Step 1: The Free Run Test

8085 Free Run Circuit Image.

The Full Free Run Circuit on the Breadboard

A Free Run circuit tricks a CPU into running a fake program that's hard-wired to its data lines. It makes the CPU run freely through its entire address space, then loop back around.

Free Run Test, continued.

<<Back to Free Run Test, Page 2

8085 Connections

Connect the 8085's leads as follows:

  • Pin 40, Vcc, to 5V.
  • Pin 39, Hold, to ground.
  • Pin 36, /RESET In, to 5V.
  • Pin 35, READY, to 5V.
  • Pin 20, Vss, to ground.
  • Pins 12-19, AD0-7, each connect to one end of a resistor from 3.3K to 10K ohms. The other end of the resistor goes to ground. (See picture, above.)
  • Pins 6-10, TRAP, RST7.5, RST6.5, RST5.5, INTR, to ground.
  • Pins 2 and 1, X2 and X1, to the clock circuit as shown on the prior page.

Pins 38, 37, 34, 33, 32, 31, 30, 29, 28-21, 11, 5, 4, and 3 require no connection. If you are using a CMOS 8085, you might want to connect pin 5, SID, to ground.

Note: Strictly, pins 7, 8, and 9 should not require connections, either, since the interrupts they cause for the 8085 are not enabled by default. Still, in case the circuit pulls a SOD instruction somehow it's not a bad idea to eliminate them as possible causes for the 8085 not behaving as expected. So I usually ground them.

Adding Lights

If you have an oscilloscope handy this is all you will need to run the test. You can see the address lines strobing as the 8085 runs by probing them. However, it's more interesting to have something you can actually see, and not everyone has an oscilloscope handy.

In the picture at the top of the page you can see that I've connected some of the high address lines to buffered LEDs. This allows me to see them flicker as the 8085 goes through its address space. If you don't have a logic board with buffered LEDs in it, you can still add LEDs very simply.

  • Connect pins 28-25 to one end of a resistor, any value from 330 to 470 ohms.
  • Connect the other end of each resistor to the anode (positive) side of an LED
  • Connect the cathode (negative) side of each LED to ground.

Voila! Instant 8085 light show.

Next: Running the Test >>

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