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AVR single chip controllers AT90S, ATtiny, ATmega and ATxmega
DCF77 async receiver and 7-segment LED driver with an ATmega324

This project is experimental. I don't know if it really works as planned here.

10 DCF77 AM direct receiver with gain-regulated OpAmps and an ATtiny25

10.8 Async serial receiver and LED display with ATmega324

The direct receiver with a regulated OpAmp outputs an async serial signal in a long format, if so desired. This receiver here reads this async signal and displays the received content on an LCD. With that, time, date, weekday as well as status messages from the receiver can be displayed, depending from the size of the attached LCD.

10.8.1 The LED display of the clock

The display used here has been described here. It works with four decimal digits with 28 pieces of 10mm-LEDs, that are organized in seven segments by four LEDs each. Each segment is attached to a constant current driver. In the middle between the four digits a double point is located with two LEDs, altogether 114 LEDs. The original works with an ATmega48.

10.8.2 Necessary hardware

Async serial receiver with large LED display ATmega324

To additionally enable adjustment of the watch via an async serial interface would have resulted in large changes to the original ATmega48 design, because the original already used RXD for a different signal. So I had to change to another controller.

After consulting the AVR selection window in avr_sim I decided to try it with a 40-pin AT324PA, which is commercially available and cheap and has all that is needed for the large watch:

two crystal pins for clocking the controller with exact second pulses, so that the watch also works without DCF77,
an 8-bit-bus for multiplexing the seven segments and the double point,
a 4-bit-bus for driving the anodes of the 7-segment display,
two INT pins for the two keys,
two ADC channels for the potentiometer and the background light sensor, and
the UART input pin RXD0 and a single I/O pin for the CTS output.

The schematic shows how all components are connected to the ATmega324. As a gimmick a red-green dual LED has been added that can display the state of the DCF77 receiver:

permanently green shows synchronization with DCF77,
permanently red signals missing synchronization with DCF77,
blinking red shows read errors on the async interface.