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SoC Broadcom BCM2835

November 05, 2011 — BarryK
The RaspberryPi has a most intriguing Soc (System on Chip) -- CPU plus GPU plus more on the one chip. It is a Broadcom BCM2835, and has the ARM1176JZF-S CPU running at 700MHz.

Raffy recently posted a link to a PDF for the CPU:
http://infocenter.arm.com/help/topic/com.arm.doc.ddi0301g/DDI0301G_arm1176jzfs_r0p7_trm.pdf

However, the GPU remains a mystery. I can't find any details specs on-line, only this:
http://www.broadcom.com/products/BCM2835

Here is another board that has the BCM2835, the Roku2:
http://www.mycablealternatives.com/2011/07/roku-2-xs-teardown/

I would love to locate Xorg driver source for the GPU. I presume that the RaspberryPi people have such a thing, and will release the source.

Comments

Pi and chips
Username: lobster
Still using Slacko on a X86 awaiting Pi . . . :cry: I compiled Brandy Basic for Slacko, So BBC Basic (one of Rpi aims) is available as is the superior Linux only Bacon Basic. Vovchik is taking an active Bacon steak [eh . . . stake] and others are also hamming it . . . John (aka Judge Dredd our beloved and beleaguered forum uber meister) who created MurgaLua might well update and develop Lua on this board MU might come out of Puppy retirement (unless he hangs out in the German section of the forum) and offer Puppy Basic . . . We also have Vala and Genie as potential alternatives to C and Pythonesque languages NetLogo (requires Java) would be a great teaching language. Runs OK in Puppy. X-basic I have had running on Puppy etc, etc . . . I look forward to devx-arm :happy:

Booting Sequence
Username: Raffy
"Here's how the [url=http://elinux.org/RaspberryPiBoard]elinux.org wiki describes the booting sequence in R-Pi: [i]"[b]BootRom[/b] The boards do not include NAND or NOR storage - everything is on the SD card, which has a FAT32 partition with GPU firmware and a kernel image, and an EXT2 partition with the rootfs. We're not currently using a bootloader - we actually boot via the GPU, which contains a proprietary RISC core (wacky architecture ;) . The GPU mounts the SD card, loads GPU firmware and brings up display/video/3d, loads a kernel image, resets the SD card host and starts the ARM. You could replace the kernel image with a bootloader image, and that would work fine."[/i] Am still looking for the GPU binary...

Re boot sequence
Username: BarryK
"Yeah, I read that, it is fascinating.

framebuffer graphics
Username: tempestuous
"ARM devices are generally aimed at dedicated small devices like mobile phones, and as I understand it such devices use direct framebuffer access, with the display interface usually custom written. There's no X server, and no window manager. Once you introduce an X server and window manager, the streamlined ARM CPU/GPU then incurs a processing penalty that was not generally intended by the ARM developers. I'm really a bit surprised about the hype on the forum about getting Puppy onto ARM devices, since it may take a lot of optimisations for it to work properly ... but anyhoo, I think the Xorg solution in this case will be fbdev/uvesafb. See - http://www.murga-linux.com/puppy/viewtopic.php?t=67166 Note: the uvesafb kernel module is not enabled in most Puppy versions to date.

RasPi GPU
Username: Dougal
"Barry, this was answered in the Q&A: http://www.raspberrypi.org/archives/169#comment-2441 One of the guys is the head of graphics in Broadcom, so he has the knowledge (and connections) which enabled him to create that hybrid SoC. It looks like a decent toy, but not being able to use it with a VGA monitor kind of makes it useless for me (I'm certainly not going to connect it to the TV...).

Koji
Username: mavrothal
"Through the OLPC project, Fedora has done major strides in ARM building. Today they have almost identical builds for x86- and ARM-based XO laptops They use the koji build system (http://fedoraproject.org/wiki/Koji) and currently their ARM branch has 11000+ packages (http://arm.koji.fedoraproject.org/koji/packages). Cross compiling tools can be found in http://ftp.linux.org.uk/pub/linux/arm/fedora/cross/latest/ You may want to take a look.

A GPU with ARM11
Username: Raffy
"To help explain the boot sequence, here is a useful quote from the [url=http://www.raspberrypi.org/archives/169]Hardware Q&A: "jamesh on September 14, 2011 at 12:24 pm said: As Liz said the 2835 is basically a Videcore IV with the Arm11 bolted in (And Gert was on the team that did the bolting). Its why it has a rather unusual boot sequence the chip is a GPU with added Arm goodness, rather than an Arm with added GPU goodness."

Embedded SW Engineer
Username: Jim Manley
"Folks need to understand that the primary purpose of the R-Pi is educational at the _lowest_possible_cost_, not a be-all, end-all computing device for geeks doing high-end development. The cost factor eliminated VGA support which, in the grand scheme of things, is an end-of-life analog technology, as virtually no devices to be made in the future will support it, and the R-Pi could easily have a lifespan of a decade, or more, in various improved forms. Some may question why, then, is there support for composite video and the answer is that is much more common than VGA on analog TVs, VCRs, DVD players, etc., found at the lower end of the economic ladder around the world. The single signal line (plus ground) required for composite also makes it much simpler to support than the 10 lines associated with VGA. I'm guessing that composite comes free on the GPU's ouput, so, bringing it out to a connector is trivial. It should also be noted that, since the BCM2835 is a GPU with an ARM11 CPU tacked on, and not the other way around, high-resolution color graphics are going to be a primary focus in educational applications for the R-Pi, just as they were when the Apple ][ debuted 35 years ago (yeah, I'm an original owner of one, and picked it over the Radio Shack Model I, which only had lo-res B&W block graphics). The fact that it supports 3-D via OpenGL ES 2.0 shouldn't be lost on anyone, as it's pretty obvious that kids will be able to develop some pretty nifty games and virtual world simulations at 1080p/60 at the XBox 1 level, on a $25+ system. It will be interesting to see what the kids gravitate towards, and we can only hope the educators expose them to as many options as possible, if the kids don't ferret them out themselves.

TV support.
Username: Roger Wolff
"Jim Wrote: [i]Some may question why, then, is there support for composite video and the answer is that is much more common than VGA on analog TVs, VCRs, DVD players, etc., found at the lower end of the economic ladder around the world.[/i] I don't think that's it. the RPI is basically a breakout board for the BCM2835. The BCM2835 supports CVBS out, so the RPI does too. Why does the BCM2835 support CVBS? Because it is a chip intended to be used as a mobile phone or media center. Both of these traditionally have TV out. And also CVBS is a very "slow" signal, easily integrated onto a chip like the BCM2835. VGA can reach quite high bitrates so it is not something that is easily added with a few gates here and there.


Tags: general