Re: 16 MB REU clone

From: Marko Mäkelä (
Date: 2001-08-22 15:22:10

On Wed, 22 Aug 2001, Rainer Buchty wrote:

> Now, how about a power-fault detection which automatically saves the DRAM
> content to FlashROM when the machine is powered off; similarly the
> contents will be restored on power-on.

In my dreams, I had reserved the Flash ROM for PuCrunch compressed images
of games and utilities, like in the VIC-20 cartridge.  But in any case, a
feature that copies data from the SDRAM (or even from the C64 RAM) to the
Flash ROM would be very useful.  It would make copying to the Flash ROM
much easier.

> Do you circumvent that by some clever design trick I was too blind to see?

I'm not so familiar with DRAM refresh that I would have thought of such
problems.  And I still have to learn VHDL to write the logic equations for
the VIC-20 cartridge once a working prototype has been assembled.  (A
friend of mine soldered the chips to one board the wrong way around.)

But is the swap operation really different from the other three operations
(C64->REU, REU->C64, compare REU==C64)?  The operation takes 2 bus clock
cycles per byte, unlike the other operations which require one cycle per
byte.  In my opinion, the only difference is that you need to buffer two
bytes between the memory access cycles instead of one.

Christer Palm wrote:

> The Xilinx XC9500 CPLD series might be a better choice. It's available
> in smaller configurations, it can handle 3.3 and 5V I/O, it's flash
> based so you don't need the external EEPROM, but it's still in-circuit
> programmable.

That sounds nice.  According to the XC9500XL data sheet, the XC95144XL in
100-pin or 144-pin TQFP package could do the job.  There should be enough
pins for implementing a freezer cartridge like Action Replay as well.  
The software from Action Replay could be adapted rather easily to relocate
the RAM/ROM page from $df00 to $de00 and to move the configuration
register from $de00 to somewhere in the $dfxx page.

Ideally, the cartridge could consist of four ICs: the logic array, the
SDRAM chip, the Flash ROM chip and a 3.3 volt regulator.  Add some
capacitors and maybe switches for "reset" and "freeze", and a programming
connector for the logic, and you're all set.

> Working *together* on *free* VHDL models of the various custom chips (and
> the CPU) is something I'd highly appreciate.

Me too.  Do you know if the tools are freely available?  I like the Atmel
AVR family of microcontrollers, because everything can be done with free
software.  I use the avra assembler and the uisp programmer.


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