I'm experimenting on a breadboard how to use the programmable interval timer 8253 (actually I'm using the new 82C54, it's the same thing). datasheet: http://www.intersil.com/content/dam/Intersil/documents/82c5/82c54.pdf

what I don't understand is what kind of clock should I use with it. I only see 1 input pin for each clock channel, but usually I use 2 pins crystal resonators (as clocks for microcontrollers). so what kind of clock should be connected to this timer IC? I guess I can't use the usual 2 pins crystal.

thank you


1 Answer 1


Those clock input pins are just that, logic inputs that need to be driven with a logic clock signal. There is no oscillator circuit provided on the IC. You will need make your own oscillator, so take your crystal resonator and wrap it round an inverter with suitable Cs to ground and a big feedback resistor for DC bias. Alternatively you could buy a ready-packaged clock oscillator, they are not ruinously expensive. I see from the data sheet that at 5v supply, the clock logic thresholds are 2v and 0.8v. This means your clock signal can be TTL or CMOS levels, either will work. If you happen to have a clock signal kicking around somewhere else in your system, then use that.

  • \$\begingroup\$ thank you, so what about this device: maximintegrated.com/en/products/digital/… looks like it can provide a fixed clock without external components (and yes, it comes in SMD package). would it be ok to provide a logic clock? \$\endgroup\$ Sep 26, 2015 at 13:19
  • \$\begingroup\$ Yes, the MAX7375 looks great to provide a clock, as long as a) one of the standard frequencies is right for you and b) its frequency accuracy is good enough for you. That's initial 2%, another 0.5% over temperature, and another 0.5% over voltage. I don't know if you were hoping as as it's a Maxim device, you could get it for free, but apparently they don't sample those. \$\endgroup\$
    – Neil_UK
    Sep 26, 2015 at 16:28

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