Raspberry Pi - Hardware Flaws and Fixups?

Not  a flaw, but I'd like to know if in future it'll be possible to power more demanding usb2 peripherics from the onboard hub, having the right power source; I'm referring to a RPF blog entry where protections on power input were discussed.

My interest would be the possibility of plugging a 2,5 harddrive, and have it powered directly by the RPi, without the use of an hub; right now this is not possible, since the spinup currents would fry the tracks, 500mA max if I remeber it right, where a spinup is >1A.

Is this feasible? Sure it's more for non-educational users, but it'll be a nice plus.

Unrelated question: I read that the usb and the lan share bw, is that true? If it is, can you give some details about how that is managed? Like usb attainable speed, lan attainable speed, both combined...

Thanks.

piovrauze wrote:

Unrelated question: I read that the usb and the lan share bw, is that true? If it is, can you give some details about how that is managed? Like usb attainable speed, lan attainable speed, both combined...

Thanks.

The SOC chip is meant to be used  inside a phone. So whereas modern CPUs have high speed busses to connect to peripherals either on the motherboard or on expansion cards, that simply isn't going to happen for a chip inside a phone or tablet.

So the SOC has... USB. The model B has an USB-ethernet dongle integrated on the board. And because there is just one USB port on the chip, there needs to be an USB hub on the board as well.

Roger Wolff wrote:

The SOC chip is meant to be used  inside a phone.

 

The BCM2835 SoC wasn't developed for use inside a phone, at least not as a primary target.  Not many phones have an HDMI socket.

Broadcom describes the target applications clearly here: http://www.broadcom.com/products/BCM2835

• High Definition 1080p Embedded Multimedia Applications Processor
• The BCM2835 is a cost-optimized, full HD, multimedia applications processor for advanced mobile and embedded applications that require the highest levels of multimedia performance. Designed and optimized for power efficiency, BCM2835 uses Broadcom's VideoCore IV technology to enable applications in media playback, imaging, camcorder, streaming media, graphics and 3D gaming.

In other words,  applications like the Roku 2 media player which uses the BCM2835, of which Roku has sold 4+ million now.  The "advanced mobile" probably refers to the "camcorder" in their description, although I'm sure they're not ruling out mobile gaming devices too which connect to HDMI TVs, but probably not tablets because the 700MHz ARM1176JZF-S is rather weak compared to the 1+ GHz Cortex-A8/9 of almost all modern tablets. "Phone" is really quite a long shot, and isn't even hinted at by Broadcom.

Morgaine.

So, is the 2xUSB+1xETH limited to the speed of 1xUSB? That is what I understood, but how is that managed internally? And what are then the results in speed terms on the other side? The ETH is advertised as 10/100, so the usb will be slower? Or it's set up to share BW between the two? That's what I am interested in.

Yes, ALL usb+ethernet are limited to 480mbps total.

"how is this managed internally?" I don't know what you mean. It's just an USB bus. Internally there is some polling going on. There are endpoints. USB is complicated.

I suspect that the ARM will be able to provide enough data so that sustained transmitting at 100mpbs is possible. I suspect that the ARM is fast enough to stash the incoming data such that sustained receiving at 100mpbs is possible.

Doing both at the same time is probably going to bottleneck somewhere. CPU horsepower? Effective bandwidth on the USB? I don't know.

" Or it's set up to share BW between the two? That's what I am interested in. " What is "it"? The ethernet chip is just an USB device. The host has the option to schedule bandwidth between different devices. Linux does this reasonably.

The BCM has a single USB2.0 connection to the SMSC9512 (take a look) that provides the 2 USB + Ethernet.  So yes from a simplistic hardware viewpoint you have 480Mbits/s to share between all three.  The SMSC looks like a hub for three devices so dynamic sharing will go on.

From the design point of view this was seen to be more than adequate for the educational target (an yes I will keep harping back to that - sorry) and was a nice (low cost and smallish) solution.  I may still moan a bit about power consumption but welcome to mixed signal.  In a general setup where you have a keyboard and mouse - clearly not a problem.  I've seen some people wanting to put Drives up on the USB.  So if you were say streaming off the internet or a usb cam and recording on the fly you might run into limits that will cause you problems rather than just being sluggish.

Before anyone bails in - I have not considered software/driver overhead, bit 'wastage' through protocol and other good stuff.  In reality this will need measurement in real applications to come to a firm conclusion.

I'll have a hunt and see if I can dig up some more data (but my bandwidth is very limited over the weekend) .. you could cross post onto raspberrypi.org and a software guy may be able to enlighten us as to what we should really expect, if it's actually been measured yet.

Pete

Thanks both of you for making it clear for me.

I don't plan to push the usb/eth to the limits, maybe not even half of it combined, so it shouldn't be an issue.

What about the power related question (more amperage on the usb port)?

PeteL,

   Thanks for the responses and educated answers.  I'm actually really liking this thread since we are discussing something other than ship dates.

    I agree with your statement on the intent for the RPi, I don't remember a home streaming media server being on the list.  ;)

    I do see though many users finding the limitations through experimentation.  The first 10k recipients hopefully will be filling these forums with data and stories so the rest can modify or tweak their plans.  I would also hope that we could affect future versions by providing documented "deficiencies".

Kevin - I agree, people will find limitations and probably some issues that will need a workaround.  I'm keen to get input on what people have found and what should or should not be included in future versions / revisions.

I keep saying that the roadmap of activities do not extend beyond the educational release, but we all know it's fun to consider what would be possible in the future. 

Innovation - 1 part enabling technology, 1 part funding, 100 parts dreaming what if !  If we can get the kids thinking then I'll be a happy man rather that a grumpy 'ol engineer!

Pete

Generally, the best throughput you can get out of USB 2.0 Hi-speed is about 2/3 of the signalling rate, or about 320Mb/s, half-duplex. In HDD terms, that's about 40MB/s.

Generally, the best throughput you can get out of USB 2.0 Hi-speed is about 2/3 of the signalling rate, or about 320Mb/s, half-duplex. In HDD terms, that's about 40MB/s.

Perfect, I estimated the same numbers, so now they shuld be right.

This is more than enough BW for my needs, and I just need the HW to test it live now... XD