Step 1: The Free Run Test

The 8085 on Logic Lab in Free Run Circuit.

Check out the 8085 and its support circuits.

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.

One Piece At A Time

The Free Run Test

The free run circuit is easy to build and lets you check out your 8085, its system clock, and the control wiring. If you've already got a power supply, the chips, solderless breadboard and wires ready to go you can put this together in a short evening or less.

Parts Needed:

  • An 8085 CPU. Any type will do.
  • Crystal Oscillator, any value from 1MHz to 6.25MHz. Lower is better.
  • 74LS00. A non-LS type will work as well, or a 74XX04 or 4049 (see below.)
  • Two 4.7k Ohm resistors.
  • Eight resistors, anything from 3.3K to 10k Ohm will work.
  • Jumper wires and a solderless breadboard.
  • Current limited LEDs, or bare LEDs and 330 to 450 Ohm resistors.

The Plan

The 8085 will be provided with power connections and a clock. Its address/data lines (DA0-DA7) will be pulled low using resistors to allow the 8085 to put its own signals on the bus, while putting a value of 00 on the bus when the 8085 reads the bus.

This will make the 8085 think that every memory location it accesses has an instruction of 00, or NOP, in it. So it will see NOP instructions filling its entire address space.

A close-up of the 8085 free run circuit.
Click Image for a Close-Up

Next: The Clock >>

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