Quoted from G-P-E:
Yep -- looks like a few changes and the NVRAM is a definite improvement. R137/U15 is also an issue waiting to happen with that board. U15 is a 74LS37 -- with R137 at the low resistance of 150 ohms (almost looks like 100 ohms in photo) his gate #4 is sinking about 38mA yet a 74LS37 is rated to sink a maximum of 24mA. While you have the board out, probably should take the time to change U15 out to an MC3459, 74F37 or 74S37.
I would have used a totally different approach for the clock gen and stayed with a plain vanilla 6800... But at least he did do away with the 9602. A bit of creativity with the memory map decoding would have put both 6810 and 5101 into separate banks within the NVRAM. ...sigh... I wish I had time for that sort of thing again.
Getting a little off topic here... but using a 6800 CPU would make no sense to me.
First of foremost the 6800 is rare compared to the 6802. They made the 6802 in way higher quantities, probably 100x more 6802 was produced than 6800. That makes it cheaper and easier to get.
The 6802 has internal ram in the right place to eliminate the 6810 at U7. Pointless to re route the U7 ram into the NVRAM. Just open up the NVRAM so homebrew software guys can use the entire thing. Saves a dip24 socket and ram or extra circuitry to reroute enabling to the nvram.
6802 also has internal clock stuff. Sticking a crystal across p38 and 37 is so simple. You can use a complete oscillator if you find getting a 2mhz crystal to start up fast enough is a problem. Using an external inverter and clock circuit is just more parts to stuff.
Back to the NVRAM... when using a FM16W08 you don't need the diode blocked VUA-Q2 pulling up the d3-d7 since there is no battery backed ram. Sip resistor pull up all eight data bits. That means you don't need a stiff 150 ohm pullup resistor either. When not pulling d3-d7 vua-q2 and you are not buffering a clock gen you can easy a cheap common gate here like 74HCT00. When you go down to just one ROM you might even find an extra gate elsewhere to completely eliminate the need for u15. That move saves at least nine parts.
Also the NVRAM can be put right on the main vcc rail. It does not need to get powered off of the reset delay. FM16W08 plays nice in this system. Saves that dumb 82 ohm resistor circuit dropping the 12v down for the ram thats wastes power.
MCP130-460 reset monitor simplifies things even more saves another 10 parts or so.