Another CPU in the Atari XL / XE series, how and why?


Features of the complete PCB design.
- A 65C02 CPU with buffered address and data bus.
- A second PIA. One port with a RTC. The second port
  with buffering to create a printer port.
- PCB may be cut to only use CPU or PIA.
- Device driver software for the Real Time Clock.
  (Including source code- only on paper yet, is tested.)
->{Driver software for the printer may be available too. I'll check}
- Board replaces the original CPU and PIA. Both have to
  be removed, the PCB occupies their previous location,
  PIA can be reused on the PCB.
- The CPU may also be replaced by a 65C816 with
  some minor modifications.


How to replace?
- Remove the old CPU and PIA.
- Replace the old PIA in the board.
- Plug the board in place of the old IC's
Done.


Why the second PIA?
To be able to use the RTC and add a printer port, without
compromising compatibility. If the extension is not
wanted, don't load drivers and it's not there.


Why replace the CPU?
- The original 6502 and also the ATARI 6502C have some minor bugs.
- The 65C02 fixes these and also adds some very useful instructions.
- Also now the CPU address and data busses are buffered.
- The 65C02 is CMOS and draws less current.
- Some Atari CPU's, manufactured in Mexico are defective. They
  are temperature sensitive and make the computer hang.
- Test: In Basic: 10 FOR X=0 TO 100000: ?X; " Atari": NEXT X and
  use a hair dryer to heat up the processor (CO14806) If the
  screen freezes, there's your problem.
And finally, it's fun to build hardware...


Design history:
 The schematics of the old 400 / 800 contain the CPU part already.
So this was just simple to do. And back in 1986 it was expensive
to buy an Atari CPU. The 65C02 was less expensive and could also
be buffered. This is useful if you want to add more hardware to
the system. And more hardware was something that was wanted. And
when a PCB has to be designed, why not do something more. So also
a Real Time Clock was added. But this called for an extra PIA.
This could be done in the cartridge port also, but this would be
used up then. And if someone decides to open up his computer, why
not do more at once.  This extra PIA would then have one port left.
In turn it would be good to use this port for a centronics printer
driver. This driver would be a chance to call for a contest.

 Why hardware was needed? It's good material to get a magazine
filled. And the ATARI Magazine, by the SAG (Foundation of Atari
Users in the Netherlands) could use some good information. In the
interest of 7000 users it's nice to help keep prices low. This has
been the philosophy of the SAG all the time. If you publish
something and offer parts, it keeps prices low.