Commodore Free 12/Commodore_Free_Issue_12_2007_Commodore_Computer_Club.d64/t.minimig

Text File | 2023-02-26 | 11KB | 343 lines

u
-Minimig
An Amiga in an FPGA

What is Minimig?
Minimig stands for Mini Amiga. Minimig
is an FPGA-based reimplementation of
the original Amiga 500 hardware. In
it's current form, Minimig is a single
PCB measuring only 12*12cm which makes
it the smallest "Amiga" ever made & the
first new "Amiga" in almost 14  years!

Minimig is available for download as
an open-source/open-hardware design
under the GNU public license. This
page describes the architecture and
the inner working of the Minimig. All
design files can be downloaded from
the download section.

History
The idea to make Minimig started
around january 2005. The C64DTV had
just been released and the Amiga
forums were buzzing disccussing the
possibility of putting a complete
Amiga with games inside a single
joystick. Things like ASIC's, FPGA's
and VHDL were discussed and being a
hardware engineer, they immediately
caught my attention. I remember that
the discussions ended with the
conclusion that it should be possible
to put an Amiga in a joystick but that
it would be a very difficult task. The
first step of such an undertaking
would be to reverse engineer the Amiga
chipset & run it inside an FPGA.

The following weeks I discussed this
idea with a collaegue who also
happened to be an Amiga enthusiast. He
did some FPGA programming during his
previous job.The more he told me about
FPGA's and the more I dug into my old
Amiga literature, the more I became
convinced that it could indeed be done
And so it started, I learned Verilog,
bought an FPGA board and started
coding! It took me almost a year to
get the Minimig to boot it's first
game (which was Lemmings, by the way).
It was and is the largest hobby
project I have ever started.

The first version of Minimig was
built around a Digilent Spartan-3 FPGA
starter board, which I expanded with a
real 68000, an upgraded vga output and
a PIC- controller based floppy
emulator. That version can be seen in
the picture to the left. Later I moved
the design to it's own custom-designed
PCB which I called rev1.0. That is the
version that is described here.

Minimig rev1.0 technical description
The Minimig rev1.0 is built on  a
single 12*12cm PCB that contains all
components to make up a complete Amiga
It has no floppy drive or harddisk.
Instead it is equipped with a MMC
flash card slot and a microcontroller
based floppy emulator.

The flash card holds the disk-images
which can be "loaded" into the Minimig
using a convenient on-screen-display.

The (physical) hardware of the
Minimig consists of 4 major parts:

The FPGA
The 68000
The RAM
The PIC controller
The FPGA
The FPGA is the heart of the Minimig.
The FPGA used is a 400Kgate Spartan-3
byXilinx. All the other major
components (RAM and 68000) connect
directly to the FPGA. The FPGA
implements the Amiga custom chips
Denise, Agnus, Paula and Gary as well
as both 8520 CIA's. It also implements
a simple version of Amber so that VGA
monitors can be connected. Besides
this, the FPGA also acts as an
automatic joystick-mouse-switcher, a
PS2-to- Amiga-keyboard converter,
PS2-to- Amiga-mouse converter and as
an OSD (on-screen display) generator.
All of these function were not present
in the original Amiga, but make life
much easier now that we are living in
the 21th century! The Spartan-3 is a
ram-based FPGA and must be loaded with
a "core" upon startup. This is done by
the PIC controller described below.

The 68000
The 68000 is the Minimig's main
processor. The Minimig uses a special
version of the 68000: the MC68SEC000.
This version runs at 3.3V and is
completely static (so it can run at
any frequency between 0 and Fmax).
This makes it an excellent companion
for the Spartan-3 FPGA as there is no
need for level-shifting between 3.3V
and 5V levels. The MC68SEC000 connects
directly to the FPGA.

The RAM
The Minimig rev1.0 board contains
2Mbyte of 70ns static ram. The RAM is
organised as 2 524288*16 banks. Each
bank has seperate enables for the
upper and lower byte. The RAM is used
to implement the 3 types of memory
needed by the Minimig, namely:
kickstart rom area, chip ram and
(ranger) fast ram. As the Minimig has
no kickstart socket, the kickstart
image must be loaded upon startup.
This is done by the PIC controller
described below. Once loaded, writes
to that area of the RAM are disabled
and the area acts like a read-only
memory. The remaining part of the RAM
(1.5Mbyte) is divided up between chip
and fast ram. NOTICE: The ST RAM chips
used in Minimig rev1.0 are obsolete.If
you want to build a Minimig rev1.0,
please check the availability of all
used parts first. A suitable
replacement for the ST type is the
ISSI IS62WV51216BLL-55TLI. These chips
can still be bought from Digikey
(part number 706-1048-ND)

The PIC controller
The PIC controller fullfills the role
of "bios". It is a single chip 8-bit
microcontroller from Microchip. The
PIC controller configures the FPGA
(by loading a core into it), loads the
kickstart image into the kickstart ram
area and acts as an Amiga floppy
emulator. Thus, the PIC controller
really starts the system up as soon as
power is applied, hence the "bios"like
function. The Minimig uses a PIC
controller type 18LF252/SP.

FPGA general internal architecture
Besides the physical hardware, there
is the "programmed" hardware inside
the FPGA.This hardware is described in
Verilog. To keep the project
manageable  have kept the same
organization as a real Amiga. That is,
I have kept the Denise functions in a
Denise module, Agnus functions in an
Agnus module etc.I have even kept a
lot af the signal names the same, so
there is a dmal signal(as well as an
extra dmas signal), an int2 signal,
an ovl signal and so on. Besides these
standard modules, there are also 2
bridge modules to connect the FPGA
hardware to the RAM and 68000 chips.
The code for the FPGA has been
synthesized using the free webpack
tool V9.1i from Xilinx. FPGA internal
bus structure and clocking scheme
This needs some explanation as it is
quite different from a real Amiga 500.
Whereas the Amiga 500 had a seperate
chipram bus and fastram bus, the
Minimig has only a single, synchronous
multiplexed bus. To compensate for
this, this bus is clocked at
7.09379MHz or twice the speed of an
PAL Amiga 500 bus. This clock
is the Minimig's main clock (called
"clk" in the code). It is the clock
far ALL Minimig sub-systems, including
the CIA's. As the CIA's are normally
run at the so-called E clock
(709379Hz), special circuitry has been
added to Gary to slow down CPU
accesses to the CIA's to approximately
E clock speed. Clk is also used as the
Minimig's pixel clock. For hires
screen-modes clk is "double pumped",
with new pixels put out at both the
rising edge and the falling edge of
clk giving an effective pixel rate of
14.18758MHz. Besides clk, two other
clocks are generated; qclk and vgaclk.
Qclk is clk shifted by 90 degrees.Qclk
is used by the SRAM bridge to control
read/write timing. Vgaclk is used as
the pixelclock for the Amber
scandoubler module. All clocks are
derived from a single 4.433619MHz PAL
crystal using the FPGA's DCM module
(Digital Clock Manager)

The Minimig internal bus is used as
both the chipram bus and fastram bus.
All modules (including kickstart area
and CIA's) connect to this bus. This
bus is time-multiplexed between
chipram and fastram/kickstart/CIA's.
The mulitplexing is controlled by
Agnus and the horizontal pixel counter
"horbeam" The lower 2 bits of horbeam
define 4 types of bus "slots" :
slot 2'b00: fastram (68000)
slot 2'b01: chipram (disk, bitplanes,
copper, blitter and 68000)
slot 2'b10: fastram and blitter (non
standard, gives cpu some more cycles
in chipram to fix some compability
problems) slot 2'b11: chipram (disk,
bitplanes, sprites, audio and 68000)

The Agnus module passes the signals
dma,dmapri and dmawr to the Gary
module to indicate the type of bus
slot. Dma indicates that Agnus is
doing a bus cycle
(read or write) and dmawr indicates
that that cycle is a write cycle.
Dmapri indicates that Agnus only holds
the bus bus does not write or read it
(Agnus does a "dummy cycle). If both
dma and dmawr are inactive, the CPU
can use the bus if it wants to.

Because the FPGA does not supports
internal tri-state busses, all
devices are connected together using
'or" gates. The convention is thus as
follows; when  a device is not
selected, it drives it's outputs low.
When the device is selected,
it drives it's outputs with the data
it wants to write.

Boot sequence
The boot sequence is a 2-step
process. The first step is to
configure the FPGA.
Like said, this is done by the PIC
controller. The second step is to
load the Kickstart image. This is done
as follows;

Once the FPGA is configured, the
system is booted in a special state.
In this state, a small bootrom is
overlayed at addresss #0. This bootrom
loads the kickstart through the floppy
emulator. Once the kickstart has been
loaded, the bootrom resets the system.
The bootrom then disappears from
address #0 and the system boots as if
it were a normal Amiga. The code from
the bootrom is written in 68000
assembly. I have used the freeware
AS32 assembler from the Freescale
website. I have made it available for
download here as I can't find it
anymore on their (again...) redesigned
website. PIC controller firmware and
FPGA to PIC communication

The PIC's firmware is written in Hi-
Tech Ansi C. The firmware contains
MMC (Multi Media Card) and FAT16
drivers to control the flash card. The
firmware also handles the user-
interface and on-screen-display. The
PIC communicates with the FPGA through
an SPI interface. The MMC card is also
connected to this SPI bus. In it's
current form, the FPGA has 2 SPI
"addresses". Address #0 is selected by
the fpga_sel0 signal and control the
floppy emulator. fpga_sel1
controls the on-screen-display.
fpga_sel2 is currently unused.

Disclaimer 1: About the Kickstart
To function, Minimig needs  a
kickstart rom image. The Kickstart is
a copyrighted piece of software.
Therefore, you are not allowed to just
download it anywhere from the web. You
must own the original kickstart ROMs &
make an image of them using the method
described in the UAE archives.

Disclaimer 2: If you decide to built
it. Do so completely at your own risk.
This is not a beginners project.

What does the future hold for
Minimig? I don't know. My hope is
that due to the GNU public license
people will debug it, expand it and
generally make it better.
What I would like to see first is the
implementation of some form of
harddisk support, ethernet support and
offcourse a debugged sprite engine
:-). It would also be nice if the
verilog sources of Minimig would make
it into a sourceforge project. I could
really need some help there. And the
rest? Only time will tell!

Reprinted with the Copyright holders
permission information taken from
http://home.hetnet.nl/{$de}weeren001/
Commodore Free would like to thank
Dennis van Weeren for allowing the
reprint of the article