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Old 12th Sep 2017, 12:06 am   #2
MrBungle
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Join Date: Jun 2016
Location: London, UK.
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Default Re: Help with 6802 Microcontroller

6802 is a 6800 with built in clock gen. nothing more.

Startup vector is 0xFFFE and 0xFFFF which usually has a JMP to wherever the entry point is.

I'd concentrate on looking at the ^RESET and ^HALT lines to start with. I've had some stuff go weird on the power in reset before and not reset the CPU state properly. After that it's possible the EPROM is dicky and it's skipping the startup vector and then PC is rolling over into RAM at 0x0000. I don't think the 6800 had PC overflow traps. Been many years since I had anything with one in it.

If you pull the EPROM out, A0 should pulse out if it doesn't trap. (No logic analyser for me - just a crappy old logic probe )

http://www.andysarcade.de/data/elect.../6802_6808.pdf contains a full startup flow on page 9.

Before everything though get a scope on the 5v rail and look for any spikes and ripple. May be old tantalum playing up or the power supply failing as well.
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