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Real-Time 'Stopwatch' 2012-09-10

This is a simple electronic "stopwatch" you can use to time an event. I used it when developing a device that needed to accurately measure a time interval (a camera shutter-speed tester). I used this to check the accuracy of the project, using a microcontroller (the SX-28).

Pretty simple, really. The first 74HCT4040 counter generates the clock (I used a 1 MHz oscillator and "tapped" the counter at Q7: you can choose these settings to suit your particular needs). The AND gates are used to control the clock and counters through the three control lines, N0-N2, which start and stop the clock, and read and reset the counters.


The 24-bit clock value is read as 6 4-bit nybbles, selected through N0-N2 as shown.

Now, you might look at this and say, "All this is is a timer that any idiot could make for their project! And why would I want to read it out one nybble at a time?". To which I say: I did it this way because my microcontroller (SX-28) had a limited number of I/O port pins available, most of which were already being used for the LCD display. So the seven I/O lines required by this circuit were perfect.
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