Intel 8085 minicomputer

Step 02: Automatic Clock

The Intel 8085 requires either a Crystal Quartz oscillator or a oscillating IC. I’ve developed a few options in short little breadboards that will give the 8085 the clock it needs on pins x1,x2.

Automatic 555 clock

[Image Full Board] [Image Mini Board] [Bread Board] [Fritzing Layout]

This clock will give you a nice slow automatic tick that will give you time to actually read the address LEDs and see what the processor is up to. Its very good for debugging your application until we get to the point where we have input/output. I’m currently running this clock at about 2 ticks per second. 4 instructions take about 4 seconds to execute so its great for watching exactly what happened step by step. Even at 1mhz 4 instructions complete faster than you’ve even realized you’ve turned the power on.


  • Ti 555 timer chip (or compatible)
  • 10 uF Capacitor
  • .1uf Capasitor (104)
  • LED (with resistor that is in spec with the LED)
  • 1x8.7Kohms resistor (Grey, Red, Red)
  • 1x10Kohms resitosr (Brown, Black, Orange)
  • Misc little bits of wires from drawer by Planet Express ship

Putting it Together

NOTE: The Fritzing images have 22Kohm resistor. There is not an 8.7Kohm available in Fritzing. Using 22Kohm results in a different frequency, no harm is done. It may be slower or faster so you can use the calculator linked below to adjust to a speed you are comfortable with.

What is Needed

What we want to do is create a nice clean square clock signal with very clear edges. We can do this with the 555 chip by placing it into one of it’s 3 modes called Astable. Astable mode makes the 555 continuously output the signal at the configured frequency. To «configure» the 555 to a specific frequency 2 resistors and 1 capasitor is used. Resistor 1 (10Kohms) goes from Pin-7 to Vcc. Resistor 2 (8.7Kohms) is connected to pin 2, 6, 7 and Ground. This causes the Capasitor on the same lines to be charged by both Resistors but discharging only through Resistor 2. Changing the values of the resistors and the capasitor is how we modify the frequency of the signal.

You can find the formula for the High Time, Low Time and frequency of the signal on the 555 Wikipedia page. Also there is a very nice calculator that actually does what you want (you type in a frequency and it gives you the components) at House of Jeff. Most calculators you insert the component values (not to useful if you are after a specific freq).

I may write more about this at a later time but for now these images show how to wire the 555 Timer into the loop that will produce the clean clock signal for the clock ticking of the 8085 on Pin0 (x1).


Notes about these images. Pin 3 (on bottom counting toward the right) is the output pin for the clock. This gets hooked up to pin x1 (pin 0) on the Intel 8085. The x2 pin is not used for ic generated clock signals unless higher than 6mhz is wanted where x2 should be used as a «pull down»

Automatic 555 Clock Full Board

Automatic 555 Clock Full Board

Automatic 555 Clock Mini Board

Automatic 555 Clock Mini Board

Automatic 555 Clock (Fritzing Image)

Automatic 555 Clock (Fritzing Image)