[Coco] DECB -> Pi2/3

[Gene Heskett]

On Monday 06 March 2017 00:36:10 James Ross wrote:

> I’ve wondered recently, specifically w/ the Pi2/3, it seems many
> people in the CoCo community have one, would there be an appetite for
> a project that ports the CoCo’s 6809 DECB code directly to the Pi’s
> native ARM code.
>
> What made me think of this was Dave Philipsen comment on boot times of 
the Linux distro on a Pi:
> > If the Pi only had to do what a CoCo does it could boot even faster
> > than a CoCo.
>
> The idea is to have a device that is instant on, running on
> bare-metal, sitting there w/ the flashing cursor ready to run Disk
> Extended Color Basic (DECB)  at full Pi2/3 speed – not emulated.  You
> could have two modes: one where the CoCo’s hardware (peeks/pokes) is
> emulated and an Enhanced Mode with all the modern capabilities of the
> Pi's I/O's … etc … Must have's would be a full screen editor,
> structured basic mode w/o line #’s (Basic09'ish?) ...
>
> The problem I see w/ the Pi running bare-metal code, you would not
> have the luxury of the OS drivers to talk to the Pi’s hardware. And
> that could be the real show-stopper due to the learning curve of the
> hardware.  I wonder just how trimmed down you could get the Linux
> Kernel + just the essential Pi drivers (screen/sound/USB/IO/etc) --
> but then does it lose it's appeal if you're still having to boot
> Linux?
>
> Does the idea fit the nostalgic hobby (retro-ish) criteria? Anyway,
> not even sure if I would interested in using such a device, or if it’s
> the challenge of the project that sounds fun! LOL! :)  It's just an
> idea I have thought of recently.
>
> James
One should remember that with the pi, it has neither a serial port, nor a 
parallel port, the hardware just isn't there, so you have 3 choices for 
an expansion interface. Obvious is 4 normal usb2's, and the otg port is 
I think usb3 but is used as the power connector in most cases. Then 
there is a 100MBit ethernet rj45 that puts it online with the big boys, 
and no reason it can't address other ethernet hardware about the 
homestead. But an SPI port is the shining star in the pi world.

It does have some gpio that can be quite fast, and in my case its running 
a serial port called an SPI using just 4 lines of GPIO. This is 
currently set to exchange any number of 8 bit words, in my case 4 at a 
time for a 32 bit "packet".  And it runs at 32 megabits a second when 
connected to a Mesa 7i90HD interface card!  This card is an fpga design, 
and the firmware currently loaded gives me 4 pwm generators to drive 
servos, 4 encoders to track where the servos are, and 8 stepmotor step 
generators. With 3 each 50 pin ports, I have around 50 pins left over 
from all of the above, to use for home switches and other toys. All I 
have to do is write the hal code. For me, literally I/O to throw away in 
a 35 dollar pi 3b and a 62 dollar interface card.

But those are the I/O choices on the pi's. USB, write your own GPIO 
stuff, or build an fpga card that talks spi. The latter will give you 
access to all the power of that quad core, 1.2 GHz clocked arm A53 
processor.  And the ability to do most anything such as running nitros9 
at a higher speed than it can run natively on the coco3 once an 
assembler has been written.   And thats just the starting target. Much 
like the original coco, your imagination is the limit of what you can 
do. And do it at 20% of the original coco's $400 for a 4k memory 
machines price.

Cheers, Gene Heskett
-- 
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Genes Web page <http://geneslinuxbox.net:6309/gene>