I'm making a digital clock out of a crystal oscillator, and 7490 counters in cascade (also some 7447, BCD to 7 seg).

I think I can handle it, but I'm having problems at choosing the hour setting mechanism.. I thought of simple pushbutton/ interrupt, but I'm afraid of "bouncing effect"... Any ideas please??

 

You can build a hardware debouncer using a capacitor and resistor.

 

Yes, I've figured it out. Anyway I have another more complex problem.

Each 7490 has its own in-clock pulse, coming from the previous 7490. right? But if I want to add a second clock line, coming from a pushbutton, so when it is pressed, it also counts as a clock pulse (for manually advance the minutes or hour "hand"). I find a problem. If I connect both pushbutton and previous 7490 output into the same clock input, I need to do something for them both to work properly.. If not, when one of them is activated and the other one falls from 1 to 0, no transition will occur...

Any idea??

 

Maybe a multiplexer?

 

You need OR gates for each inputs.

 

That would not work, because whie you force a clock pulse into the or gate, the pulse coming from previous stage might hold the output high, so no transition will occur

 

Use an 'Ex-OR' gate ... (exclusive OR) to mix the two.
When the normal clock is low the 'setting' pulses will pass unhindered,
when the normal clock is high the 'setting' pulses will simply be inverted.

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