArduinoClock_SSD1322.ino
git commit -m "short description"
git push