Disk II Interface: AVR vs. CPGA

[dilbert]

Hi folks, Mike!
Recently I was evaluating how to build a "modern" Disk II interface.
So I got a copy of " understanding the Apple ][", and started to thing about the
possibilities - and found some comments in the web, that an AVR micro-controller
would not be fast enough for this task. Therefore I started to implement a version
using VHDL and a XILINX CPGA - and thought about using a micro-controller only
for the RWTS and other "higher" functions.

Then I found your project

Nice work!
(Sometimes the Idea to rebuild a replica of the motherboard did cross my mind - but then I considered it
as much too challenging (for me) - Mike did it - fascinating! My highest compliments!)
Now I am wondering about my project - One the one hand it is a good start to learn VHDL, so I will likely
continue anyway... but on the other hand: Would there be any advantages using a CPGA?
Any opinion highly appreciated.

About the power supply:
That was an issue for me, too. When I started I was surprised that I would need -12V.
For now I am using a external power supply, whichs delivers all necessary voltages,
but I never thought about creating -12V internally (like you did using the 7662).
Thanks for that idea, now I have ordered some 7662 for testing - but looking at
the data sheet, I am uncertain about that the 7662 could deliver the requested
"juice" stable enough. Did you do some testing about that?

To forstall any questions: My project is not advanced enough to show any details or
results, I am still fighting with VHDL and other details (and unfortunately my spare time is limited).

Greetings from Germany,
Detlev

[mwillegal]

Hi Detlev,

I am coding in assembly and an 8MHz AVR is plenty fast for writing, though the code is tight in a few spots. Reading is more challenging (and fun), by far. I created a spreadsheet that compares read performance to the 2MHz sequencer that WOZ created as documented in "Understanding the Apple II". Currently the WOZ sequencer is slightly better in terms of data recovery, but I've got a few tweaks that haven't been implemented, yet. I'll likely post the spreadsheet onto my website, when I get done with the tweaks.

At this point, I'm pretty confident that the 8MHZ AVR will be fine, but if for some reason, I run into insurmountable difficulties, I could always add an external crystal and run it at 16 or 20 MHZ, in which case I'll have cycles to spare.

In terms of power supply, if I remember right, the -12V is only used in a voltage divider circuit on the analog board and current draw is very small (at one point I measured it). Though the 7662 is not regulated, it is running it off of the 12V regulator. Without no dynamic loads, I figured that it would prove sufficient as is. I believe I'm actually having issues with +12 Volts, during stepper motor operations, which has a lot more dynamic load. I haven't had time to investigate in detail, but I'm expecting that I'll need to beef up the +12 volt section up. Looking at all those disk interface bus lines dedicated to +12volts certainly is an indication of what is going on there, but I haven't actually measured it.

Regards,
Mike Willegal

PS Making the rev0 motherboard, has been a great experience.