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Board assembly part II - Free run

A project log for T030

MC68030 based single board computer

Tobias RathjeTobias Rathje 03/02/2016 at 22:210 Comments

With the CPLD, crystal oscillator and reset circuitry in place, it's time to mount the CPU and try to get it to cycle through the address space by grounding the data lines.

This is also known as free running mode, more on that here:

http://www.bigmessowires.com/2014/10/29/68k-free-run

As Steve puts it: "It seems that blinking an LED in free run mode is a rite of passage for 68K projects."

And this is exactly what I am going to do next.

I have buffered address lines A21 to A25 through the CPLD in order to avoid driving LEDs directly from the address bus. DSACK0 and DSACK1 are grounded.

And it works! :-D

Here with the LED connected to A21 and the logic analyzer hooked up to four of the lower address lines. The plug with the frayed ribbon cable is a makeshift adapter to ground the data lines.

The result is one of the worlds most complicated LED blinking circuits. But it proves that the CPU is functional.

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