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
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
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
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
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.
Calculation basics are in the Libre-Office-Calc file