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:

 

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.

I know you mention not using a hub, but consider this: A tiny 4-port USB hub with a 2A power supply (the norm) could power both the hard drive and the RPi (with a USB type-A to micro-B cable).  As long as the overcurrent monitor on the hub is ganged (most are, since it's cheaper), there would only be a problem with overcurrent if the total draw is more than 2A. And you'd have two extra USB ports to boot.

I understand the point, and I'll have to use a hub, still I find it more attractive to use less HW.

Would it be possible to add the 5V power source directly on the onboard usb port, in parallel wit the board supply? (using the same power source)

If I use only one of the 2 usb ports, I could do it without soldering.

Is this feasible? what happens to the rest of the RPi, does this bypass the board overcurrent protection?

PeteL has said that you can bridge the fuses. You'll be stuck with "just" the 700mA entry-fuse on the microUSB port. And as long as the total doesn't exceed that, you'll be fine.

I won't be doing this mod before Farnell has RPI's in stock for "next day delivery".

That's for sure! I won't be doing HW endangering stuff till there is a costant supply of RPi ready.

This one will be kept clean of mods, and I'll use it for testing the sw and limits of the HW.

@PeteL: I remember Arch distro "overclocks" the SoC to 800MHz, how does this affect power consumption, temperature and emissions?

There is a report of an expensive TV being zapped when connected

to the RPi hdmi port.

http://www.raspberrypi.org/phpBB3/viewtopic.php?f=6&t=5632

perhaps prudent to use only inexpensive monitors until more is known

about this issue.

There is a report of an expensive TV being zapped when connected

to the RPi hdmi port.

http://www.raspberrypi.org/phpBB3/viewtopic.php?f=6&t=5632

perhaps prudent to use only inexpensive monitors until more is known

about this issue.

You know, I'm beginning to have my doubts about the whole power philosophy in the Pi's design.  Not for the next minor board update(s), but for the next major board design, I think the entire question of power should be reopened from scratch.

The concept of using cellphone chargers for power sounded like genius originally, but in practice it has turned out to have a very ugly underbelly.  These chargers are made for charging, not for delivering constant power to downstream USB peripherals, and they're made to such a rock-bottom price that a very high proportion of them has pretty dreadful electrical performance.  This has been a source of many problems for Pi users already, who commonly go through several chargers before finding a good one.

It's no surprise really, with the benefit of 20/20 hindsight.   Chargers bundled with cellphones are only expected to work with one specific load, their cellphone.

The polyfuse issue has exacerbated this.  It's bad enough using a wonky charger as a power supply, but when you then kill off voltage regulation almost completely by putting such high-resistance polyfuses in the supply line then you're just asking for trouble.  For all intents and purposes, the Pi is supplying an unregulated USB supply.

It's not quite a disaster just yet, but I'm pretty sure that it's possible to do far better than this and still remain within the price niche, particularly now that BOM costs have dropped hugely as a result of volume manufacturing.

Morgaine.

I've only had a little experience with polyfuses.  That was a product where part of the manufacturing process included tweaking the 5V power supplies to compensate for the voltage drop across the polyfuses.  Not a very nice way to manufacture products.  My suggestion (which was adopted) was to replace the polyfuses with automotive mini-fuses, which have very low resistance and are easily replaced.  Sure, self-resetting fuses sound wonderful and convenient, but we're seeing lots of problems with them in practice given the variability of power supply voltages, micro USB cables, and polyfuse characteristics.

I agree with Morgaine that the increased volume suggests that choices made to save pence should be revisited.  Personally, I like the idea of a built-in switching power supply with 3.5 mm jack and a bridge rectifier on the input so that you can hook up any 6V to 12V wall wart without worrying about polarity.

I agree 100%.

While I accepted it, I've got never convinced that using a charger and the USB connector for input power were the best approach.

I asked Pete more than once if they ever considered using a power management IC instead LDOs, Eben at some time talked about it, but never got/saw a response.

There are plenty of low cost/acceptable quality power supplies out there, and by the volumes/interest we are seeing with the Pi I'm sure the distributors can get a

very good price and bundle as an optional a power supply.

Get rid of the fuses, there are input voltage protection chips like the NCP349 used on the Beaglebone and USB power switches with protection can be used

in replacement of the fuses.

-J

Replacing polyfuses F1 and F2 by wires leading back to the power input point VIN is quite likely to become Raspberry Pi Hardware Hack #1.

Morgaine.

Coder

I can't seem to reply to that post. ( http://www.raspberrypi.org/phpBB3/viewtopic.php?f=6&t=5632 )

I suspect his TV decided to quit, and it seems thats not unusual for these models.

http://www.badcaps.net/forum/showthread.php?p=205335

The good news is the link above may help him fix the TV, but he may need to login to get the pics and cct diagrams.

I can't see how the Pi could cause the power issue he had.

It took out the mains supply to the house, rather than just stopping the TV from working.

Even if the Pi 5v was high (or low) the mains supply will not be effected in that way. The worst would be the TV fuse blowing, or the magic smoke escaping from a component, and therefore it stops working.

On the charger note, my son is waiting for his Pi, and we tested his charger supplied with his new smartphone, and it was 4.6v across TP1 and 2.

So I would suggest that someone develop a small 5v supply that takes a 9v to 12v Wallwart, and end this varying 5v situation once and for all.

Mark

I used to think that the Foundation's choice of cellphone chargers for power was "genius", but that's now changed to "worst idea ever".

It's only in the light of experience that we now know that cellphone chargers are a complete disaster, and don't really qualify as "USB power supply" at all.  They've taken the concept of voltage regulation back in time by 3 decades.

I strongly believe that Pi v2 needs a completely different power source and power management strategy.

Morgaine.

OMG, it's just been pointed out to me on the Pi forum that the USB spec for "chargers" allows the voltage to drop to 3.6V up to the rated current, as long as it supplies 5V at up to 500mA.  Forum link:  http://www.raspberrypi.org/phpBB3/viewtopic.php?f=24&t=6187&p=80651#p80651

If so, it's no surprise at all then that people are experiencing problems.

UPDATE: It may not be so, the Wikipedia entry is not consistent with the spec.  See the update on forum link above.

Morgaine.

I had very good results using Texas Instruments SWIFT DC-DC converters to step down from a wide voltage range (sort of 7-24V) to a stable

5V output. Here is a sample circuit using the TP54231, very efficient, can handle up to 2A and if you wish you can put a bridge rectifier on

front and not worry about polarity.

-J

Thanks jamodio, that could be very useful.

In looking up more resources on the TPS54231, I found an application node / user guide with a fairly similar circuit but designed for 3.3V operation, which may be useful in other applications  --- http://www.ti.com/lit/ug/slvu263/slvu263.pdf  .  Circuit diagram is on p.17 of the PDF.

very nice posts jamodio and Morgaine.  I will have to try this in future experiments i do.