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USB dev cart & SD card interface on a single cartridge

Written by cafealpha 1 comment
Last week, I populated the second "USB & SD card" dev cart ever

data/images/20160112_usb_sdcart.tb.jpg
USB & SD dev cart Rev 3.2

Why "second" ? Because the first "USB & SD card" dev cart I made is the one I'm currently using for dev :
data/images/20150716_memcart_rev2f.tb.jpg
USB & SD dev cart Rev 2f

PCB revision is different ... can you spot the differences ?

It can't be seen on the picture, but Rev 2f have a 128KB SRAM chip soldered on its rear side.
I actually don't use it, this is just a remain of the times I though extra RAM was required in order to get "SD Memory Cart" features to work.
After many tests, I could verify that it was not needed : SH-2 code for "SD Memory Cart" feature runs directly from Flash ROM, and 16KB+8KB of work RAM provided by games for Backup library is enough for my needs.
So I removed the routing for this SRAM chip on Rev 3.2 PCB

Another story, but about Rev 3.2 cartridge pictured in this article : I was about to trash it because I couldn't flash its CPLD firmware, and hence thought it was broken.
I first though that CPLD itself was broken (this may rarely happen when overheating it during soldering), so I removed it and soldered another CPLD, and ... same behavior
CPLD itself is detected by Quartus, but flash always fails at 98% of the flashing process. I (finally ) start to think it may be a power supply related problem, and ... bingo : CPLD wasn't 3.3V as expected.
In fact, this board features two 5V -> 3.3V power converter units (on PCB rear side, can't be seen on the picture above), a "spare" one not connected anywhere (just here in case of), and the "real" one, and I populated the "spare" one
In order to fix this, I didn't populated the "real" one, but instead connected the spare one to SD card and CPLD power supply line, and that's why there is a grey wire nearly USB connector



Final note : as I consider USB port for debug only, I don't plan to improve the architecture and routing of such "USB & SD dev cart", so I suppose its design will remain "messy and fat" for a while ...
But in the hypothetical case many talented developers will suddenly arrive in Saturn Homebrew scene and will want to develop games using both CD ROM for music and SD card for huge game data (yeah, I really want a Saturn game with gameplay/level/whatever data so large that it can't fit a CD-ROM !), I will re-design it in order everything fit in a standard cartridge shell

1 comment

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