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30th-Aug-2012 02:44 pm
Let there be light:
2012-08-30 GD-EMU proto V2 #1

And there was light:
2012-08-30 GD-EMU proto V2 #2

More pictures to follow soon :)

Status so far:
Voltage regulators - check
MCU starts - check
Bootloader operational using 3V3 UART - check
MCU JTAG - check
C runtime stub + simple exception handling - check
Status LEDs - check
UART 115200 8N1 console - check
Interrupts - check (need to investigate if registers are really properly saved though)
External RAM - check (problem found, should be fixed now)
High speed SD interface - in progress

Random fun fact: Many SD/SDHC cards exhibit various little quirks in SPI mode so the code needs to be aware of those to work properly in every case. One would think the native SD protocol is so tightly standardized that there should be no such surprises. Well, I just found a bunch of 2GB Kingston SDs that respond to ACMD41 with bad CRC7...

EDIT: Turns out the R3 answer is the only one not protected by CRC7, that space is marked as reserved and just filled with all-ones. I'm still not getting the busy bit within reasonable times on these Kingstons but I suppose reading the docs few more times might teach me something new again.

Anyway, here's the actual thing:
2012-08-30 GD-EMU proto V2 #3

Now it's a proper prototype, with all these wires and blinking LEDs. A few things are still missing on the PCB but right now I need to get SD protocol working so I can fetch FPGA configuration image and test it.


High speed SD interface - check
DMA on SD i/f - check
Basic FAT support - check
FPGA - in progress

I'm using my own FAT library, which has no write support but it was designed to be fast while consuming as little RAM as possible. In fact current SD cards are so fast it makes sector buffering impractical, since the lookups and LRU queues kill any gains with additional overhead. I suppose it'd be different if the CPU was clocked above some 400MHz and had some fast L1 cache.
Right now I get average of ~10MB/s in test that seeks to random part of 1.2GB file and reads 1-3500 consecutive 2352-byte long chunks. This is to simulate RAW image reads for GD-ROM. So pretty well I'd say, a nice boost compared to 2.5MB/s I got over SPI.

The native SD interface required a pretty much complete rewrite of some code, so I'm not 100% sure it's stable and all, but seems to work for hours without problems so far.
1st-Sep-2012 05:37 pm (UTC)
ISO - yes, but keep in mind this will not boot. So, ISO is only good for homebrew and/or with boot disk. See below.

CDI - I want to support it but there are some legal issues (it's not a free format, it belongs to a company that made DiscJuggler)

GDI - it's free :) But the fact that it has one file for each track is a bit problematic, especially while trying to play CDDA without breaks between tracks.

So, initially I will support GDI but I want to make my own format that will work better with this device. There will be a converter for GDI and CDI, so that anyone can make SD card with games on it.

I plan to have several games on one SD card. But I want to make it a simple solution. So, it will be a structure like this (4GB card):

\ (root folder)
GD1 <- first game
GD2 <- second game, disk 1
GD3 <- second game, disk 2

The images can be switched with the two buttons, and the LEDs show which image is selected. So there is going to be a limit of about 16 images or so. I plan to support hot-swapping the cards as well.

With this solution you can also have boot disk image as "game 1", and then the rest can be non-bootable images.
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