Michael Zucchi

 B.E. (Comp. Sys. Eng.)

  also known as zed
  & handle of notzed


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Tuesday, 05 January 2010, 18:09

Wrong UART

Well I got serial output working - was using the wrong UART (should've read the docs before assuming UART1 - it's UART3). And Das U-Boot leaves it properly configured so the output function is only 3 lines of code.

Then I hit an issue as soon as I started to use static data - my code wasn't being loaded at the right address (which took me some time to realise, since the code still ran). I don't really know why it works but I fiddled with the Das U-Boot mkimage line and now it's loading everything to the right address - it should be loading at 0x80008000 but I need to tell it to load at 0x80000000 to make it work. *shrug*

I got sick of ripping the SD card out to setup a new 'kernel' image (and a little worried, the SD slot on the beagleboard isn't really designed for repeated use), so I rigged up a rather nasty `runscript' (from minicom) script which uses ymodem to upload the image whenever I reset the machine. While the image is small this makes the testing cycle pretty easy. Just need a serial port thingy for my workstation and then I wont need the laptop anymore either.

Also along the way I wrote up some basic graphics output - just to clear the screen and display text, including a simple printf type thing. I converted my favourite font 'fixed' to a c file and can now dump text to the framebuffer. Originally I just wanted it to get some debugging output since I couldn't work out what was up with serial, but it's still handy to have.

And lastly I had a bit of a play with the DMA engine. And after a LOT of mucking about, finally got it filling rectangular regions. I couldn't for the life of me work out why double-indexed DMA just did nothing - in my haste to avoid reading the docs too closely I missed the bit which says exactly how it calculates the next address after a 'frame'. I had the alignment mucked up so it did absolutely nothing. If nothing else, it can be used for a very basic 2D accelerator - 90 degree rotations and flips, rectangular moves/copies, solid colour fills, and even masked sprites (colour keyed). So now my 'random filled box demo' runs a lot faster than it did before - around 10x actually (not that it was particularly optimised or anything).

I did some research on USB. Man what a mess that is. A few books i 'found' on the topic were useless too, just written from the perspective of windows driver writers at best (when a book uses 'visual basic' for 'portability' you know there's something very very wrong). I found some USB host stacks but they're proprietary, GPL2.x only, or BSD + advertising clause. And in any event tightly coupled to a given OS (as they must necessarily be). The Haiku one is at least X11 licensed, and clean and simple, so apart from being C++ might be the place to start if I decide to hurt myself one day with that pain in the head.

Hmmm, perhaps interrupts next, will have to get my hands dirty with assembly at some point.

Tagged beagle, hacking.
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