This program receives the european DCF77 time signal and syncs it with the external RTC-clock. It is displayed on a 8-digit 14- or 7-segment display with display-controller HT16K33. The MCU used is an ATtiny 814 or 1614. Without external RTC, serial debugging and OneWire, an ATtiny 412 can be used (99 % of flash space used). The circuit can be powered by a LiPo-cell. If the voltage drops below 3.0 V, the voltage is displayed as a low voltage indicator. A push button switches between different display modes: Time with date, time with seconds, time with temperature, time with battery voltage. A sync to DCF-time is performed in the background every 30 minutes, and immediately after power-up. The software uses a layer-based approach. Low-level: DCF-object with interrupt handler, mid-level: receiving data state machine, upper level: display handling. No blocking code exists, fully asynchronous operation.
MCU-clock: 8 MHz, Timers used: TCA0 (DCF signal processing), TCB0 (timing for OneWire), TCD0 (millis), RTC (2 Hz generation via interrupt)
External RTC: DS3231 with battery backup, supplies 32K clock for internal RTC, 8-digit display (I2C): DFRobot 7-segment, or custom built 14-segment. External DCF77-receiver: ELV DCF-2 (MAS6180 AM-receiver) Temperature-sensor: DS18B20 with external pullup
Pins used:
PA3 - Input, DCF-signal, input-pullup enabled, low-active signal
PA4 - Input/Output, used by tiny OneWire-implementation (temp sensor DS18B20),
external pullup 4,7K
PA6 - Output, LED which reflects the DCF-input signal
PA7 - Input, mode select button, active low with input-pullup enabled
PB0 - I2C CLK for display
PB1 - I2C DATA for display
Compiles with MegaTinyCore on Arduino IDE. External libraries required: RTClib.h (Adafruit fork)
Custom built alphanumeric display used: http://www.technoblogy.com/show?2ULE
