Stopwatch with digital display in KiCad

Recently we were busy realizing a stop watch with digital display for a customer in the field of special machine construction. The customer wanted a display for minutes and seconds. The display had to consist of 4 seven-segment LED displays. They should disappear elegantly under a screen and be mounted via a frame in the machine front panel. The clock should count up to 59:59 and then reset to 00:00. Via a control input the stop timer should be able to be started, stopped and reset via the customer’s PLC. A nice little project started…

Previous status

The customer has already installed a BETESO “Digi”. This is a circuit board with evaluation circuits for current and voltage measurement combined with a numerical display on 7-segment basis. The look and feel of the stopwatch should be similar to the Digi. So the board dimensions, the type 7-segment display, the optical aperture used and some other basic conditions were set and binding for us.

The realization of the stopwatch

No sooner said than done. After we at BETESO consequently switched to KiCad as software for board layout at the turn of the year, this was my first 4-layer board in KiCad. What can I say: it works!
The layout was a bit tricky, especially since I wanted to keep the supply plan as intact as possible after the flooding. After a few initial hurdles the routing in KiCad worked out quite well. Therefore here are a few impressions of the board from KiCad:

Impressionen Board Stoppuhr aus KiCad
Front view
Impressionen Board Stoppuhr aus KiCad
Rear view
Impressionen Stoppuhr aus KiCad
Front view with 7-segment displays

The technology

The heart of the board is an ATmega32A in a 44-pin QFN housing. The board is also equipped with an ISP interface for easy programming of the controller. We also released the JTAG interface and UART of the ATmega. This facilitates firmware development and on-chip debugging.

We programmed the firmware for the microcontroller in C using the AVR-GCC toolchain and the library avr-libc for the ATmegas. As clock generator for the stopwatch we have chosen a 11.0592 MHz baud rate quartz. This feeds the 16-bit timer/counter of the ATmegas in CTC mode via a prescaler.

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