Clock board

Digital LED Clock

A 6-digit 7-segment clock (HH:MM:SS) built around an ATmega328P, an external RTC, and a constant-current LED driver — split across two custom PCBs I designed in KiCad. This was mostly an excuse to actually use everything I've learned so far about digital electronics and embedded C in one project I could see and touch, instead of another breadboard that gets torn down at the end of the semester.

What it does

• Shows time as HH:MM:SS across 6 seven-segment digits
• Keeps time with a DS3231 RTC (accurate even when the board is powered off, thanks to a coin cell)
• 4 buttons: set hour, set minute, reset seconds, and toggle to show the DS3231's built-in temperature reading instead of the time
• Multiplexed display — only one digit is physically powered at a time, cycled fast enough (~166Hz) that it looks fully lit

Hardware

Two boards, connected by a short ribbon cable:

• clock_board — the "brains." ATmega328P, DS3231 RTC, a 74HC595 shift register + 6 transistors for digit selection, an ISP header for programming, and the 4 push buttons.
• led_board — the "face." A TLC5916 constant-current LED driver feeding three 2-digit common-anode 7-segment modules (6 digits total), plus the transistor-switched power lines coming in from the clock board.

Full schematics and PCB layouts are in hardware/, one subfolder per board. Both were designed from scratch in KiCad — no dev-board shields, no premade modules other than the RTC/driver ICs themselves.

Firmware

Bare-metal AVR C — no Arduino core, straight avr-libc and direct register access. I wanted to actually deal with I2C, timers, and interrupts myself instead of hiding behind Wire.h and digitalWrite.

• twi.c — hardware I2C driver for talking to the DS3231
• ds3231.c — RTC register access (BCD conversion, get/set time, temperature)
• display.c — 7-segment font table + the Timer1 interrupt that handles digit multiplexing (bit-banging both the 74HC595 and TLC5916 shift chains)
• buttons.c — debounced button reads with auto-repeat on hold
• main.c — main loop tying it all together

All of it lives in firmware/, buildable with either the included Makefile or platformio.ini.

Status

Schematics and PCBs are designed; firmware is written and reviewed but not yet validated on the assembled board (still waiting on boards/soldering as of writing this). A couple of polarity assumptions (digit-select active-high/low, segment driver active-high/low) are flagged with #defines in firmware/src/config.h specifically because they're easier to confirm with a multimeter on the real board than to guess from a schematic.