XBoard
The Xboard is a multifunctional
expansion board for the Atari 7800 gaming console. The board is equiped
with 128Kbyte of fast, banked SRAM that can be used for program code,
graphic data and of course plain old game data ;-)
It also has a socket for an Atari POKEY
sound chip. This is the same sound
chip found in the Atari 400/800 and other Atari 8-bit computers as well
as the Atari5200 game console.
The XBoard was designed to sit in the
place of the MARIA chip inside the A7800. This poses a problem for many
NTSC A7800's as the MARIA chip is soldered onto the PCB. For PAL A7800's
this is not a problem as the MARIA is socketed and the XBoard is simply
dropped into the place of the MARIA.
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The X-Board is exclusively sold by AtariClassics
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Memory mapping:
The XBoard introduces three new devices into the memory map of the
A7800. The first device is the control register of the XBoard. This
register is used to enable or disable the two other devices, the RAM and
the POKEY. The control register also contains the bank select bits which
allows you to select one of the 8 16KByte banks.
XCTRL bit description
| 7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
| NU |
NU |
NU |
POKEYEN |
MEMEN |
BS2 |
BS1 |
BS0 |
| POKEYEN |
Enable POKEY
support (1 = POKEY's enabled) |
| MEMEN |
Enables SRAM and
banking support (1 = SRAM enabled) |
| BS2, BS1, BS0 |
Selects 1 out of
8 16KByte banks |
| All
bits are reset to 0 at power on, which means both the POKEY
and SRAM is disabled when the system is started. |
This is how the XBoard maps into the
A7800 memory map.
| Description |
Start
address |
End
address |
Size
(bytes) |
| POKEY1 |
$0450 |
$045F |
16 |
| POKEY2* |
$0460 |
$046F |
16 |
| XCTRL |
$0470 |
$047F |
1 |
| SRAM |
$4000 |
$7FFF |
16384 |
* = The control PLD on the board has a
chip select for the second POKEY routed to a connector on the board.
Sound output section:
The output signal from the sound chip is fed through an analog
switch which is controlled by the POKEYEN bit in the XCTRL register. The
reason for this output switch is that the POKEY chip does not have any
external way of reseting it and could theoretically start up playing a
sound. To avoid this the analog switch disables the sound when POKEYEN
bit is zero.
The board is also prepared for adding a
second POKEY with a connector that connects to the POKEY sound output.
More information about this will be available later.
Software:
Due to the complexity the board needs some setting up before it can
be used. Here's a
snippet that will help you to enable the memory and do a quick memory
test. The code is only to give you an indication on how it can be done,
if you have a better algorithm please feel free to share with the rest
of us.
You also need to completely inititalize
the POKEY before starting to use it. This
shows you how.
Processing:
All boards are carefully tested before shipping and are guaranteed
to work, however I can not assume responsibilty for installations not
performed by me.
If you have any question you can always
contact me.
(C) Pontus Oldberg 2006
- Last Updated: 2008-11-01
You can freely use any images, source code, text or other material from
this site as long as it is for a non- commercial purpose. If you want to
use something for any other purpose please contact me.
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