Step 1: The Free Run Test

8085 Free Run Circuit on Breadboard

8085 Free Run Circuit, No Logic Lab Req'd

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.

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Final Checks

Check all connections. A good practice is to use a socket on the breadboard as a placeholder, then wire up to its pins before inserting the IC to be actually used. This allows you to check your connections with a multimeter for shorts or other incorrect connections before inserting the IC.

A warning: we're engaging in one bad practice here. We don't have a real RESET circuit on the 8085 yet. This means there's a chance that the 8085 won't start running properly. Usually it will, but occasionally it won't. If the lights on the address lines don't light up, or only light up at full brightness, when you apply power to the complete circuit, this may be the reason.

In that case, simply pull the /RESET line on pin 36 out from where it's plugged in to 5V while power is still on. Leave the end connected to the 8085 alone. Then touch the loose end to ground, and finally plug it back in to 5V. This will reset the 8085 after the power is up, and it will run properly if all else is correct in the circuit.

Running the Test

Put in your 8085 chip if you haven't already. Do a final visual check of connections. Switch on. If all has gone well, you will have the LEDs on the highest address lines (A14 and A15) flickering on and off. Those on the address lines below the two highest may flicker quickly, or simply appear to be lit up half-bright because they are turning on and off too quickly for the flickering to be seen.

If you have more than one crystal oscillator in the required range (1MHz to 6MHz), use the lowest value for the free run test. With a 6MHz oscillator, A15 will be turning off or on about 13 times per second. This is fast enough to be hard to see, it might just look like it's on, but dim. Going to a 4MHz oscillator will reduce that to about 8 times a second. Much easier to see, but still fast. Better is to use a 1MHz or 2MHz oscillator, which will give nice blinks on both A14 and A15. a visible flicker on A13, and a barely visible shimmer on A12.

The lights you see are the 8085 addressing memory locations above 4K when the lights come on. You can also reverse the sense of the LEDs if you're using loose LEDs. Turn the LEDs around, swapping the anode for the cathode. Then take the lines that connect the LEDs to ground and connect them to 5V instead. Now the LEDs will be on when the address lines are low, and will darken briefly when the address line they connect to goes high.

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