Step 1: The Free Run Test

8085 Clock Circuit Using NAND Gates.

The Clock Circuit with NAND Gates

8085 Clock Circuit Using Invertors.

The Clock Circuit with Invertors

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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The Clock

Start with the crystal oscillator. If it's a standard four-legged unit like what I used (see photo, above), pin 1 is no connection, pin two is ground, and pin four goes to +5V. The signal is on pin three. Use an oscilloscope or logic probe to verify that you have a clock signal. If you're doing this without a logic probe or o-scope, you can use a multimeter. Look for a change on the output pin of the oscillator when you power it up, versus no power.

Next, hook up the signal to your inverter IC. I used a 74LS00 Quad NAND gate as my inverter. I did this by tying together the two inputs from the same NAND gate and putting the crystal oscillator's signal there. Then I took the output of this gate and put it to both inputs of another NAND in the same IC. You can do this with any version of the 7400, either a 7400, 74S00, 74L00, 74C00, etc. The only thing to look out for is the CMOS versions, the 74C00, 74HC00, and 74HCT00. If you use one of these all inputs that you are not using must be tied to ground or to 5V. CMOS chips get unhappy if you leave inputs floating with no connection. They don't care about the outputs, though.

You can also use an actual inverter, like a 7404 or one of its various versions, or a CMOS 4049 inverter. In this case, you don't need to tie any inputs together. You put the crystal oscillator's output to one of the invertor inputs, then run that invertor's output to another invertor's input. Note that if you don't use a NAND gate you'll need to add another NAND chip to the MAG-85 later, when we add I/O decoding.

Build the circuit as shown in the schematics at the top of the page, but don't connect the outputs to the 8085's X1 and X2 yet. Check to make sure that you're getting oscillation with an oscilloscope or logic probe. Check both outputs that will go to the 8085's X inputs. If something's not working, check your circuit for errors, and make sure that your crystal oscillator is working.

This type of clock, with two inverted signals, is called a "push-pull" clock. At frequencies below 6MHz, the 8085 can work with just a single oscillator signal at X1. At 6MHz and above, however, it is recommended that the 8085 be used with a push-pull circuit like this one here. The 8085 takes this clock signal and divides it in half for its internal clock. So when we feed the 8085 with a 6MHz signal on X1 and X2, it will be running at 3MHz.

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