612 Simple Stopwatch

612 : Simple Stopwatch

Design render
  • Author: Fabio Ramirez Stern
  • Description: A simple stopwatch counting in 100th seconds and outputing it via SPI to a MAX7219 chip controlling an 8 digit 7-segment display.
  • GitHub repository
  • Clock: 1000000 Hz

How it works

A clock divider turns 1 MHz into 100 Hz, which drives a stopwatch going from 00:00:00 to 59:59:99. To achieve this, a chain of two types of counting circuit, one per digit gives it's output to an SPI master that encodes the result to be displayed on a 7-segment display with at least 6 digits.

How to test

The start/stop button toggles the clock, the lap time button pauses the display, while the clock keeps running in the background. Pressing it again re-enables the display. The time can be reset with the reset button on input 2, or with the chip/PCB wide reset. The PCB wide reset affects everything, the input pin driven reset does only resets the counters.

External hardware

2-3 buttons, one for start/stop and one for lap times. For the reset, either a third button or the dev board's reset for the whole chip can be used. 1 MAX7219/MAX7221 driven 7-segment display, or something that can interpret the SPI signal according to the MAX's specifications.

IO

#InputOutputBidirectional
0start/stopSPI MOSI
1lap timeSPI CS (active low)
2reset (active high)SPI CLK
3stopwatch enabled (counting up)
4display enabled (goes low when showing lap time)
5
6
7

Chip location

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